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What would you do with these deck heights?
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rodeking
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PostPosted: Wed Oct 03, 2018 11:57 am    Post subject: What would you do with these deck heights? Reply with quote

I'm finally getting around to assembling this 1679. I had the case line bored and decked. I'm getting the following deck heights without any base gaskets:

#1 .035"
#2 .035
#3 .041
#4 .036

The options as I see it are:

1) run it as is
2) use .010 shims under all, not sure .005 are available, maybe paper gaskets?
3) take .005 off #1, #2 and #4 pistons in the lathe

I'm shooting for 8.3 - 8.4 CR with new Mofofco heads. I like option #3 but maybe I'm worrying about this too much and should just get some .010 shims and proceed?
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mcmscott
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PostPosted: Wed Oct 03, 2018 12:05 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

I'd cut and re-balance.
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PostPosted: Wed Oct 03, 2018 12:36 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

I would recheck and find out what is rong.
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Brian_e
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PostPosted: Wed Oct 03, 2018 12:44 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

First swap the pistons with #2 and #3 and remeasure. If the numbers stay the same, then swap the rods. If the numbers still stay the same, try swapping just the barrels around. If the numbers stay the same, then your crank is ground off, or your case needs to be decked.

Or just cut the piston tops and re balance.

Brian.
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rodeking
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PostPosted: Wed Oct 03, 2018 6:46 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

I'm going to start swapping pistons, jugs, rods if I get some time. The crank is original, never ground, and I just had the cases decked, hopefully correctly. I measured the jug heights, they are all within .001.

I guess my other option is get some .005 copper shim stock and cut a couple shims for 1 and 2 and only cut #4 piston and rebalance.

Guess I don't have to swap jugs since they are all so close so it's either pistons, rods or bad deck job.
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Dale M.
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PostPosted: Wed Oct 03, 2018 7:18 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

As super quick check I would compare one of the .035 pistons against the .041 piston and see if there is variance in piston deck (top) to pin height....

It's possible to have a "short" piston...

What does straight edge across cylinders tell you???

Dale
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vwracerdave
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PostPosted: Wed Oct 03, 2018 7:48 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

rodeking wrote:
I'm finally getting around to assembling this 1679.
#1 .035"
#2 .035
#3 .041
#4 .036


I'd add a .010" barrel shim and build it. If I had a lathe I might cut #3. I would not pay to have it done.
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Howard 111
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PostPosted: Wed Oct 03, 2018 7:48 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

Did you use a deck height measuring tool?
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mcmscott
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PostPosted: Wed Oct 03, 2018 8:36 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

Most "engine builders" have a lathe, so if you dont, then take your parts to an engine builder. But it sounds like you have a lathe.
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PostPosted: Wed Oct 03, 2018 8:37 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

Are we to guess what your:
Bore
Stroke
CC of heads
desired compression ratio
Cam
If so I agree with:
vwracerdave wrote:

I'd add a .010" barrel shim and build it. If I had a lathe I might cut #3. I would not pay to have it done.

if the deck measurements are repeatable.
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mcmscott
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PostPosted: Wed Oct 03, 2018 8:37 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

Howard 111 wrote:
Did you use a deck height measuring tool?


He probably used a yard stick to come up with measurements within .005"
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PostPosted: Wed Oct 03, 2018 8:50 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

Whichever way works for you.
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rodeking
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PostPosted: Wed Oct 03, 2018 9:02 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

88 thick walls, stock 1600 crank, desired CR is 8.3, CB cheater cam

as i said, new Mofoco heads so they will set the CC I need to get 8.3.

yeah, I have a lathe

"deck height measuring tool" is 1/4'' 6061 strap clamping down the jugs and feeler gauges

I know I can just slap in some .010 shims but I like to run deck as tight as possible, not a real necessity for this engine but that's what I like to do
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Slow 1200
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PostPosted: Wed Oct 03, 2018 11:06 pm    Post subject: Re: What would you do with these deck heights? Reply with quote

do the barrels sit at the same height when installed?
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PostPosted: Thu Oct 04, 2018 1:05 am    Post subject: Re: What would you do with these deck heights? Reply with quote

Brian_e wrote:
First swap the pistons with #2 and #3 and remeasure. If the numbers stay the same, then swap the rods. If the numbers still stay the same, try swapping just the barrels around. If the numbers stay the same, then your crank is ground off, or your case needs to be decked.

