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hulbyw
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Joined: April 24, 2021
Posts: 196
Location: Australia
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 Posted: Sat Oct 18, 2025 3:14 am Post subject: Another Deck Height Question |
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Seems to me that most folks on here recommend a 40 thou deck height. I have used a 90 thou shim and a 10 thou shim to get the following deck heights on my 1904cc build:-
Cyl 1 = .048"
Cyl 2= .048"
Cyl 3 = .050"
Cyl 4 = .048"
Question for engine builders is do I call it done or remove all the 10 thou shims so I have
Cyl 1 = .038"
Cyl 2= .038"
Cyl 3 = .040"
Cyl 4 = .038" |
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Alstrup
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Joined: July 12, 2007
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Location: Videbaek Denmark
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Victor H
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Joined: November 28, 2004
Posts: 114
Location: Lexington, South Carolina
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| Posted: Sat Oct 18, 2025 5:12 am Post subject: Re: Another Deck Height Question |
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Stroke 74
Bore 90.5
Displacement 1904 |
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Glenn 
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Posts: 80390
Location: Sneaking up behind you
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Brian_e
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Posts: 4232
Location: Rapid City, South Dakota
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| Posted: Sat Oct 18, 2025 7:56 am Post subject: Re: Another Deck Height Question |
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I feel the deck height should be similar to something like the oil clearance on the mains. There is a high, and a low tolerance, and you don't go outside those numbers. On the deck height, low would be .037", and the high would be .045". I won't go outside those numbers on any build.
Cam and compression are somewhat irrelevant at this point. Get the deck height into the .037-.045 range. After that, you figure what compression you will need based on the cam, application, altitude, and fuel you want to run.
You then adjust the compression ratio with the volume of the combustion chamber.
The deck height should not be used to adjust the compression ratio!!
If it were me, I would ditch the .010" spacer and let it rip.
Brian
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BFB
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Joined: November 03, 2014
Posts: 3099
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| Posted: Sat Oct 18, 2025 8:29 am Post subject: Re: Another Deck Height Question |
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I think thats really straining at a gnats ass at this point, I mean were talking .2 difference in CR w/ or w/o the .010 shims.
assuming ( like a lot of people do it ) the heads are already purchased? and OP probably wouldn't be opening up or fly cutting his heads, it is what it is at this point.
although, I would agree with Brian_e and id leave out the .010 shims , and get as much compression as I could ( even though its minimal ) because unless the heads have really small chambers were only talking 8 - 8.5 CR.
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raygreenwood
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Joined: November 24, 2008
Posts: 23464
Location: Oklahoma City
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| Posted: Sat Oct 18, 2025 11:56 am Post subject: Re: Another Deck Height Question |
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| BFB wrote: |
I think thats really straining at a gnats ass at this point, I mean were talking .2 difference in CR w/ or w/o the .010 shims.
assuming ( like a lot of people do it ) the heads are already purchased? and OP probably wouldn't be opening up or fly cutting his heads, it is what it is at this point.
although, I would agree with Brian_e and id leave out the .010 shims , and get as much compression as I could ( even though its minimal ) because unless the heads have really small chambers were only talking 8 - 8.5 CR. |
While 0.2 compression ratio increase is small....depending on what you are doing its not "miniscule" or insignificant.
If you look at the 1.7L type 4 engine from its first iteration of 66hp with 6.8:1 compression and twin solexes....then the next variation with the same compression and fuel injection made 69hp.
So now you know what the small difference that injection can make (except that D-jet tunes very poorly at low compression so it could have better at higher compression).
The next variation, injected and 7.3:1 compression made 69 smogged for Cali and 73hp in the few 49 states that it came that way.
The final variation was 8.2:1 compression domed pistons and 82hp. Having run this same set up with flat top pistons at 8.0:1 compression...it made ~77-78hp.
The cams while a little bit different working into injection...were largely the same in most respects. The primary increase in hp was due to cmpression. Most of the small tuning moves that were done to ignition and injection were not even really possible without an increase in compression due to the type of injection system used mainly.
