[78 chevy truck]

these are the numberes off the pistons that were in my block.
H423Np
08700-16-8
they are 4 reaif vavel and they are sunk down in the piston. the hole center isown from the outer side of the piston. the sides of the piston are even with the block. what comp ratio are thes pistons

[NorthStar]

I recognize that number the dish is -13 CCs

works out to about 8 to 1 with a 76 CC head

[78 chevy truck]

are summits pistons kit like this? so it these pistons are even with the deck does that mean the block has been decked

??????????????????????

[DoubleVision]

it would mean the blocks been decked .040 or so, since these pistons have .020 off the tops, as for the pistons summit carry`s they carry a lot, which pistons? what are the numbers? look on the blocks deck on the passengers side, if the suffix code isn`t present the blocks been decked.

[78 chevy truck]

there is a c all bye it self no other letteres

[DoubleVision]

ok then, the blocks been decked. what are the numbers of the pistons from summit?

[78 chevy truck]

idk it is just a rebuild kit with all the stuff to rebuild it there are no numberes

[DoubleVision]

the basic kit from summit with cast pistons has the same pistons avalible in flat top or dish top, both have .020 off the tops.

[78 chevy truck]

so will this work with my block then

[NorthStar]

Since its probably impossible for you to know how much was taken off the block its important for you to measure where youre pistons are at when on TDC right now with the dished pistons in there.

Your old piston H423NP has 1.550 compression distance.

If you buy the ones I told you about earlier H345NCP they have a slightly taller height of 1.560 so they will only work if the dished ones are down at least .010.


If the dished pistons are from .010 to zero try this piston H597P its a flat top too but has exact same height as the dished ones you have now 1.550 but they use a different ring set. And they probably cost more.


Heres the math

stock deck height 9.025

piston height 1.560
rod length 5.7
half the stroke (3.48/2)=1.74

add those all up = 9.000

So you see they are allowing you .025 to be decked off the block before pistons are popping out the holes. If you figure it with the 1.550 height pistons then you could deck .035 off. Ideally you want them to be flush with deck but dont sweat too much if theyre down a little bit. But you dont want them to go past.

Also the rod length and stroke of each journal probably arent exactly to spec so there will be variation from cylinder to cylinder.


You can also use a thin head gasket to achieve the "magic" .040 quench distance if pistons are not at zero deck height. Thats a whole other bunch of digging to see what thickness of gaskets are available.


Savvy?

[78 chevy truck]

can i use my old pistons and put new rings in? what would my comp be the have been milled to 72 cc. i cant aford a new block on a tite budget

[NorthStar]

Sure you can. How many miles are on them? It important to make sure that the carbon that builds up in groove is removed. Or you could end up with rings not being able to fully compress and be forced against cylinder wall.

Compression Ratio Calculator

[78 chevy truck]

i got 9 to 1 one with the pistons i have and a 0035 gasket hight that right

i never market the pistons takeing them out will this matter if i re ring them and not put them in the same holl

[astroracer]

Yes it will matter if the engine was rebuilt with those pistons. If it was done correctly each bore would have been sized to a specific piston and swapping them hap-hazzardly around may get you some tight bores and some loose bores. Look at the pistons and you may find a bore number already stamped in them that was done during the previous rebuild.

[Goldduster360]

Has anyone measured the true compression height of a H345 piston. I noticed some other threads on this site referenced these as "sucker pistons" with .020 lopped of the piston top. That would mean the compression height is actually 1.54 and not the 1.56 listed by Federal Mogul. I am in the process of ordering a kit for my 30 over 350 block but don't want to get stuck with "sucker pistons" since my block has never been decked and am also running 76cc heads.

Oddly, the Federal Mogul catalog shows the H345 piston with a compression ratio of only 8.34 to 1 with the 76cc head. Seems kind of low for a 1.56 compression height and .040 thick gasket unless of course the piston is destroked .020 which I think is another term for ".020" lopped off the top off the piston".

When I use .085" for deck height I come up with 8.4 to 1 instead of the 9.0 to 1 ratio with a .040 deck height. 8.4 is pretty close to their reference of 8.34 leading me to believe the H345 has a compression height of 1.54. I came up with .085" as an estimate of .045" down the hole and .040" thick head gasket.

Federal Mogul lists the H631p30 as a 8.6 to 1 piston it only has two valve notches but still lists dish volume as -5cc's the same as the H345. The H631 must have a true compression height of 1.56?