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Old 09-04-2013, 09:37 AM   #1
arc40s
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Default piston hits head

I resurfaced the heads and the block so both would be true. Now after warm up I get a low knocking sound. Only 9 miles on engine. I suspect piston is expanding and hiting the head. Can you use two head gaskets to fix or will head dome have to be reshaped?
What is usual damage done? piston breaks? piston bushing destroyed? Rod bearings out of round due to pounding? Main bearing harmed?
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Old 09-04-2013, 09:39 AM   #2
Ross F-1
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Default Re: piston hits head

Unless they cut the heads .060" or so to "true" them, very unlikely. What type engine is this? Stock pistons, aftermarket, Egge by any chance?
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Old 09-04-2013, 09:41 AM   #3
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Default Re: piston hits head

engine is 59AB new Egge pistons .060+
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Old 09-04-2013, 10:48 AM   #4
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Default Re: piston hits head

what was your piston to head clearance when the motor was assembled?

some egge pistons have an odd pointed dome that doesnt match heads that could be the problem?
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Old 09-04-2013, 10:58 AM   #5
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Default Re: piston hits head

You will probably need to get the piston domes flattened a little. Some careful measurements using rolled up aluminium foil balls will tell you what is happening.

I put some Offenhauser heads on an otherwise stock French engine and had the same problem. One side seemed tighter than the other. I needed to do a quick fix as I was racing the next weekend so I filed the pistons down a bit and licked the domes out with a grinder. Yup, I'm a bodge artist. I wouldn't recommend you do that but it is one way around the problem.

From what other people have reported, the Egge pistons seem to have the same pointy dome as the French pistons.

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Old 09-04-2013, 01:34 PM   #6
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Default Re: piston hits head

I use Egge pistons all the time and never had that problem. My 280 has .040" milled EAB heads and they did hit. I had to grind some clearance in the head. Tis was done by placing a head gasket ofer the combustion chamber and using a marker pen scribe a line around the dome area. Then using a die grinder blend in this area..
Never assemble an engine without checking this.
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Old 09-04-2013, 05:16 PM   #7
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Default Re: piston hits head

You may be looking at something like this at piston dome top to tight ..

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Old 09-04-2013, 05:53 PM   #8
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Default Re: piston hits head

If you are gonna take the heads off just get a machine shop to flycut the center of the head a little bit. If the piston is hitting you will be able to see it.
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Old 09-04-2013, 05:55 PM   #9
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Default Re: piston hits head

Vergil, I like your cutaway,sure makes a pic worth a thousand words.
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Old 09-04-2013, 05:58 PM   #10
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Default Re: piston hits head

Quote:
Originally Posted by Lawrie View Post
Vergil, I like your cutaway,sure makes a pic worth a thousand words.
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Thanks Lawrie, they have sure helped me out.

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Old 09-04-2013, 06:41 PM   #11
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Default Re: piston hits head

Yes Vergil that's a great visual aid. I try to get everyone to tighten up their piston to head clearance. Makes the engine run more efficient.
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Old 09-04-2013, 06:54 PM   #12
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Default Re: piston hits head

I know other say to the contrary but the EGGE piston saga raises its head again, or more correctly hits the head. We know of three local motors ,one being ours that EGGE pistons hit the head on new edelbrock and offy heads.
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Old 09-04-2013, 09:06 PM   #13
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Default Re: piston hits head

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Originally Posted by arc40s View Post
What is usual damage done? piston breaks? piston bushing destroyed? Rod bearings out of round due to pounding? Main bearing harmed?
I took apart a 37-38 truck engine with earlier heads, it had domed pistons and flat top heads. The pistons hit so hard you couldn't turn the engine past TDC with a long bar. I looked like it ran for a long time like that, I can't imagine the noise it made. It had a 12V conversion, that's probably why it started. Or alse they towed it to start. The pistons were all broken up, but still in place intact in the cylinders. One head was cracked in the chamber. Otherwise there was no visible damage to the rotating assembly or block, the bearings looked fine except for being worn out.
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Old 09-04-2013, 10:03 PM   #14
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Default Re: piston hits head

My, My! Vergil, what a great aid that cutaway is. I experienced the same thing on my '52 F1 V8. I used soft solder to figure out the squish.
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Old 09-04-2013, 10:07 PM   #15
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Default Re: piston hits head

I have no past experience here, but it has to be hard on the piston, gudgeon bush, rod, bigend. It surely must shorten the life expectancy of the motor.

Mostly I came in here to say thanks to Vergil for the picture. It certainly clarifies things for me. Now I follow what Ol' Ron keeps saying about the 0.040" to 0.050" clearance.
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Old 09-05-2013, 01:06 AM   #16
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Default Re: piston hits head

My +60 8BA Egge pistons hit the stock cast iron heads. You could see the marks. No apparent damage. Quick fix with the angle grinder on the heads. Runs beautifully now.
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Old 09-05-2013, 02:34 AM   #17
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Default Re: piston hits head

Quote:
Originally Posted by Bluebell View Post
I have no past experience here, but it has to be hard on the piston, gudgeon bush, rod, bigend. It surely must shorten the life expectancy of the motor.

Mostly I came in here to say thanks to Vergil for the picture. It certainly clarifies things for me. Now I follow what Ol' Ron keeps saying about the 0.040" to 0.050" clearance.
And Ol Ron is on the button. I had my heads machined to give me 45 thou over the pistons, and it makes a very noticeable difference.
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Old 09-05-2013, 01:35 PM   #18
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Default Re: piston hits head

Before you pull the heads off (if you haven't already), I'd suggest pulling the plugs out and checking to see if the valves are hitting them. We had that problem on a hot-rodded flathead in one of my father's cars. A change of plugs to a different brand/model was the remedy.
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Old 09-08-2013, 04:43 PM   #19
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Default Re: piston hits head

EGGE piston story. I put together a 8BA .060 over with merc crank. EAB heads which were never cut. I had to cut .030 off the tops of the pistons on a radius down to .000 at the piston edge. This gave me .045 piston to head clearance. Then I measured with Edlbrock and also some Sharp heads and came out even tighter.
I like to run a little more clearance .050 to .060 cause up around 6000 RPM I think the rods treach a little
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Old 09-08-2013, 07:35 PM   #20
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Default Re: piston hits head

Yes, If yoy're going to turn 6000 with stock rods, you'll need more clearance than that.
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