From early to late there was a total rise in compression of 1.4:1 and this helped produce a total increase of HP of 19.5%.
While its not a straight line....its pretty close to the truth that up to a point, the type 4 1.7L can increase hp by ~1.39% per 1/10th of a point of compression.
That 0.2:1 compression ratio rise along with the tuning it affords....allowed the 1.7L to gain right at 5hp (the difference between 7.8:1 with flat tops and 8.2:1 with domes).
But...all of that said....having the really tight deck if nothing else keeps more of the combustion heat up in the chamber and out of the cylinder.
As a last note using the 1.7L type 4 example....removing the "head killing" 0.023" thick head gaskets and the 0.009" cylinder base shim reduces the deck around the edge of the domed piston ...which is typically about 0.079"...to about 0.047.
This brings (usually) the compression up to ~8.4:1 and sometimes 8.5:1. This along with the better injection tuning that is now possible and a more accurate ignition thrown in can net right at another 5hp.....getting you to 87-88hp.
0.2:1 compression is not to be sneezd at unless you have no way to tune to use that increase.
If this all sounds trivial...just remember how many of the type 1 and type 3 guys for decades fought for increases above 30, 40 and 50 hp and each one was GOOD.
A 20+ percent increase in hp is good anytime Ray |
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Alstrup
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Joined: July 12, 2007
Posts: 7948
Location: Videbaek Denmark
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| Posted: Sat Oct 18, 2025 12:24 pm Post subject: Re: Another Deck Height Question |
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| Victor H wrote: |
Stroke 74
Bore 90.5
Displacement 1904 |
Then 0,038".
And correct, it is not so much the 0,2 - or whatever difference in CR as it is the improvement in squish and thereby mix quality as well as exploiting what comes in and finally burn temperature.
The better burn quality and the lesser unburned, - or mix ignited so late in the cyklus that it is virtually useless, the better power you get as well as less heat transferred to the heads and surrounding parts.
Going from 0,048 to 0,038" deck will 90% certain not be noticeable when you drive it, but it will definitely be measurable at the sniffer, and if you could compare the two side by side you would notice that the tighter deck will handle higher power for longer periods before the heads reaches unsafe temps.
To follow up on Ray´s analogy with type 4´s. Back in the days when we would install a bone stock (or close to) type 4´s in a beetle to get 1 more power, and 2 more reliability when driving fast, We took a 80 hp 1,7 W engine and tightened the deck, bumped the CR to 9-1 and gave them a better valve job. Used 36 or 40 mm dual carbs and typically a cut down and modified stock muffler. After that small engines typically pulled about 90 hp and when coupled up with a dialed in SVDA distributor they could do really really good fuel economy, like 33-35 mpg average. and 40 mpg if you drove it with respect.
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DesertSasquatchXploration
Samba Member
Joined: April 16, 2021
Posts: 1060
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| Posted: Sat Oct 18, 2025 12:48 pm Post subject: Re: Another Deck Height Question |
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Dude its perfect with the .038-.040 deck and a 55CC head your at 8.8CR and 8.6CR with the extra .010 shims
Looks like you have decent fuel 95/98 RON I wouldn't go lower then 8.6CR
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jpaull
Samba Member
Joined: February 22, 2005
Posts: 3678
Location: Paradise, Ca
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| Posted: Sat Oct 18, 2025 1:16 pm Post subject: Re: Another Deck Height Question |
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| hulbyw wrote: |
Seems to me that most folks on here recommend a 40 thou deck height. I have used a 90 thou shim and a 10 thou shim to get the following deck heights on my 1904cc build:-
Cyl 1 = .048"
Cyl 2= .048"
Cyl 3 = .050"
Cyl 4 = .048"
Question for engine builders is do I call it done or remove all the 10 thou shims so I have
Cyl 1 = .038"
Cyl 2= .038"
Cyl 3 = .040"
Cyl 4 = .038" |
Just to make sure, how are you checking your deck height? Do you have all 4 nuts torqued down?