Or just cut the piston tops and re balance.

Brian.


^^ This. Figure out what each part is doing. Take one rod, one piston and one barrel and try them on each cylinder position and mark each big end +002" or -0.04" etc as the deck measures relative to the first one you measure which you'll mark 0.000" - it's actually a combination of crank inaccuracy and case machining but that's not important coz you cant change it. Do the same for the rest of the parts ie. use same cyl hole, piston and barrel to check the rods and mark them. At the end stack the numbers up to be as close to each other as possible for each combination of crank pos, rod, piston and barrel. You'll be surprised at what you can manage. I went from somewhere around +/-0.005" to < +/-0.001" using this method. Good luck!
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PostPosted: Thu Oct 04, 2018 5:03 am    Post subject: Re: What would you do with these deck heights? Reply with quote

Don't tighten your deck tool down too much, and tighten evenly with same side down always, the 1/4 strap could be flexing up a little in the middle, maybe teetering a little each time you check. I think your close enough with the tools that you are using. Better yet use your tool to hold cyl. down then use a good steel rule on edge to check deck. 1/4 aluminum bar isn't a good reference surface for checking a couple thou.
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rodeking
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PostPosted: Thu Oct 04, 2018 7:25 am    Post subject: Re: What would you do with these deck heights? Reply with quote

It's the piston.

I put that #3 piston/jug in all the holes and I get a consistent .039-.041 deck in all of them. I label my pistons/jugs with letters, it's "C" in the pics.

"A" was in #1, "B" in #2, "C" in #3 and "D" was in #4 when I measured the deck in the first post.

Then I measured all the pistons from the top down to the pin with my Mitu 571 digital depth gauge. That takes some time to make sure you get on the very top of the pin but it consistently showed "C" piston as being short.

My deck strap is actually 3/8 and it measures within 1 thou no matter what side I have up but I always put the same side up.

Now to get some .005 copper shim stock to shim 1 and 2. And make a fixture to hold "D" piston to cut it .005. That should give me a 40-41 deck on all holes. Very Happy
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Brian_e
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PostPosted: Thu Oct 04, 2018 9:10 am    Post subject: Re: What would you do with these deck heights? Reply with quote

All of your measuring and figuring looks correct from here.

If I were building it for myself, instead of fumbling with a very fragile .005 shim, I would trim the other side barrels .005" and run them all .035".Twisted Evil

I like your building style. Might as well push the limits some and experiment with small details rather then tossing big shims in and taking the sloppy easy way like everyone else.

What carbs and cam are you going to be running in this thing? Are you doing the super street 37x32 heads?

Brian
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Brian_e
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PostPosted: Thu Oct 04, 2018 9:14 am    Post subject: Re: What would you do with these deck heights? Reply with quote

vwracerdave wrote:
rodeking wrote:
I'm finally getting around to assembling this 1679.
#1 .035"
#2 .035
#3 .041
#4 .036


I'd add a .010" barrel shim and build it. If I had a lathe I might cut #3. I would not pay to have it done.


If he cut the #3 piston, it would then be an even larger difference in deck height. It is already further down the hole. Cutting another .005" off the top of #3 will then make it a .046" deck height on just that one hole.

I am only mentioning this because I did your same math once and created a bunch more work for myself getting it straightened back out.

Brian
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vwracerdave
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PostPosted: Thu Oct 04, 2018 9:45 am    Post subject: Re: What would you do with these deck heights? Reply with quote

Brian_e wrote:
vwracerdave wrote:
rodeking wrote:
I'm finally getting around to assembling this 1679.
#1 .035"
#2 .035
#3 .041
#4 .036


I'd add a .010" barrel shim and build it. If I had a lathe I might cut #3. I would not pay to have it done.


If he cut the #3 piston, it would then be an even larger difference in deck height. It is already further down the hole. Cutting another .005" off the top of #3 will then make it a .046" deck height on just that one hole.

I am only mentioning this because I did your same math once and created a bunch more work for myself getting it straightened back out.

Brian



You are correct and I appreciate you correcting my mistake and pointing this out. For this engine and because he does have a lathe, I would cut the other 3 pistons to equal #3 with .041" deck and build the engine without any shims.



On a little 1679 with 8.3 compression you will never be able to measure the difference on a dyno worrying about .005 deck height.
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