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Old 10-10-2011, 06:42 PM
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Deck height variations between cylinders

Hey guys building a 383 stroker. I've got the entire short block assembled now and proceeded to "re-check" the deck heights with it all together. Well....... the deck height figures I am getting now with everything installed are not what I got when I originally measured it in all 4 corners with no rings on the pistons. The block was decked when I got the machine work done, how much they took off I don't know but it was probably only a couple thou. to restore the flatness. Anyways below are how far the pistons are sitting in the cylinders at TDC, measured yesterday.

#1- 0.028"
#2- 0.024"
#3- 0.026"
#4- 0.024"
#5- 0.029"
#6- 0.025"
#7- 0.028"
#8- 0.025"

When I did my original measuring I used one pistons/rod combo on all cylinders to eliminate differences in rod length & compression heights between pistons and rods. My measurements then were within 0.002" of eachother. With these official measurements I have a spread of 0.005" from the highest (cylinders 2&4) to the deepest (cylinder 5). When comparing sides to themselves it is closer and front to back is almost identical. I guess what I am trying to figure out is how come I got such a variance in deck height. I am going to assume that not all my pistons are actually a true 1.125 compression height and or not all my rods are a true 6 inches. I am going to go ahead with the build as it is already assembled and it isn`t coming back apart but do you think having deck height variations as posted above would be noticable. I would have slightly variating compression in some cylinders which I am going to calculate but how much of a difference this would make I do not know. Also, I need to find a damn head gasket for this thing (aluminum head compatible) that will give me the lowest possible quench figure. I am a little puzzled, however, how I could have a piston depth of any greater than 0.025 since this is what I should have if no decking was performed. To have a deck height greater than 0.025 when my piston, rod, crank combo measures 9 inches AND the block has been decked is beyond me.........

Keith

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Old 10-10-2011, 08:29 PM
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Your 1-3-5-7 cylinders are within .002 + -. 2-4-6-8 are also. Double check the no. 3 measurement. If you just had block decked "flat" that explains the difference between sides. If you are seeking equal distance between sides the block deck must be "squared" and "parallel milled". Or milled to your spec. Considerably more expense.

Factory production blocks are seldom square or equal deck.

The rest can be tolerance stack.
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Old 10-10-2011, 08:48 PM
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Thanks for the reply Bob. So what is the max allowable variation then between individual cylinders is my question I guess. Will I be okay to put the rest of the engine together like this and run it. I used a compression calculator and between my highest reading (0.024) and my lowest reading (0.029) I would drop 0.1 point of compression. I would be at 9.9:1 and 9.8:1, respectively. Is that going to cause problems. Also I need to find some sort of ultra thin (0.015) head gasket that is compatible with aluminum heads to arrive at a good quench. I don`t think such a gasket exsists. Or I could use a popular fel pro MLS gasket with 0.028 compressed thickness and I would be at 0.057 quench on my worst cylinder. Is that acceptable. What would you suggest I do with this.

P.S.- My question mark button is not working, that`s why the punctuation is not correct.

Keith
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Old 10-11-2011, 06:13 AM
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Run it.. I've seen "Race" motors built by big name brand shops with more variation between cylinders. Max variation allowed is set by the builder. Will be expensive to get closer if you can't just swap bores/piston/rod assys.

As for the gaskets. You will probably have to live with a bit of excessive quench.. Take that in mind with machining on next build .. Shoot for .040 total.
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Old 10-11-2011, 09:02 PM
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Awesome, I wasn't going to take it back apart no matter what........ this build is taking forever and I am going to work with what I've got at this point. I did drop the ball on the decking however, I wish I had read the proper way to do it before getting all my block machine work done. Oh well, gonna have to go ahead with it. I still don't understand how I could be any more than 0.025" in the hole, especailly since the deck has been resurfaced!! I also understand that quench distance isn't so important when using aluminum heads since aluminum transfers heat out of the chamber faster than iron heads. I also have noticable piston rock at TDC and a larger piston to wall clearance for my forged pistons..... 0.004" for all 8 cylinders. With this I would want to widen my quench a bit. As far as the gaskets are concerned...... I'm still not sure what the absolute thinnest compatible gasket is for my build, guess I'll keep looking. Oh and I am going to go recheck the #3 deck height, just to be sure.

Thanks,
Keith
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Old 10-12-2011, 06:11 AM
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Many Rebuilder Pistons have reduced Compression Height but, you're using Forged so that's not going to be the problem with the Deck Height. Your Block must have had an excessive amount of clearance. I've seen several with up to .032" to .037" stock.
How about using the FelPro 1094 gaskets? > http://www.summitracing.com/parts/FEL-1094/
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Old 10-12-2011, 06:32 AM
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When you assemble several components together the tolerance differences "stack" as you just found out. A thousandth here and there sure add up. You could move pistons and rods around until things get closer or you could just assemble it and not sweat it. You have what, .005" between the shortest and tallest? I wouldn't worry about it!
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Old 10-12-2011, 08:13 AM
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Pistons rock at TDC so measuring them could be tricky. I wouldn't worry about it, but if you can't find the gasket you are looking for, you could shave your heads couldn't you?
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Old 10-12-2011, 08:43 AM
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No, shaving the heads does nothing as far as changing the quench. As SSedan64 already mentioned the gasket to use would be FelPro Q1094 which is a coated .015 thick gasket.
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Old 10-12-2011, 09:56 AM
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I totally agree with you on the gasket, def aluminum compatible and would yield the desired quench. I was thinking on terms of his compression he could shave his heads if he had to run a larger gasket, so he can get compression back up, I don't think it is an issue though.
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Old 10-12-2011, 04:49 PM
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Are you checking Deck Height inline with the Piston Pin?
Example >> http://www.santabarbaramusclecars.co...e-build/16.jpg
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Old 10-13-2011, 11:13 PM
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Thanks for the replies. I am measuring the piston height at TDC at the pin centerline, right in the center of the piston to be more exact. See pic below to view how I am performing this measurement.