The .038 would be preferable if you have all 4 nuts torqued just like it would be when its built. If not, and you only used 2 nuts or not torqued down fully, your .038 might be .030. In that case, your first readings might end up closer to .040
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chrisflstf
Samba Member
Joined: February 10, 2004
Posts: 4220
Location: San Diego
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| Posted: Sat Oct 18, 2025 1:48 pm Post subject: Re: Another Deck Height Question |
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| What Jeff said above. Ive found the deck to decrease about .005" to .006" comparing all 4 studs fully torqued to just the cylinder sitting there, or lightly snugged down |
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BFB
Samba Member
Joined: November 03, 2014
Posts: 3099
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| Posted: Sun Oct 19, 2025 8:30 am Post subject: Re: Another Deck Height Question |
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| raygreenwood wrote: |
| BFB wrote: |
I think thats really straining at a gnats ass at this point, I mean were talking .2 difference in CR w/ or w/o the .010 shims.
assuming ( like a lot of people do it ) the heads are already purchased? and OP probably wouldn't be opening up or fly cutting his heads, it is what it is at this point.
although, I would agree with Brian_e and id leave out the .010 shims , and get as much compression as I could ( even though its minimal ) because unless the heads have really small chambers were only talking 8 - 8.5 CR. |
While 0.2 compression ratio increase is small....depending on what you are doing its not "miniscule" or insignificant.
If you look at the 1.7L type 4 engine from its first iteration of 66hp with 6.8:1 compression and twin solexes....then the next variation with the same compression and fuel injection made 69hp.
So now you know what the small difference that injection can make (except that D-jet tunes very poorly at low compression so it could have better at higher compression).
The next variation, injected and 7.3:1 compression made 69 smogged for Cali and 73hp in the few 49 states that it came that way.
The final variation was 8.2:1 compression domed pistons and 82hp. Having run this same set up with flat top pistons at 8.0:1 compression...it made ~77-78hp.
The cams while a little bit different working into injection...were largely the same in most respects. The primary increase in hp was due to cmpression. Most of the small tuning moves that were done to ignition and injection were not even really possible without an increase in compression due to the type of injection system used mainly.
From early to late there was a total rise in compression of 1.4:1 and this helped produce a total increase of HP of 19.5%.
While its not a straight line....its pretty close to the truth that up to a point, the type 4 1.7L can increase hp by ~1.39% per 1/10th of a point of compression.
That 0.2:1 compression ratio rise along with the tuning it affords....allowed the 1.7L to gain right at 5hp (the difference between 7.8:1 with flat tops and 8.2:1 with domes).
But...all of that said....having the really tight deck if nothing else keeps more of the combustion heat up in the chamber and out of the cylinder.
As a last note using the 1.7L type 4 example....removing the "head killing" 0.023" thick head gaskets and the 0.009" cylinder base shim reduces the deck around the edge of the domed piston ...which is typically about 0.079"...to about 0.047.
This brings (usually) the compression up to ~8.4:1 and sometimes 8.5:1. This along with the better injection tuning that is now possible and a more accurate ignition thrown in can net right at another 5hp.....getting you to 87-88hp.
0.2:1 compression is not to be sneezd at unless you have no way to tune to use that increase.
If this all sounds trivial...just remember how many of the type 1 and type 3 guys for decades fought for increases above 30, 40 and 50 hp and each one was GOOD.
A 20+ percent increase in hp is good anytime Ray |
while I see the point you and Alstrup make i still say its straining at a gnats ass. for one, none of the examples you gave were of a .2 increase in compression, it was in .5 increments with the exception of the 1.7 that went from 7.8:1 to 8.2:1 which is .4, not .2 and only gained 5hp. while that 5 hp is definitely good, if you halved that ( I know it doesn't work exactly this way ) you'd be talking 2.5 hp for .2 increase in compression.
which leads me to my second point, you said the 1.7 went from 66 hp to 69 with fuel injection. I think a lot of times ya'll make the assumption that someone asking these questions will get the tuning right on the money, I dont. ( no offense to OP intended ) yes in a perfect scenario that .2 with proper timing , fuel management , and everything else being in order will make a difference but with all the vw engines ive worked on for other people I think performance should typically be deducted by their tuning and maintenance.
