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Piston to valve clearance one more time, new guy
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Author:  KCampbellVA [ Sat Feb 07, 2026 2:51 pm ]
Post subject:  Piston to valve clearance one more time, new guy
I know this topic has been beat to death but even though I have searched and read dozens of posts over a number of other sites I am still not clear on the answer.
I am building an engine for the front engine dragster I built. My top priority was never to set any worlds records for low ET of a slant six powered vehicle, but rather to build something that looked like a dragster you would see around a track in the early 60's, and have it get down the track respectably,, reliably and at a modest budget. So no custom aluminum connecting rods or supercharger etc.
This is my set up: stock forged crank and connecting rods, flat top .060 over Sealed Power pistons. The camshaft is the original cam reground by American Custom Cam, Glendora CA to 242 @ .050 with a modest .470 lift. I decided against larger valves and opted for good porting with original valves with a back cut.
So far I have milled the block .060 and am considering milling the head .040 and using a .020 steel head gasket. So total reduction of .120.
Probably leaving a lot on the table but all this removed and a small give back on the base circle of the cam and this shouldn't result in having to worry about pushrod length or rocker angle. Yes?
If I stick to this formula, do I need to worry about or check the vallve/piston clearance??
My machine shop guy says he cut .060 off a head and block years ago and had contact ???
Anyway, what do you think. Thanks for the wisdom.
Author:  Dart270 [ Sat Feb 07, 2026 4:17 pm ]
Post subject:  Re: Piston to valve clearance one more time, new guy
Nice to see someone else is a fan of Occam's Razor.

You will be on the edge for stock pushrod length, but it is worth trying with that setup before you buy custom ones.

I cannot imagine you will hit valve-piston with only 0.120" reduction, but it is probably worth checking just to make sure. I have built many motors with more cam and milling than that with plenty of clearance (well over the rule-of-thumb minimum of 0.090"). I put play dough across the top of #1 piston, then plop the head on with smooth washers spacing the head above the block by about 0.100" thick (measure them with caliper). I sand the washers so they will leave no marks and lightly torque the head to 30-40 ft-lbs. You only need the 2 head bolts at the very front, then a couple halfway down the head. No head gasket. Run the crank around a few revolutions slowly, then pull the head and slice through the dough with a razor blade and measure min thickness with a caliper. Subtract the head gask thickness from the thickness of washers to get the excess, then subtract that from the thickness of the compressed dough. My guess is you will be at 0.150-0.200" clearance. The 225 is a very forgiving motor in terms of valve-piston, even with biggish cams.

What is the lobe separation angle of your cam? That is a great cam profile for about 9.5:1 or up to 10:1 comp with pump premium. You should advance the cam 4-6 deg from the LSA number.

Feel free to ask more questions and happy building!
Lou
Author:  hyper_pak [ Sat Feb 07, 2026 5:06 pm ]
Post subject:  Re: Piston to valve clearance one more time, new guy
I agree with Lou.
It is easy to check with the clay method.
Also I run a 4 speed and missed gears gets kissing close.
Author:  Rick Covalt [ Sat Feb 07, 2026 8:32 pm ]
Post subject:  Re: Piston to valve clearance one more time, new guy
I have cut the heads .090 & one .120 and never had any problems but worth checking. I also used stock pushrods on these.
Author:  KCampbellVA [ Sun Feb 08, 2026 5:01 am ]
Post subject:  Re: Piston to valve clearance one more time, new guy
Yeah, and forgot to mention that fly cutting valve reliefs are an option too.
Author:  hyper_pak [ Sun Feb 08, 2026 6:20 am ]
Post subject:  Re: Piston to valve clearance one more time, new guy
I had my forged pistons flycut on my last build.
Big cam and head+block cuts.
Author:  Greg Ondayko [ Sun Feb 08, 2026 6:32 am ]
Post subject:  Re: Piston to valve clearance one more time, new guy
When I built my '69 Dart with the MoPar Performance 0.528" Cam (specs Below)

P4529343
Intake Duration, Advertised: 284° Exhaust Duration, Advertised: 284°

Intake Duration @ .050 221° Exhaust Duration @ .050” 221°

Intake Lobe Centerline 102° ATDC Exhaust Lobe Centerline 106°BTDC

Lobe Separation Angle 108° Overlap 68°

Intake Lift 0.528" Exhaust Lift 0.528"

I milled 0.100" off of the block and 0.100" off the head. It resulted in ~10.25:1 Static Compression Ratio with stock 225 Pistons and rods. It needed 0.100" shorter pushrods and when I went to that cam, I needed to notch the pistons 0.090" for valve clearance.

Some documentation about that is here.

Greg
Author:  Charrlie_S [ Sun Feb 08, 2026 7:25 am ]
Post subject:  Re: Piston to valve clearance one more time, new guy
Quote:

If I stick to this formula, do I need to worry about or check the vallve/piston clearance??
My machine shop guy says he cut .060 off a head and block years ago and had contact ???
That might have been a 170 engine, but the cam will make a differance. A 170 engine is a "zero deck" motor. The piston is close to the top of the block at TDC. The 225 engine is about .10-.180 down in the hole, so not as critical.
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