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Deck height tolerance for compression ratio?
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sslick
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PostPosted: Sat Oct 29, 2016 4:36 pm    Post subject: Deck height tolerance for compression ratio? Reply with quote

So, my deck height is within .003 of the specified height given on the vw engine calculator to achieve an 8.5:1 compression ratio. What is an acceptable tolerance for deck height between each cylinder and also what is an acceptable deviation from a given deck height on the engine calculator?
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ps2375
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PostPosted: Sat Oct 29, 2016 8:01 pm    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

Depending upon which calculator you use, as some only go one decimal place and others may go two. That much is pretty close. You'll want to check that the tops of cylinders are even with a good straight edge.
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Glenn Premium Member
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PostPosted: Sat Oct 29, 2016 8:05 pm    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

Run the numbers yourself: http://www.glenn-ring.com/tech/cr_calc.xls

Changing the deck fro, .060"(7.74:1) to .063"(7.69:1). That's just .05 difference... works for me.
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sslick
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PostPosted: Sat Oct 29, 2016 8:13 pm    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

ps2375 wrote:
Depending upon which calculator you use, as some only go one decimal place and others may go two. That much is pretty close. You'll want to check that the tops of cylinders are even with a good straight edge.

The CB performance calculator goes three decimal places (.xxx).
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sslick
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PostPosted: Sat Oct 29, 2016 8:23 pm    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

Glenn wrote:
Run the numbers yourself: http://www.glenn-ring.com/tech/cr_calc.xls

Changing the deck fro, .060"(7.74:1) to .063"(7.69:1). That's just .05 difference... works for me.

I did, Glenn. I plugged the numbers into the CB calculator. Here are my figures: 64mm stroke, 83mm bore, one head is 43cc, the other is 44cc. I am looking for 8.5:1 comp ratio. The calculator says .023 deck for the 43cc head and .016 for the 44cc head. My actual measurements of deck height on my engine are: cyl#1&2=.019 and #3=.022, #4=.025. so, I am wondering if using the 43cc head on #3&4 and the 44cc head on #1&2 would be recommended?
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sled
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PostPosted: Sat Oct 29, 2016 11:18 pm    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

Those are very tight deck heights. Maybe you want to flycut the heads slightly to achieve your desired comp ratio?

You definitely run the risk of having the Pistons collide with the chamber at .016 deck (as well as the engine going "solid" at higher Rpms) Many people say .030-.040 is minimum safe limit.
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sslick
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PostPosted: Sun Oct 30, 2016 4:18 am    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

sled wrote:
Those are very tight deck heights. Maybe you want to flycut the heads slightly to achieve your desired comp ratio?

You definitely run the risk of having the Pistons collide with the chamber at .016 deck (as well as the engine going "solid" at higher Rpms) Many people say .030-.040 is minimum safe limit.

Ok, I hear you. Here's another idea...if I run 8:1 comp (instead of 8.5), my deck for the 43cc head calculates out to .047, and .040 for the 44cc head. What if I remove a few thousandths from the barrels and add shims to get the deck where it needs to be? #1&2 measure the same deck height but 3&4 are .022 and .025. Is that .003 difference going to give me problems? I know I cannot remove an unequal amount from the barrels on that side of the engine...the head would not sit flat.
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sled
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PostPosted: Sun Oct 30, 2016 9:05 am    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

Im not sure why you would need to remove anything from the cylinders, just use shims between the cylinders and the case to increase your deck height.

what engine is this for, seems like a 40 horse? what cam are you running that you want 8.5:1 comp?

I'd shoot for .045 - .050 deck height for a 40 horse with stock internals. .050 is plenty tight to give you some squish. The chamber design of 40 heads is more of a limiting factor when being concerned with squish.
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sslick
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PostPosted: Sun Oct 30, 2016 9:27 am    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

sled wrote:
Im not sure why you would need to remove anything from the cylinders, just use shims between the cylinders and the case to increase your deck height.

Because shims are only available in .010 increments, in order to achieve the 8:1 compression and the required .047 deck (on the 43cc head) and the .040 deck (on the 44cc head), I would have to remove some material from the cylinders and then shim accordingly. I listed my current deck height above.
sled wrote:
what engine is this for, seems like a 40 horse? what cam are you running that you want 8.5:1 comp?

This is for a big bore 40hp with an Engle w100 cam. I have had second thoughts on the 8.5 compression based on the low deck height and possibility of collision

sled wrote:
I'd shoot for .045 - .050 deck height for a 40 horse with stock internals. .050 is plenty tight to give you some squish. The chamber design of 40 heads is more of a limiting factor when being concerned with squish.

I am using early type 3 heads which have been flycut for this engine (maybe flycut too deep?)
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Kjell Roar
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PostPosted: Tue Nov 01, 2016 10:06 am    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

A good tinsmith can make you some spacers in different thickness...
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PostPosted: Wed Nov 02, 2016 1:08 pm    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

sslick wrote:
ps2375 wrote:
Depending upon which calculator you use, as some only go one decimal place and others may go two. That much is pretty close. You'll want to check that the tops of cylinders are even with a good straight edge.

The CB performance calculator goes three decimal places (.xxx).


The CR only goes 1 decimal place.
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PostPosted: Tue Nov 22, 2016 9:52 pm    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

.042-.058"
no more than .008 variance

"racing" or 'blueprinted" tolerances, cut that in half
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PostPosted: Sat Jan 07, 2017 7:39 am    Post subject: Re: Deck height tolerance for compression ratio? Reply with quote

My deck height is .017 on a 69 x 83 40hp with 7.9:1 cr . Mild cam with 1.1:1 rockers too. 2 years and over 3000 miles with no issues. I got my .100 inch cylinder shims made by Rimco.
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