I bought an engine kit a while ago from PAW that came with Speed Pro H345NCP pistons. Basically they're flat top hypereutectic pistons with a coating on the skirts for a .030 over 350.

There weren't any specs in the box that mentioned compression height, nor on the box anywhere. I went online and did a search and found many places that say the compression height is 1.54"

I'm thinking, "That's really low. I don't want that."

So I measured them (I'll go into that later) and got 1.562". I'm confused.

Right on the piston there's a number stamped near the pin hole that says "H345N" and under it is "H645P". I looked up both and the only difference is the H645P is for 4.3 V6 engines. Not really helpful.

I don't quite have the money right now to get one pressed onto a rod to find out the true height. Besides, I don't want to spend the money to have them pressed on just to find out they sit an additional .020" in the hole.

What I want to verify is how I measured them, and see if it's right....

First I measured the pin OD with a dial caliper and got .928" Stock is .9272 correct? So I'm darn close there without a mic.

Then I measured from the top of the pin hole to the top of the piston and got 1.098"

So..... I divided the pin by 2 to find the centerline, then added that to the distance from the top of the pin hole to the piston top.

.928" / 2 = .464" + 1.098" = 1.562"

I figure there's ~.002" clearance from the pin to the piston so it can actually move, and of course for oil. Which leaves me with 1.56" compression height.

I know it's not rocket science, but I just want to make sure they're what I want before I spend the money (which should be in a few weeks) to have them pressed onto the rods.

Can anyone tell me if I'm doing something completely wrong in my measuring??

 

Thats how you figure out the compression height. Dont forget that the pin bore isnt centered in the piston so you cant calculate your deck clearance using that number.

 

Right.

I'll do that after I get them pressed on. It's just puzzling how you read something on ~10 different sites that sell them, then you measure them and get totally different measurements.

Kinda makes you question yourself.

 

Looks right to me.

Goes to show you, you can't believe everything you read.

 

Unless things have changed FM speed pro H345HP and
NP pistons have a 1.54" compression height.

A H645P may be an entirely different beast made from the same casting. could very well have a different 1.56" compression height. (pin hole location)
As I recall 4.3 V6 pistons use metric rings. So a FM replacement may use metric rings too.

Better check out exactly what piston you have.
Check the ring grooves too.

 

Speed Pro uses the same pistons for a 4.3 as they do for a 350 except that they use matched sets of 6 instead of matched sets of 8. Factory 4.3L pistons may well be different but aftermarket 4.3 pistons are just a set of 6 350-style pistons (no metric rings or anything funky like that).

Also, Speed Pro pistons are not side-specific in most (all?) varieties. There's no "notch" that has to point towards the front of the engine. As such, they do not have offset wrist pins like many stock pistons do.

Your method of measuring wrist pin height is valid. If you measure them at 1.56 then I would beleive they actually run .025 in the hole on an unmilled block. I beleive most (all?) Speed Pro hypers for a SBC are designed to run .025 in the hole. I have a set of pop-up Speed Pro hypers in a motor on my stand right now and they spec out at .027 in the hole from actual measurement.

 

Thanks for the responses guys.

I know most everything about my pistons except the compression height (well, I know now ).

This is what is says on the box...

The minimum bore clearance is .001"
Bore range 4.030" - 4.031"
Size point 1.83" from top of head
H345NCP 030

Then there's all the other scan bars and crap like that.

I have no clue what the "Size point 1.83" from top of head" means. Is that were to measure the OD of the piston? 1.83" down, opposite the pin direction?

Just so you know, the piston face doesn't have a "notch" per say, but there is a dimple which signifies which way to face the front of the engine. Whether or not that means they're offset pins, I would imagine so. If that's not what it means, better safe than sorry. May as well press them on that way, right?

 

TTT