I see this listed under piston discriptions, but I don't understand what it is, or how it relates to piston selection.

Thanks Bill

The distance from the center of the pin hole to the top of the piston.

This distance is affected by the crank stroke, rod length and deck height.

The distance from the center of the pin hole to the top of the piston.

Distance to actual piston face...not to the top of dome or base of dish.

http://www.rosspistons.com/images/comp_1.jpg

http://www.rosspistons.thinkhost.com/custom_compression_height.php

Thank you, I only saw it listed for some pistons I was looking at. I now see it is relationship to rod length.

Thanks Bill

Example......

Chevy just shortened the rod in the 400 SB to 5.565" and left it's CH the same as the 350 with a 5.7" long rod to give you an idea how this works..

pdq67

Example......

Chevy just shortened the rod in the 400 SB to 5.565" and left it's CH the same as the 350 with a 5.7" long rod to give you an idea how this works..

pdq67

Do you what the reasoning was behind this idea? I used a slightly higher CH with the stock 400 rods.

Back again, my bad...........................

Anybody know for sure??

pdq67

PS., and you can look at SB's like this...

265, 283, 302 = 3" stroke;

307, 327 = 3.25" stroke; and

267, 305, 350 = 3.48" stroke;

As a general rule 350 pistons and 5.565 (400) rods won't work with a 400 stroke crank. The 400 counter weights will hit the bottom of the 350 pistons ala 383.

Not only is the 400 rod shorter the big end was redesigned to give clearance between the rod bolts and cam.

As to why GM didn't use the longer 5.7" rod I'm not sure. Maybe the decreased CH didn't allow for ring spacing they liked.

302 Chevy is 1.8
327 is 1.675
350 is 1.56
The stock bore is the same for all these blocks, but the stroke is different, so the compression heights are also, meaning the pin locations are different. If you run a 327 piston in a 350 block, with a 350 crankshaft, the piston sits further down in the bore, and you don't develop compression. The relationship is more to stroke, as well as deck heighth, and somewhat to rod length. All must be factored when assembling the engine. The actual meaning is: The measured area from the top of the pin hole to the FLAT top of the piston (not any dome heighth), then you add 1/2 of the pins diameter to get the compression heighth measurement.

As a general rule 350 pistons and 5.565 (400) rods won't work with a 400 stroke crank. The 400 counter weights will hit the bottom of the 350 pistons ala 383.

Not only is the 400 rod shorter the big end was redesigned to give clearance between the rod bolts and cam.

As to why GM didn't use the longer 5.7" rod I'm not sure. Maybe the decreased CH didn't allow for ring spacing they liked.

That makes sense, but then they left the piston .025" down in the hole and used a 76cc head chamber. It is amazing the 400 had as much power as it did in stock form. :confused: I used a piston that had a CH .005" greater than stock and decked the block .020" to get a zero deck height.

That makes sense, but then they left the piston .025" down in the hole and used a 76cc head chamber. It is amazing the 400 had as much power as it did in stock form. :confused: I used a piston that had a CH .005" greater than stock and decked the block .020" to get a zero deck height.

The general feeling is that the total quench or squish distance should be about .040". The quench distance is the compressed thickness of the head gasket plus the deck clearance. The stock GM shim gaskets are in approx .020". Add that .020" to the.025" piston to deck clearance and you are in the .045" range. Common after market head gaskets such a Felpro are around .040" thick. With a zero decked block and the .040" gasket the quench distance your still in the desirable range.

[QUOTE=

As to why GM didn't use the longer 5.7" rod I'm not sure. Maybe the decreased CH didn't allow for ring spacing they liked.[/QUOTE]

So is there any advantage to go with the 5.7 rod with a 1.425 CH vs a 6" rod with a 1.125 CH as relationship to ring spacing, sealing, in building a 406? Looking a SRP flat tops, .030 over.

Thanks bill

Dan I could not find the steel shim head gaskets, that was the reason for the deck height change. The next best thing was to adjust the block to what was available.

So is there any advantage to go with the 5.7 rod with a 1.425 CH vs a 6" rod with a 1.125 CH as relationship to ring spacing, sealing, in building a 406? Looking a SRP flat tops, .030 over.

Thanks bill

The advantage/disadvantage has to do with rod length and its effect on power. Here is one article. http://www.stahlheaders.com/Lit_Rod%20Length.htm

Once CH is determined the piston designers then locate the rings as best to accommodate the pin location.

The more experienced engine builders can expound further with their thoughts and preferences.

64elcamino

Dan I could not find the steel shim head gaskets, that was the reason for the deck height change. The next best thing was to adjust the block to what was available.:thumbsup:

My 406 had .020" Mr. G. shims in it b/c I mocked it up and reached up inside it and scribed all the places by the big valve intake valves that they overhung, removed them and carefully removed the offending overhanging material.

Worked fine, but was a bunch of hand work..

pdq67