Minimum thickness of valve relief to bottom of piston

[winr1]

Measuring from the lower part of valve relief that is

Will be mocking up my mill soon to check to see if the valve reliefs will need to be deepened

Plan is to whack around .083 off the piston to get zero deck

Gonna put piston at zero deck, install head/ gasket with intake/exhaust and see what I have before cutting piston

The valve reliefs should be in the same position but higher after cutting .... correct ??

Just bench racing till while I finish the house



Ricky.

[My427stang]

Diamond says .120 min crown thickness, seems thin, however I would also check clearances with clay and/or check interference starting at 20 degrees BTDC and every 5 degrees before it starts growing. TDC  is not usually where they hit

[jayb]

I don't think I'd go below 0.200" thick in the reliefs.  The pistons I've cut in half to check this have all been at least that thick or thicker, and these have been lightened by the manufacturer (CP).  Seems like if it was safe to go thinner, they would have done that...

[CaptCobrajet]

It depends on your situation.  Some conditions require more material than others.  Obviously an 8:1 low power engine with low cylinder pressure can live thinner than a boosted 14:1 piece.  I think for general street use, 500-ish hp, that .120 is a good number.  I had CP thicken up the pistons we made for you Mr. Jay, because I had endurance in harsh conditions on my mind.  It also depends on the area and the "area" of the thin spot.  I wouldn't do it myself, but I have taken apart some engines that had been raced hard for lots of runs, and seen .070-.080 thick in small areas. 

I also remember Kip Martin pulling a head once for teardown, and the flat tops had turned themselves into dishes!  I was young, and I didn't ask him how thick those were, but safe to say.....he had gone too far!

Generally, the piston folks are not scared to go .150-.180 on NA pistons for just about anything.  Sometimes, when a dish is really deep, and you are running out of room in the forging, things get tried, and most of the time it works as long as the tune is good.

[winr1]

Thanks guys :-)



Ricky.

[Barry_R]

When we were doing the old Speed-Pro forgings we had a lot of folks that would lighten them up.  We used to try and stay at +/- .170 on larger areas, but would let folks go to .120 in the "crotch" of a valve relief.

[Joe-JDC]

Back in the late '80s when super charging started to become the in thing with the EFI crowd, we asked about the thickness minimum on forged pistons, and usually got the minimum of .180" as the very minimum if considering over 5# of boost.  Also the minimum distance for the top ring kept getting lower on the piston due to heat from the forced induction.  When I ran a TRW 427 piston in my MR with 428 crankshaft, we cut the top of the piston, and when we asked TRW, they also indicated no less than .180" for durability.  I raced that combination with .039" quench, and had witness kiss but no carbon on my pistons for several years.  Joe-JDC

[gt350hr]

    Ricky ,
       My day job for the last 20+ years has been in the custom piston industry. In general , the minimum "deck" thickness should stay at .180 or more. The minimum thickness in the valve relief area depends a little on the specifics of the valve pocket. An "inline" valve pocket ( like an FE has) can be as little as .120 on the intake side BUT .175 on the exhaust side due to the higher localized heat. The "area" of the thin spot is also critical. This come into play on "canted" valve pocket designs where on of the pin bosses "flairs" into the center of the valve pocket. This produces the "oil can" effect which over time will initiate a crack.
   Common sense will say that lower power/ compression / natural aspiration is more forgiving than higher power/compression/boosted applications. Taking .083 off of the top of most pistons will cause a failure in 'my " personal experience.
   Randy