I rechecked piston #3 height, measured out to 0.026" again or damn close to it. So..... all of those measurements provided in my original post are accurate. I have a 0.005" spread, which isn't great but for a street machine I don't think it would matter much. My stacked height should be 9" (1.875" crank, 6" eagle H beams, 1.125" wiseco forgies = 9"). With that in mind I must have a freak tall deck that exceeds the 9.025" it should be. It is a 3970010 GM block, early 70's IIRC.

Question: That gasket you suggested would be the PERFECT match for this build, however, I do not believe it is compatible with aluminum heads since it is not a MLS gasket. That is just a rubber covered steel shim and I don't think those gaskets allow the movement between the two different metals and cause brinelling, I could be wrong. I was under the impression that you had no choice but to use a MLS gasket between a cast iron/aluminum sandwiche. If I am able to use that gasket I am all for it as that will give me a quench of 0.039" to 0.044"............... right on the money!! Could anybody please explain if this gasket would work with the aluminum heads or not. Oh and I wouldn't read much into fel pro's description of the gasket on summit, theyve got that same description for ALL gaskets.

Thanks again,
Keith
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Old 10-14-2011, 01:03 AM
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Victor Reinz Nitroseal p/n 5746- Compressed thickness is 0.025", 4.1” bore. NAPA carries Victor.

GM 10105117- Multi-layered stainless steel gasket with a 4.1 bore, .028" thick, will handle some surface irregularities of the deck and head surfaces (supposed to be the “revised” gasket, better seal- Post #23 ). $21/ea. @ Summit.

GM 14096405- Stainless steel layers over a graphite core (or graphite on one side, SS on the other), is .028" thick, 4.1” bore.

For use w/aluminum heads the fire ring needs to be annealed or preflattened- I do not know what the fire ring specs are for these HG so they need to be verified.

The rubber coated shim will prevent galvanic action but the question then becomes if they will have enough "give" to keep sealed w/the different amounts/rates of change from heat between iron and aluminum.

machine shop guys, a question about aluminum heads and head gaskets.
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Old 10-15-2011, 12:10 AM
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I've used the 1094's on several Engines w/Al. Heads with no problems. I also retorque these after a heat cycle.
A little cheaper >> http://www.flatlanderracing.com/felprochevysbhead.html
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Old 10-15-2011, 02:10 PM
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Quote:
Originally Posted by 383SBC
Hey guys building a 383 stroker. I've got the entire short block assembled now and proceeded to "re-check" the deck heights with it all together. Well....... the deck height figures I am getting now with everything installed are not what I got when I originally measured it in all 4 corners with no rings on the pistons. The block was decked when I got the machine work done, how much they took off I don't know but it was probably only a couple thou. to restore the flatness. Anyways below are how far the pistons are sitting in the cylinders at TDC, measured yesterday.

#1- 0.028"
#2- 0.024"
#3- 0.026"
#4- 0.024"
#5- 0.029"
#6- 0.025"
#7- 0.028"
#8- 0.025"

When I did my original measuring I used one pistons/rod combo on all cylinders to eliminate differences in rod length & compression heights between pistons and rods. My measurements then were within 0.002" of eachother. With these official measurements I have a spread of 0.005" from the highest (cylinders 2&4) to the deepest (cylinder 5). When comparing sides to themselves it is closer and front to back is almost identical. I guess what I am trying to figure out is how come I got such a variance in deck height. I am going to assume that not all my pistons are actually a true 1.125 compression height and or not all my rods are a true 6 inches. I am going to go ahead with the build as it is already assembled and it isn`t coming back apart but do you think having deck height variations as posted above would be noticable. I would have slightly variating compression in some cylinders which I am going to calculate but how much of a difference this would make I do not know. Also, I need to find a damn head gasket for this thing (aluminum head compatible) that will give me the lowest possible quench figure. I am a little puzzled, however, how I could have a piston depth of any greater than 0.025 since this is what I should have if no decking was performed. To have a deck height greater than 0.025 when my piston, rod, crank combo measures 9 inches AND the block has been decked is beyond me.........

Keith
Pretty normal, you're looking at production tolerance. Between here and perfect is blueprinting. While that is effective in an engineering sense it isn't financially unless your income hinges on winning races.

X2 on Cobalt's gasket recommendations, the stainless faced, graphite core jobs are super good. They keep the quench/squish tight while having the inter-layer shear absorption to keep aluminum and cast iron parts leak free.

Bogie
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