Measuring piston-to-head clearance

[MopaR&D]

After assembling my 360 shortblock with KB-107 pistons I found with a dial indicator that they come out .008" above deck. I have a set of Fel-Pro Q8553PT head gaskets which I'm told are .045-.055" thick, but to be sure I want to bolt on a head with the gasket and measure the true assembled quench distance. I am using factory Magnum heads (closed chambers) which is why I need to get this spec just right; from what I've read .035-.045" is the optimal range, above negates the squish effect and below runs the risk of contact.

What's the best way to measure this? I was thinking modeling clay but I can't visualize an accurate way to measure the thickness of something soft like that once it's been compressed.

Also I'm having trouble finding affordable gaskets that are between .043" and .048" thick, they all seem to fall outside of this range.

 

[Purple Haze]

Try here http://www.cometic.com/default.aspx

 

[Prine]

Plastigauge? Maybe?

 

[oliver]

i use oil and playdough. then use a depth guage, oil is just so the playdough doesn't get stuck to everything.

 

[MopaR&D]

i use oil and playdough. then use a depth guage, oil is just so the playdough doesn't get stuck to everything.

What kind of depth gauge are you talking about? I've never seen one and Google shows a few different types...


Also "Purple Haze" (like the name haha) those Cometic gaskets are a little too expensive for me, and that was just looking at the prices for standard off-the-shelf ones. I'll probably still call to see how much a pair of .045" thick ones would be but I'd like to go with a cheaper option if it exists.

 

[oliver]

well, if you dont want to blow 50-150 bucks on a depth mic, you can also use the depth end of a vernier caliper if you are careful.

 

[MopaR&D]

well, if you dont want to blow 50-150 bucks on a depth mic, you can also use the depth end of a vernier caliper if you are careful.

i don't own either of those unfortunately newb. i actually think I might be able to do it with a dial indicator; use a hard flat piece of metal or something on top of the clay to baseline the dial then remove the piece and let the tip go down to the bottom of the clay...?

 

[oliver]

hit up harbor freight and get one of the cheapy digital calipers. just as accurate as a mitotoyo.

 

[MopaR&D]

OH duh forgot calipers have depth gauges on the other end... Looks like I'll order one tomorrow. Thanks and hopefully it turns out within the range I want it so I don't have to order new gaskets...


BTW just an aside oliver but I visited San Francisco last month for the Outside Lands festival... it's a beautiful place with lots of cool stuff (beautiful people too at least from what I saw) but a little too overcrowded and expensive for me, how do you like living there if you don't mind me asking?

 

[Big Jim]

The most accurate way to check is with a dial indicator. Need to put lightweight checking springs on valves, have piston @ TDC. Zero indicator with valve closed, measure distance until valve hits piston. Take that reading, subtract (valve lift of cam x rocker ratio- for instance .450" liftx 1.5 rocker ratio=.675") remainder is piston to valve clearance...a little complicated the first few times but more accurate than clay method.JMHO

 

[Guitar Jones]

That is not an accurate way to check piston to valve clearance as the closest contact is about 10-20 degrees BTDC for the exhaust valve and about 10-20 degrees ATDC for the intake. The valve is never at full lift when the piston is at TDC.

As for measuring piston to head clearance, you've already done it the right way, a dial indicator and the gasket compressed thickness. You can't always trust the advertised compressed thickness though.

My old engine had the pistons out of the hole .011 with a .038 compressed head gasket and it was fine. Turned it over 7000 all the time and the pistons never touched the heads.

 

[oliver]

BTW just an aside oliver but I visited San Francisco last month for the Outside Lands festival... it's a beautiful place with lots of cool stuff (beautiful people too at least from what I saw) but a little too overcrowded and expensive for me, how do you like living there if you don't mind me asking?

Awesome, i had vip tickets to the outside lands festival (helps being freinds with current mayor). Maybe even seen you there even though it was *** to face everywhere! haha, i actually do not like living in the bay area. I hate cities, i went to school at purdue in indiana and had a house with an acre. Gimme 15 acres and a decent house and i'll be happy.

 

[Purple Haze]

What kind of depth gauge are you talking about? I've never seen one and Google shows a few different types...


Also "Purple Haze" (like the name haha) those Cometic gaskets are a little too expensive for me, and that was just looking at the prices for standard off-the-shelf ones. I'll probably still call to see how much a pair of .045" thick ones would be but I'd like to go with a cheaper option if it exists.

Gave you that in case you need some thicker one's.

 

[MopaR&D]

That is not an accurate way to check piston to valve clearance as the closest contact is about 10-20 degrees BTDC for the exhaust valve and about 10-20 degrees ATDC for the intake. The valve is never at full lift when the piston is at TDC.

As for measuring piston to head clearance, you've already done it the right way, a dial indicator and the gasket compressed thickness. You can't always trust the advertised compressed thickness though.

My old engine had the pistons out of the hole .011 with a .038 compressed head gasket and it was fine. Turned it over 7000 all the time and the pistons never touched the heads.

I'm not too worried about valve-to-piston clearance as the KB-107s have valve reliefs and my cam has only .503" lift with 1.6 rockers. That's good to know your engine worked well with so little quench height, maybe I can also get away with a shorter gasket.