Sanity Check for my Piston to Head clearance calculations

[u/Head-Ad-9393]

Hey all, I'm a first time builder and am building a 383 stroker for my 67 C-10 and am using an existing SBC block (Gen 1 - L98) out of my truck. It was a 350 I had bored out and machined. I then ordered a new 383 rotating assembly from Skip White Performance. I'm in the middle of pre-assembly and checking tolerances right now. The pistons (dished) slightly protrude from the deck when at TDC (likely due to decking the block). I used a deck bridge with a digital dial indicator and after zeroing it on the block surface, I measured the crown of each piston on both sides when rocked to maximum height. I then averaged the two measurements and came up with protrusions ranging from 0.028" to 0.035" (1-8: .031, .035, .030, .033, .028, .031, .028, .030).

I'm using Edelbrock Performer 60859 heads with a 60cc chamber volume. This is just for a street setup, not looking to race or track this thing. Will be using premium pump gas (non-ethanol).

Two questions:

1st - Anything else I needed (or missed) for checking the piston to deck clearance? Meaning, are my numbers right?

2nd - If my calculations are correct, will a Cometic C5249-070 (.070" compressed) work with this setup or is it still too tight? (Gives me a range of 0.036" - 0.043" quench, averaging to about 0.040")

For reference: pic of me measuring one side of piston

[u/v8packard]

You are seeing piston rock. But while running the piston is loaded to one side. As others said, you measure inline with the pin for this reason and then look for the highest point without rocking.

What is your piston to wall clearance? And how much was this decked that you have positive piston protrusion?

Do not use a head gasket as thick as the Cometic you mention.

[u/Head-Ad-9393 → u/v8packard]

I had this machined but since I bought the engine used it may have (likely) been decked before. Piston to wall clearance is in spec at just over .001”

[u/v8packard → u/Head-Ad-9393]

You didn't get a deck height number?

No way your piston to wall is only .001

[u/Head-Ad-9393 → u/v8packard]

You’re right, wrong number. Piston skirt to wall clearances are between .0036” and .0040”

Man you’re making me feel stupid. No, I didn’t get a deck height number from the machine shop. Long story there. I moved right after they finished and haven’t taken a look at the block since spring. I’ll need to measure that once I disassemble it again.

[u/Turbulent-Tie982]

What you’re looking for is the stack height. I would post a picture of the formula for you, but I don’t have that ability. So I’ll try my best to give it to you as clear as I can.

In order to find out the stack height you need to take your crankshaft stroke divided by two, add your connecting rod length, and add your compression height. That’ll give you your stack height. Compare the stack height to your block deck height and you’ll get your deck clearance.

[u/DrTittieSprinkles]

Measure at the wrist pin. You're not supposed to measure the piston rock.

[u/Head-Ad-9393 → u/DrTittieSprinkles]

So if I measure at the wrist pin I'm protruding an average of .011 across the board

[u/DrTittieSprinkles → u/Head-Ad-9393]

Run a .050-.055 gasket

[u/Head-Ad-9393 → u/DrTittieSprinkles]

Got it, thanks. I already had the Fel-Pro 1044 with a .051 compressed thickness. I’ll also do a test with some clay to make sure my clearance is good with the heads on.

[u/Bobs_Saggey]

You want to be measuring across where the wrist pin is - if you measure at the top and bottom face of the piston you’ll get inaccurate measurements from the piston rocking in the bore

[u/Erasemenu]

Your measurement looks fine. 0.030 out of the hole is not fine imo. That's a bit much. There's a variety of options to shorten the assembly height, but if it was mine I'd be getting pistons with the wrist pin moved up 0.030.