so even if we give the benefit of the doubt that .2 adds 5hp , we can probably deduct 10 hp for any number of things even in the build like poor breathing capabilities or cooling, then theirs tuning like the carb not being jetted, or timing not optimal. then there's the maintenance things like valve adjustment, burned points or fouled plugs... the list goes on. and I know y'all have seen this as much or more than I have. Ive picked up numerous Buggys cheap because the owners thought the engine was junk, some barely ran others wouldn't start at all, 95% of them just needed a tune up, carb cleaned, and valves adjusted.
so its not that I say this like ya'll aren't correct, and nothing wrong with giving OP the optimal answer, but in all reality most people wouldn't know the difference in there engine
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Alstrup
Samba Member
Joined: July 12, 2007
Posts: 7948
Location: Videbaek Denmark
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| Posted: Sun Oct 19, 2025 9:28 am Post subject: Re: Another Deck Height Question |
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Youre comparing apples to oranges. One thing is making it run, another to make it run well, and close to its best.
When you begin on a clean sheet of paper, might as well do it right. There are so many engines out there which are "built" but really just thrown together, and will never perform even close to its potential. I had one as late as this friday. Home built and he was proud, because it was FAST, but needed a fine tune. Thought he had about 140 hp since he "copied" one of my builds, but with a different brand cam and different, but "similar" heads. (his words) At the moment of truth he had 117 hp and a wimpy torque curve. But, but, it revs to 7000... Sure, but it doesnt pull.
This is why I keep banging on those very important aspects if you want a good result.
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raygreenwood
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Joined: November 24, 2008
Posts: 23464
Location: Oklahoma City
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| Posted: Sun Oct 19, 2025 3:45 pm Post subject: Re: Another Deck Height Question |
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| BFB wrote: |
| raygreenwood wrote: |
| BFB wrote: |
I think thats really straining at a gnats ass at this point, I mean were talking .2 difference in CR w/ or w/o the .010 shims.
assuming ( like a lot of people do it ) the heads are already purchased? and OP probably wouldn't be opening up or fly cutting his heads, it is what it is at this point.
although, I would agree with Brian_e and id leave out the .010 shims , and get as much compression as I could ( even though its minimal ) because unless the heads have really small chambers were only talking 8 - 8.5 CR. |
While 0.2 compression ratio increase is small....depending on what you are doing its not "miniscule" or insignificant.
If you look at the 1.7L type 4 engine from its first iteration of 66hp with 6.8:1 compression and twin solexes....then the next variation with the same compression and fuel injection made 69hp.
So now you know what the small difference that injection can make (except that D-jet tunes very poorly at low compression so it could have better at higher compression).
The next variation, injected and 7.3:1 compression made 69 smogged for Cali and 73hp in the few 49 states that it came that way.
The final variation was 8.2:1 compression domed pistons and 82hp. Having run this same set up with flat top pistons at 8.0:1 compression...it made ~77-78hp.
The cams while a little bit different working into injection...were largely the same in most respects. The primary increase in hp was due to cmpression. Most of the small tuning moves that were done to ignition and injection were not even really possible without an increase in compression due to the type of injection system used mainly.
From early to late there was a total rise in compression of 1.4:1 and this helped produce a total increase of HP of 19.5%.
While its not a straight line....its pretty close to the truth that up to a point, the type 4 1.7L can increase hp by ~1.39% per 1/10th of a point of compression.
That 0.2:1 compression ratio rise along with the tuning it affords....allowed the 1.7L to gain right at 5hp (the difference between 7.8:1 with flat tops and 8.2:1 with domes).
But...all of that said....having the really tight deck if nothing else keeps more of the combustion heat up in the chamber and out of the cylinder.
As a last note using the 1.7L type 4 example....removing the "head killing" 0.023" thick head gaskets and the 0.009" cylinder base shim reduces the deck around the edge of the domed piston ...which is typically about 0.079"...to about 0.047.
