Valve hitting piston on mock-up


Veteran Member
Sep 10, 2009
Poughkeepsie, NY
Doing mock up on a 355 I'm building. Checked valve to piston clearance on the #1 cyl with the clay method.. and things do not look so good.

I have it set up to check both #1 valves with solid lifters and stock pushrods & rockers. The exhaust valve is coming into contact with the valve relief at about 20*btdc at the end of the exhaust stroke.

First, let me mention I am mocking it up with no head gasket, just the head very lightly snugged down with a few bolts. I'm only using a .015" shim (coated) so I figured I could take the measurement and add roughly .015 to get the actual clearance.. Didn't want to compress the gasket before permanently installing the heads.

So, I did this test 4 times, each time adjusting the rocker nuts to zero lash. First time, I thought it was just resistance from the clay so I pushed it a little too hard- went over the nose and bent the pushrod. The next 3 tries I stopped when it hit, but had similar results:




So, I'm not sure where to start correcting this. If I did have the gasket on, it may not have hit.. but if the clearance should be at least .100" it still would've been too close. The intake had plenty of room closest point was .140 without the added gasket thickness-


Here are the specs..

speed pro hyper flattops, sit .025" below deck @tdc
going to use felpro 1094 .015" compressed steel shims (coated for aluminum)
Comp 280h cam (hyd FT)
Edelbrock 64cc e-street heads
calculated CR- 10.4:1

I was really surprised to see this happen. I thought maybe the cam timing is off, but I'm positive the dots are lined up and the crank gear is keyed in the center position (0*).. Just doesn't seem right.

I've already gotten some great advice from this site for this engine, so thanks a ton for the help.. what would you do?? I'm building this for a friends '72 chevelle and I'm trying to do it right the first time and make it last.


Veteran Member
Jul 8, 2001
Phoenix Arizona
I assume you pumped up the lifter with oil? Did you use valve springs to settle the hyd. lifter? I am pretty sure you cant do that. You need valve spring pressure to sim. the pumping action on the lifter


Veteran Member
Feb 11, 2008
Olney, MD
First, I think you're supposed to do the interference check with the gasket in place.
I would check what degree BTDC the #1 intake valve starts to open. That's a basic cam timing measurement. It'll ensure your cam timing is on the mark. If it is way off, you can tell without special tools (by eyeing the timing mark as it comes around).
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Veteran Member
Lifetime Gold Member
Sep 20, 2004
the H345NPs are symmetrical reliefs. The text being upside down means nothing, you get 8 of the same piston in the box. As long as the dots all face the front of the engine, that's what you get.


Veteran Member
Nov 16, 2000
Philly area
If the big dot on the top of the piston is towards the front of the engine they are in the right way.

Having miles of clearance on the intake but none on the exhaust is usually indicative of a cam installed very retarded. The piston is "chasing" the exhaust valve back to TDC WELL before the valve is close to being closed. Such a combo should not have a problem near this bad. Especially on the exhaust with a single pattern cam that's got 4* of advance ground into it from the factory.

You should degree the cam to confirm this absolutely. But....

Quickie check you can do by eye to catch a chain with the dot on the wrong tooth (which I have run into once before)...... Set it "dot to dot" like you had when you installed it. Look at the location of the cam dowel pin. It should be ever so slightly past the 3:00 position. Like in the range of "3:15" is about where it should appear to be. If it's only in about the 2:30-2:45 kinda location your chain set is marked wrong (typically off by one tooth on the top sprocket).

If it fails that quickie visual test you can rest assured the cam is not in the right position. But if it looks OK you have no choice but to do a full degreeing of the cam to determine it's true location.
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