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President: Team Full Chat
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faaakkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkkk

Sorry OB :frown
 

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Discussion Starter #22
Well I've spent too much on it to give up just because I've scrapped a cylinder head and piston. There are some scratches on the afflicted cylinder which I think will hone out, and I have a spare cylinder head. The new piston is already ordered, and a soon as it's all back together I'll be checking VERY carefully what clearance there is between valves and piston at the minimum clearance point. If I find nothing exceptional (and I don't expect to have much more than 0.5mm clearance anyway) then I'll be looking very carefully at valve collet placement, and checking that all those little buggers, every single last one of them, are tucked up in bed nicely. The only other thing that could possibly have caused just the one cylinder to eat itself could possible have been that the valve clearances were too small, causing the valve to open just that little bit further. The rest would be down to accumulation of tolerances.
 

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I bet the valve wasn't set. I always stress out about that and use a brass drift on the top of the valve and tap it with a hammer a few times to make sure they're in. Even then, it's still worrisome. I had the timing off by SEVERAL degrees on my cams on my 93 with the slotted cam sprockets for an entire season once. I got some different numbers from different tuner and leaned it was so far advanced that I should have had interference but I didn't. just my opinion but I don't think you'll get your damage from a skimmed head and deck. Anyway, knowing how you operate here, I know you'll check all the measurements and see where you're at. That's why we appreciate your work.
 

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Discussion Starter #25
The first picture is the remind of no. 3 piston. The second is how it looked today after the rebuild started in earnest. The 4th rebuild, that is. I must be getting more patient, in the olden days I would have taken a sledgehammer to it. Either more patient or more incompetent...:unsure:
 

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Discussion Starter #27
Ok, so I decided to give in to paranoia and check how much clearance there actually is between the JE 12.5:1 piston and the valves. That was done by sticking some bluetack to the piston, greasing the valves and turning the engine over. Clearance for the exhaust valve is 3.1mm, for the inlet valve 2.2mm. As far as I'm aware, the conrod stretches by about 0.5mm at peak rpm. So I don't have a lot of room for carbon build-up. Not that it ever stays long enough in use between teardown to build anything up....
 

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