Hello,
I have a custom SBC engine. Some of pistons are traveling past the top of the deck by +0.008".

Question; Is a compressed gasket thickness of 0.045" thick enough if the piston is traveling +0.008" above the deck?

Question; Is my method of checking deck height acceptable?



Thanks in advance,
dave w

 

no you will want to zero that indicator off the actual deck then check the piston towards the outside and the inside of the block

as the pistons will tend to rock as the crank passes TDC so the high side will tend to switch from outside to inside or vice versa

you will also want a way to square that indicator tip to the deck any deflection will give false measurements on the low side

and dont use that scale measure right to the piston

 

Thank you SpitotRs305 for the input on my measurement procedure. I can re-measure as you have described.

The engine build was on a limited budget, so block was not decked. The deck measured flat with a straight edge.

My next question; Is a 0.045" thick head gasket thick enough, if I accurately measure piston travel +0.008" above the deck? Depending on which website I've visited ... the accepted compressed gasket thickness above the piston needs to be 0.036" ~ 0.038". So for my example, I would need a compressed gasket thickness of 0.046" (0.008" above the deck travel + 0.038" compressed gasket thickness above the piston).

dave w

 

well .036 might work... but you will want at least .040 distance between the head and piston face for proper quench although with the dish pistons i am not sure if that will matter much... to far off of that quench distance with a flat top will invite detonation...

if quench doesnt matter with the dish piston then it really comes down to clearance... and what you feel safe running along with piston-valve clearance

although i am not a personal fan of the piston being outside of the bore at TDC i dont see why it couldnt work although i have never tried it myself

do you have the rod torqued down for this measurement? has the block ever been machined before? are you using some type of domed piston?

i ask because it is unusual that the piston is that high in the bore

 

The engine is fully assembled, pistons / rings assembled, with the pistons pressed on to the rods. Rods torqued to the crank, main caps torqued.

I'm using 6" rods, budget type ... which I think have a wide tolerance on the center to center distance.

I'm trying to build a low budget - MPG small displacement SBC ... notice how small the bores are?

I was thinking perfect quench would be nice, but I'm really concerned that as the pistons get to normal operating temperature ... the pistons will expand and hit the head if I don't have enough compressed head gasket clearance.

 

That's an interesting way of measuring the deck height although it can only measure if the piston extends past the top of the deck. If the piston is down into the bore like it should be, you'll never get a reading.

A deck height setup tool will allow you to measure the depth to the deck then you can zero the gauge. You then move to setup tool over the wrist pin side of the piston to measure how much it comes up. On the side of the piston, you're away from the center of the piston and on a flat portion that won't be affected by the piston rocking on the wrist pin.

This is the most common tool

 

I went and spent the mega $$ for the correct tool. http://www.nicholsonperformance.com/...ts/show/230889

dave w

 

That'll work. I made one for about $5 out of some scrap steel. I use mine to check liner protrusion on diesel engines.