This brings (usually) the compression up to ~8.4:1 and sometimes 8.5:1. This along with the better injection tuning that is now possible and a more accurate ignition thrown in can net right at another 5hp.....getting you to 87-88hp.
0.2:1 compression is not to be sneezd at unless you have no way to tune to use that increase.
If this all sounds trivial...just remember how many of the type 1 and type 3 guys for decades fought for increases above 30, 40 and 50 hp and each one was GOOD.
A 20+ percent increase in hp is good anytime Ray |
while I see the point you and Alstrup make i still say its straining at a gnats ass. for one, none of the examples you gave were of a .2 increase in compression, it was in .5 increments with the exception of the 1.7 that went from 7.8:1 to 8.2:1 which is .4, not .2 and only gained 5hp. while that 5 hp is definitely good, if you halved that ( I know it doesn't work exactly this way ) you'd be talking 2.5 hp for .2 increase in compression.
which leads me to my second point, you said the 1.7 went from 66 hp to 69 with fuel injection. I think a lot of times ya'll make the assumption that someone asking these questions will get the tuning right on the money, I dont. ( no offense to OP intended ) yes in a perfect scenario that .2 with proper timing , fuel management , and everything else being in order will make a difference but with all the vw engines ive worked on for other people I think performance should typically be deducted by their tuning and maintenance.
so even if we give the benefit of the doubt that .2 adds 5hp , we can probably deduct 10 hp for any number of things even in the build like poor breathing capabilities or cooling, then theirs tuning like the carb not being jetted, or timing not optimal. then there's the maintenance things like valve adjustment, burned points or fouled plugs... the list goes on. and I know y'all have seen this as much or more than I have. Ive picked up numerous Buggys cheap because the owners thought the engine was junk, some barely ran others wouldn't start at all, 95% of them just needed a tune up, carb cleaned, and valves adjusted.
so its not that I say this like ya'll aren't correct, and nothing wrong with giving OP the optimal answer, but in all reality most people wouldn't know the difference in there engine |
While my point wasn't exactly about a "0.2" compression ratio bump.....it DID show how very small bumps in compression actually make a good percentage of difference....especially if you CAN tune for that difference.
However, to your point, the vast majority of home engine builders do not have that much resolution and accuracy in tuning....and many do not care to do the fine tweaking anyway because it runs "good enough" or they can't "feel" or measure the difference.
My post though was spot on about the fact that from start to finish on the factory builds for the 1.7L engine....the total improvement in compression from 1967 to 1974 only totalled 1.4:1....an increase that many already sneeze at, but it did make a collective ~20% increase in power with the same displacement, heads and close to the same cam.. But each minor increment brought improvement if not in wholesale HP....in throttle response.
I can also tell you that those listed engines were just factory advertised compression steps/numbers. I have built plenty of type 4 with only a 0.2:1 difference from the previously identical build of another engine that noticeably improved output and tuneability for better throttle response.
Among D-jet owners, especially in 411/412, 914, Mercedes and Volvo, we see a lot more work on the parts of owners and shop owners to do the extra work on tuning as tightl6 as they can....because without doing it to the best you can with parts and effort....the system can otherwise be miserable to drive.
A lot of them know what improvement can be made with 0.2:1 changes....once you get over a basic minimum. A compression ratio increase of only 0.2:1 will probably not help if you are only starting at 6.5:1....or even 7.5:1. But once you get to ~8.0:1.....every 0.2:1 makes noticeable improvement in tuning capabilities. Ray |
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BFB
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Joined: November 03, 2014
Posts: 3099
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| Posted: Mon Oct 20, 2025 6:27 am Post subject: Re: Another Deck Height Question |
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| Alstrup wrote: |
Youre comparing apples to oranges. One thing is making it run, another to make it run well, and close to its best.
When you begin on a clean sheet of paper, might as well do it right. There are so many engines out there which are "built" but really just thrown together, and will never perform even close to its potential. I had one as late as this friday. Home built and he was proud, because it was FAST, but needed a fine tune. Thought he had about 140 hp since he "copied" one of my builds, but with a different brand cam and different, but "similar" heads. (his words) At the moment of truth he had 117 hp and a wimpy torque curve. But, but, it revs to 7000... Sure, but it doesnt pull.
