The Stovebolt Site Forums The Shop Area Engine Shop 261 zero deck questions.

As usual for me the more I measure the more questions come up. I checked a couple engines and find an average piston top to deck value of .062” to .065”. Removing .060” there and .030” off the head sound like too much and now I wonder if the zero decking process is intended to replace head milling instead of being used with it at least in this case. I realize there will be valve train geometry issues but those aside at what point does reducing combustion chamber volume become overdone for a street engine? Thanks for replies.

"Zero deck" procedures are usually reserved for race engines. It's also necessary to have connecting rods exactly the same length, and pistons with matched compression heights if you're going to do the zero deck process. Then the combustion chambers need to be CC-matched for equal volumes. The whole idea is to have exactly the same compression pressure on all cylinders, and also getting the pistons and cylinder head surfaces close enough together to "quench" the flame out in that area as the piston reaches top dead center. This prevents "pre-ignition", where the flame in the combustion chamber lights off in two different directions, and "detonation" which is the noise created as the pressure waves from both flame fronts meet in the middle. That rattle you hear as you lug down on a hill is detonation, and it's incredibly destructive. I've burned holes in 440 Mopar race pistons the size of a golf ball in a matter of seconds when the detonation happens at full throttle and max load. Of course, the engine was turning 6500 RPM, and putting out 715 HP at the time, on the dyno.

I'd suggest leaving at least .025" of deck clearance on a street engine. There's no need to do a zero-deck procedure unless you're planning to wring the last ounce of torque out of it, and that's not likely to happen on the average street buildup.
Jerry

Sorry

My two cents, I have noticed that replacement pistons tend to sit lower in the hole for decking cleaning purposes. One can still maintain a compression ratio and still get a flat clean surface to bolt a cylinder head to after decking.
Your right Curt, I just measured the deck on my new 261 short block and couldn't believe there was .060 there, and that was after .010 was milled off the block before assembly. This got me thinking of what my compression ratio was going to look like, so I ran some numbers.Sorry to load it into your thread but it kind of goes with it all.
Swept piston area within the cylinder: 235 (.030 over)= 654cc (A)
...........................................................261 (.030 over)= 724cc (A)
Cylinder head 850.........................................................= 86cc (B)
......................848.........................................................= 79cc (B)
compressed head gasket thickness (.042)...................= 9cc (C)
Deck (.060) 235...........................................................= 10cc (D)
....................261...........................................................= 11cc (D)

CR= A+B+C+D/B+C+D

235 with 848 head= 7.7
235 with 850 head= 7.3
261 with 848 head= 8.3
261 with 850 head= 7.8
if you mill the head, add .12 cr for every .010 taken(milled off the head surface)
These are my own calculations but feel they are close, use at your own risk.

The 235 pistons (stock) sit higher in the hole then the 261 pistons. I forget off hand exactly what the numbers were the last time I actually measured but I want to say the stock 261 piston sat anywhere between .090 and .110 in the hole, and the 235 were less at like .070-.080, something along those lines. So if you are decking a 235 by .060 and cutting he head by .030, you should be getting a higher compression ratio then the similarly machined 261.

And of course, its the case that all of this machining is due to the lack of any affordable piston for either of the motors, but especially the 261, with any kind of usable compression, and the out of date piston ring package for the 261 pistons.

Good info Mike. Thanks for putting in your .02

Sorry

If the stock 261 pistons sit significantly lower in the cylinder than the stock 235, my guess would be that it was done to compensate for the larger volume of the cylinder of the 261.
If the goal of GM was to maintain a compression ratio similar to the 235, lowering the dome height of the 261 piston would be one of the easiest/least expensive ways to do it.
GM wasn't trying to make race engines back then. They were trying to make them run well on the gasoline available at the time.
Carl

Probably very true Carl.
One has to question if raising compression by adding a 848 head or decking the block/milling the head is really the right choice and will be fruitful? Especially when you take a good hard look on how the cylinder head was designed and the way mixture flows in and gas flows out. To me it is like lipstick on a pig in a way, trying to make it into something it was never meant to be.

True for sure. When I milled the head .0625" on my GMC 302 engine(similar head design) I did notice a significant HP increase, due in part no doubt to slightly advancing the spark and leaning the idle mixture due to the 5,000' altitude.

Ed

Lower compression ratio, presumably because the 261 motors were for the larger trucks, oil trucks, dump trucks, school buses, equipment that was run at low rpm, but needed some torque to do the job, not horsepower. Both pistons are flat tops. Then there is the 261 head, there are two of them that were used, can have combustion chamber volumes of 86cc and like 91 or 92 cc which resulted in an advertised compression ratio of 7.0 but most times was actually lower then that. The 56-57 motors with the 848 heads were advertised at 8.0, and the 58 and up were advertised at 8.5. Now I have never measured a 56-57 head and compared it with a 58 and up head to see if the difference is in the combustion chamber or maybe in the machining of the block or some slight difference in the compression height of the pistons for the .5 bump in compression.

[quote=Hotrod Lincoln]"Zero deck" procedures are usually reserved for race engines. It's also necessary to have connecting rods exactly the same length, and pistons with matched compression heights if you're going to do the zero deck process. Then the combustion chambers need to be CC-matched for equal volumes. The whole idea is to have exactly the same compression pressure on all cylinders, and also getting the pistons and cylinder head surfaces close enough together to "quench" the flame out in that area as the piston reaches top dead center. This prevents "pre-ignition", where the flame in the combustion chamber lights off in two different directions, and "detonation" which is the noise created as the pressure waves from both flame fronts meet in the middle. That rattle you hear as you lug down on a hill is detonation, and it's incredibly destructive. I've burned holes in 440 Mopar race pistons the size of a golf ball in a matter of seconds when the detonation happens at full throttle and max load. Of course, the engine was turning 6500 RPM, and putting out 715 HP at the time, on the dyno.

I'd suggest leaving at least .025" of deck clearance on a street engine. There's no need to do a zero-deck procedure unless you're planning to wring the last ounce of torque out of it, and that's not likely to happen on the average street buildup.
Jerry
[/quote
Thanks for explaining that Jerry.