Piston tagging exhaust valve?

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Piston tagging exhaust valve?

Post  fastashley on September 6th 2016, 9:12 am

Blew up our 460 jeep this weekend. Disassembled it to find 2 broken valves, disintegrated piston, and a chamber beat to pieces (obviously). What was interesting was that ALL the pistons had an exhaust valve tag on them. Never had trouble with this on a BBF, so I'm hoping someone has some ideas. This is a reletively low buck deal, I'll list the specs.
Stock crank and rods
Old TRW flat tops
A429s 2.19/1.72
Custom Cams flat tappet .618/.633 .252/.258@.050
1.72 Harland sharps
8.550 hardened 3/8 pushrod
CO2 shifter set at 6100 shift

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Re: Piston tagging exhaust valve?

Post  rmcomprandy on September 6th 2016, 9:39 am

fastashley wrote:Blew up our 460 jeep this weekend. Disassembled it to find 2 broken valves, disintegrated piston, and a chamber beat to pieces (obviously). What was interesting was that ALL the pistons had an exhaust valve tag on them. Never had trouble with this on a BBF, so I'm hoping someone has some ideas. This is a reletively low buck deal, I'll list the specs.
Stock crank and rods
Old TRW flat tops
A429s 2.19/1.72
Custom Cams flat tappet .618/.633  .252/.258@.050
1.72 Harland sharps
8.550 hardened 3/8 pushrod
CO2 shifter set at 6100 shift

It appears that it just did not have enough exhaust valve to piston clearance.
Did you check it when assembling it...?

Advancing the cam a few degrees will create a bit more exhaust valve to piston clearance however, it will diminish the intake valve to piston clearance.

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Re: Piston tagging exhaust valve?

Post  gt350hr on September 6th 2016, 4:22 pm

It shouldn't have hit , the cam is too small. You have a cam timing issue/ dowel pin/timing chain problem IMHO.

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Re: Piston tagging exhaust valve?

Post  FalconEh on September 6th 2016, 8:00 pm

Block or heads decked (or both?)
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Re: Piston tagging exhaust valve?

Post  jones on September 6th 2016, 8:21 pm

Is it possible the connecting rods stretched from high rpm when the Pistons let go?

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Re: Piston tagging exhaust valve?

Post  jones on September 6th 2016, 8:22 pm

Or valve float

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Re: Piston tagging exhaust valve?

Post  gt350hr on September 7th 2016, 12:39 pm

All of the exhaust valves hitting is a result of retarded cam timing with a cam that short on duration.

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Re: Piston tagging exhaust valve?

Post  Lem Evans on September 7th 2016, 12:50 pm

Do the pistons have a exh. valve relief?

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Re: Piston tagging exhaust valve?

Post  fastashley on September 8th 2016, 10:12 pm

The Pistons do not have an exhaust relief, but neither do the Pistons for our 557. That has a much larger cam

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Re: Piston tagging exhaust valve?

Post  fastashley on September 8th 2016, 10:16 pm

Neither block or heads decked

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Re: Piston tagging exhaust valve?

Post  68galaxie on September 9th 2016, 9:43 am

What heads are on the 557 with no exhaust valve reliefs?
Does the 557 also have A429 heads?
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Re: Piston tagging exhaust valve?

Post  68galaxie on September 9th 2016, 9:46 am

The later SCJ heads do not require exhaust valve reliefs for smaller camshafts. (different valve placement etc...)

The early A429 usually require an exhaust valve relief even with smallish cams - unless the pistons are well below the deck.
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Re: Piston tagging exhaust valve?

Post  Lem Evans on September 9th 2016, 10:33 am

An exh. relief with that style head would be a good thing.

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Re: Piston tagging exhaust valve?

Post  gt350hr on September 9th 2016, 11:07 am

The exhaust valve lift @ 10*s before TDC is "around" .200 , so the valve is well above the gasket surface of the head . No reason for a relief as there is plenty of "free drop" in that head.

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Re: Piston tagging exhaust valve?

Post  68galaxie on September 9th 2016, 11:20 am

If the free drop is well above the head gasket the camshaft timing must be out alot.
Possible retarded a tooth?

Was the engine assembled without piston to valve clearances measured?
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Re: Piston tagging exhaust valve?

Post  Lem Evans on September 9th 2016, 2:05 pm

The free drop should not be assumed. The head could have been milled none, .075" or somewhere in between.

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Re: Piston tagging exhaust valve?

Post  gt350hr on September 9th 2016, 6:07 pm

Come on Lem , I'm not that stupid. I checked one of my A 429 heads and even if the head was milled to the intake seat , there would be enough room. This is a flat tappet cam and the true lift @ 10 BTDC is closer to .175 than the .200 I first mentioned. With a 280@ .050 solid roller I would have a different opinion. Unless cam timing wasn't checked, something changed to cause the interference.

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Re: Piston tagging exhaust valve?

Post  Lem Evans on September 9th 2016, 6:52 pm

gt350hr wrote:Come on Lem , I'm not that stupid. I checked one of my A 429 heads and even if the head was milled to the intake seat , there would be enough room. This is a flat tappet cam and the true lift @ 10 BTDC is closer to .175 than the .200 I first mentioned.  With a 280@ .050 solid roller I would have a different opinion. Unless cam timing wasn't checked, something changed to cause the interference.

Easy big boy....I did not say or infer that you are stupid. I did not say that the absence of a exh. valve relief was the root of his issue. What I said was, an exh relief with that style head was a good thing...meaning... it is my opinion that a piston used in a racing engine with this style head should have that relief to give 'us' more rope to run with.    
I've had a A429 headed engine with a 288* @ .050" exh lobe and a .035" mill job so I'm not completely ignorant of the issues involved.
It's Friday here....is it Monday there? Very Happy

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Re: Piston tagging exhaust valve?

Post  fastashley on September 10th 2016, 8:03 am

The heads on the 557 are (newly installed) Blue Thunder B heads. The cam is .700/.715 .260/.265@.050.
The A429's used to be on this motor, with the same small flat tappet that is now in the 460.
I'm usually the one that assembles our motors, but my uncle did the 460. He's getting older and his attention to detail was never his strong suit. I can almost guarantee he didn't check bearing clearances, thrust, deck height, pushrod length or valve clearance. That being said, he's probably 'bolted together' a dozen of these low buck BBF's. The exhaust valve thing is new. I know the heads have not been cut, the block I'm not sure. Although within his circle of friends, nobody does that stuff. We've run that same head/cam set-up on the 557 to 7000 with no problems, so I doubt valve float is the issue. The motor ran the times we predicted, so I doubt timing was that far off. Scratching my head

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Re: Piston tagging exhaust valve?

Post  68galaxie on September 10th 2016, 9:56 am

The only thing we know is the exhaust valves tagged all of the pistons.
So we are left with:

a. The cam timing is way off (1 tooth retarded) - can easily be done if in a hurry.
b. Engine revved way beyond vale spring control - was spring set up verified? measured? spring load as well as correct installed height?

Just my 2cents worth


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Re: Piston tagging exhaust valve?

Post  fastashley on September 10th 2016, 4:48 pm

First try with CO2 shifter. My money is on a malfunction resulting in over rev

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Re: Piston tagging exhaust valve?

Post  BBFTorino on September 10th 2016, 7:30 pm

Over rev = valve/spring float.

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