This is why I keep banging on those very important aspects if you want a good result. |
thats exactly my point, not sure how your missing it and think its apples & oranges.
also dont see any reason to piss on someones parade , if they think its fast and enjoy it, then let them. and you should be happy, you know the saying 'imitation is the ultimate form of flattery'...
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Alstrup
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Joined: July 12, 2007
Posts: 7948
Location: Videbaek Denmark
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| Posted: Mon Oct 20, 2025 7:01 am Post subject: Re: Another Deck Height Question |
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Well, I could of course have turned the correction factor off and pulled the handbrake slightly on when measuring transmission loss, but would that be more right?
| raygreenwood wrote: |
A lot of them know what improvement can be made with 0.2:1 changes....once you get over a basic minimum. A compression ratio increase of only 0.2:1 will probably not help if you are only starting at 6.5:1....or even 7.5:1. But once you get to ~8.0:1.....every 0.2:1 makes noticeable improvement in tuning capabilities. Ray |
I absolutely hear you.
10-12 years ago, I forget, we were at a dyno day where there was among others a nice imported type3 with a 2007 Berg engine. Berg "110" cam, stock valves, Hemi heads, LOW compression, DCNF carbs and some sort of type 3 header. I do not recall the brand or exact type. That thing couldnt fall from a tree on its own. Totally ridiculess. If memory serves it was something like 92-94 hp and not more torque than what a decent 1600 would pull. But it ran nice, didnt run hot, unless pushed on the motorway for longer periods of time.
Some 3-4 years later a friend of mine bought the car, well knowing this. So he took it apart, and sure enough, The heads were Hemiéd about max, and 0,070" deck height, so the static compression was 6,9. We flycut the heads some 0,100" and gave them a little more modern valve job, corrected the deck resulting in 9-1. First we could pull 5 degrees timing, then it wanted smaller idle and more main, so a little more back and forth with timing and jetting, but we ended at about 33 degrees. Power increased to the neighbourhood of 115 hp and 175ish Nm torque . After that little surgery the car became a totally different animal. While no rocket ship it at least did something when you floored it.
Ohh, and an interesting note. Fuel economy on cruise was about the same. But on the motorway it became about 15% more efficient... (with a regular German 009)
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sidemarkers
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| Posted: Tue Oct 21, 2025 5:19 pm Post subject: Re: Another Deck Height Question |
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Clatter
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Posts: 7846
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| Posted: Tue Oct 21, 2025 7:24 pm Post subject: Re: Another Deck Height Question |
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I’m more likely to be alarmed by the .002 difference in deck between 3 & 4.
If it’s a shorter rod or piston or rod throw off,
Then no big deal.
If the heads have to deal with two cylinders being .002 different in height,
Then I’d be more concerned.
Hopefully the deck on the case is flat and the cylinders are the same length.
Just thoughts when i see those numbers….
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hulbyw
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Posts: 196
Location: Australia
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| Posted: Thu Oct 23, 2025 5:10 pm Post subject: Re: Another Deck Height Question |
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Thanks folks for all the responses.
The build is not being thrown together however I am the first to admit I am self taught and this is the first non stock engine I have built and the second engine I have ever built.
The heads are 51.5cc, the cam is a 2239 copy, rockers stock and I will be using 98 octane fuel The CR will be around 9 to 1.
Just have to find time between medical appointments and other "stuff" to re measure without the 10 thou shim. |
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hulbyw
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| Posted: Sat Nov 01, 2025 12:36 am Post subject: Re: Another Deck Height Question |
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OK so I have found some time.
After a few adjustments and consistent set up between cylinders including using a torque wrench all decks are now at .045", using both shims. Calling it done.
CR calculates at 9:1 according to the CB calculator. Now to use Brian's Rocker Geometry vid and get a friend with a lathe to cut my aluminium pushrods. Thanks again for the help. |
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