piston/block heigth

[V4F]

how much piston can go above the block ? ie: .015 ... .020 etc .
pistons are hitting head , but not sure if head has been cut .... thanks .. steve

[James Rogers]

I have run pistons that were .030 above the deck and a head that was FLAT and have never had a piston hit. The gasket is .060 and will compress .015 leaving plenty.

[MikeK]

Just for the record an original stock block with 11.500" deck height had the pistons 0.0315" above deck if all else was to spec. The blocks themselves were spec'ed at 11.500 -11.505" and usually came off the machining line nearer the high limit.

At 3,000 rpm you won't get much rod stretch, perhaps 0.005", but there are other factors. Both the aluminum piston from pin center to face, and the rod will run hotter than the block temperature and will therefore exhibit greater expansion increasing the piston protrusion. In an A this could add up to an additional 0.015".

Ideal 'squish' height is debateable, but should it get less than 0.020" it is possible to initiate detonation despite a relatively low compression ratio.

[40 Deluxe]

MikeK, Are you sure that less than .020" squish height will cause detonation? My experience has been the opposite; it will lessen detonation. Here's why: After the spark plug fires, a flame front advances across the combustion chamber, building pressure (and thus heat) ahead of it. Under high load/heat situations, this pressure ahead of the flame can auto-ignite the fuel/air mixture in the far reaches of the chamber (the squish area above the piston). So now you have two flame fronts heading towrd each other. When they collide, BAM! detonation! With a tight squish height there is less room for a second flame front to form. Also, with a small squish height, there is greater turbulence as the piston reaches TDC, which better mixes the air and fuel, making for a faster burn (less time for detonation to occur and less spark advance needed).

[V4F]

i have 1-3 pistons hitting at an idle ! thanks for the info , going to check today . im afraid the head has been shaved ............. steve

[MikeK]

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Originally Posted by 40 Deluxe

MikeK, Are you sure that less than .020" squish height will cause detonation? . . .

40 Deluxe, you have nicely explained the classic squish effect. Based on that, the closer to zero you get, the greater the benefit. But there is another factor, and that's why I said "It is possible. . ." There is a relationship between the surface shape and area of the squish and piston velocity during the actual 'squish'. If you have a large surface area or a long distance to the open combustion chamber there is a pressure buildup in the center of the squish area as the mix tries to escape to the lower pressure open combustion chamber. The squish area of a typical flathead is generally more than twice that of an OHV. The pressure in the center can exceed the detonation point with too small a squish height. What is ideal in an OHV may be too little for a flathead.

If you're really into it, properly placed Singh grooves on the head squish area will help.

[V4F]

ok , i took measurements this morning ... stock head (*maybe shaved)

head : the chambers (not the valve area , just the flat where the piston is) .. .085

piston above the block :
1) .065
2) .085 ****************
3) .040
4) .068

used head gasket : .070-.075 thick

****** any info greatly appreciated .............. thanks .. steve

[MikeK]

The huge variance indicates one of two things. Either the rod lengths are uneven or the crank was reground without indexing and no attempt was made to set the stroke to spec.

Crank each piston down to BDC and measure the depth from the deck. Add the above-deck measurements. They should all equal factory stroke: 4.250" If not, the crank is way off. If they do equal 4.250", the rod lengths are off. You may have a combo of the two.

Other contributing factors: The deck may not be machined parallel to the pan rail (datum reference zero), and/or the crank main bearing centers may have been bored above/below zero. These last two would add or subtract progressively from front to back.

[wensum]

My rebuilt motor with 100thou oversize pistons had the pistons 50thou above the block. The new Winfield 7:1 aluminum head had a flush face and so I had to get it machined to clear the pistons. the combustion chambers were prettty rough too and I got those polished.
I had always understood that the original pistons stood proud of the block but had no idea by how much.
Keith

[V4F]

apparently who ever had this redone yes ago , cut to many corners . block is cut , head is cut . quit a mess but figured out how to fix it . thanks all for your help .
THE FIX : new head , solid copper spacer & copper head gasket to clear the pistons !
so glad its not mine !! ............. steve

[1931 flamingo]

Are you making the copper spacer or are they available by someone??
Paul in CT

[V4F]

ive been told that they are made of solid cooper @ .062 by a vendor......... still looking ... steve

[ctlikon0712]

So did you measure as Mike suggested? What did you find or what is the bottom line issue as to why the piston heights are varied?

[PC/SR]

Take the necessary metal off the top of the piston. No need for a new head or custom gasket. How much to take off depends on the pop up and how much squish you want. I like .045-.055. Pistons are fine so long as there is .200 material remaining on the piston top, maybe even less depending on the Compression Ratio. Works for me with a Weiand 7-1 flat top head and a Best .060 thick gasket that compresses down to about .055.

[V4F]

i have not been back to the car today . so no measurement .
i hate cutting pistons with out rebalancing . gonna try the porrmans fix as above .... thanks .. steve

[PC/SR]

Steve: If you are still looking, Charlie Yapp at Secrets of Speed advertises the .062 solid copper gaskets.

[V4F]

pc/sr ... thank you !!! ................

[V4F]

well charlie no longer makes them . any ideas ? a good used one ! thanks .. steve

[MikeK]

any ideas?... Yes. Fix the problem rather than trying to band-aid the symptom. With pistons up and down like you describe in post #7 that thing is going to run like a cement mixer. A thicker gasket is not the answer.

[quickchange]

Had ross racing supply me with set of pistons, ordered stock but for 4" bore, they arrive but on fitting to my b block were 50thou below deck hight insted of 30 thou above which was what std original pistons were , These are 80thou less than stock, Ross pistons claimed that was not there fault ???? so had to stump up for a correct set, So anyone need a set that won,t touch the head V4F would sort of solve your problem , Another engine we did have to fly cut price head above piston some 40 thou as pistons touched on revs, Ended up 50 thou clearance, Its easy to check clearance by torque head , insert a piece of resin core solder in spark plug hole & gently turn engine over , pull solder out & measure the thickness, saves pulling the head if using plastercene. 88 days till. ??