Not quite a Knackered big end !
Discussion
So much for the initial diagnoses, once stripped the problem was obvious......
Certainly explains why the plug on that cylinder had it's electrode squashed against the plug tip.
Total result was one broken piston crown and one bent valve - so a lucky escape
Bores, shells, guides, all good so a new set of forged pistons and a new valve lapped in and we've good to go.
Certainly explains why the plug on that cylinder had it's electrode squashed against the plug tip.
Total result was one broken piston crown and one bent valve - so a lucky escape
Bores, shells, guides, all good so a new set of forged pistons and a new valve lapped in and we've good to go.
looks like when the pocket was cut in the piston, whoever did the work didn't take into account how much this was reducing the cross section between the pocket edge and ring land - slight weakness in the piston crown/a bit of fatigue + a damm good thrapping on the track = knackered piston!
Everything else was fine - no indication of detonation, overheating, fuelling or timing problems – though having said that, someone has done some work on the engine as the head had been flowed, a vernier added & a right pigs ear made of the sump !
In fact, there was so much sealant in the pickup I’m surprised we had any oil pressure….. but that’s for another thread.
Think this pic give a better view of how thin the gap was - was less than 2mm at some points
All the others pistons have the same size cut outs (as you'd expect) but look fine.
Engine builder suggested moving to forged with a thicker crown to allow the pockets to be cut without getting too close the ring land
Everything else was fine - no indication of detonation, overheating, fuelling or timing problems – though having said that, someone has done some work on the engine as the head had been flowed, a vernier added & a right pigs ear made of the sump !
In fact, there was so much sealant in the pickup I’m surprised we had any oil pressure….. but that’s for another thread.
Think this pic give a better view of how thin the gap was - was less than 2mm at some points
All the others pistons have the same size cut outs (as you'd expect) but look fine.
Engine builder suggested moving to forged with a thicker crown to allow the pockets to be cut without getting too close the ring land
I had exactly this on 2 pistons when I ran the engine with too much advance.
My plug gaps closed as well but I guessed what it was at the time!
I had schrapnel sucked back into the plenem and deposited into other cylinders.
I gave up on the 5.0 because of the weak pistons as I do a lot of track use and am reverting back to a 4.6.
I know it was self inflcted but those in the know would aways recommend a 4.6 for heavy use.
Good luck with the build.
My plug gaps closed as well but I guessed what it was at the time!
I had schrapnel sucked back into the plenem and deposited into other cylinders.
I gave up on the 5.0 because of the weak pistons as I do a lot of track use and am reverting back to a 4.6.
I know it was self inflcted but those in the know would aways recommend a 4.6 for heavy use.
Good luck with the build.
Griffith77 said:
looks like when the pocket was cut in the piston, whoever did the work didn't take into account how much this was reducing the cross section between the pocket edge and ring land - slight weakness in the piston crown/a bit of fatigue + a damm good thrapping on the track = knackered piston!
To get CR, squish & large valves there is little else you can do. Plenty of cars are running with this piston without problem. I'd take a second opinion on the traces in that piston & show the inside of the heads as well. However I suspect you may have cleaned up the evidence that would give better clues to the root of the problem. Forged pistons are likely to be different weights, significant rebalance required, as well as commonly requiring sliding pins so rod little end hone... Even if you replaced the piston like-for-like I'd advise having the pistons+rods matched & balanced end-end as well as the crank balanced to suit (there can be quite a lot of difference in weights of rods & pistons that are supposed to be the same part). Might as well have one of Robs 4.6cum5.0 top hatted lumps & be done with it if you are that way inclined?
Whilst it's 'all in bits' check that the crankshaft pulley is a tight fit on the woodruff key in the crankshaft. Mine wasn't which resulted in the pulley moving around and wearing down the nose of the crank! That then took out the crankshaft oil seal.
Dulfords had the engine out, stripped, crank machined, welded and machined again....
They also had some 'stepped' keys made to allow for the difference in slot sizes so now my pulley is rock solid.
Expensive way to find out about this though!
Dulfords had the engine out, stripped, crank machined, welded and machined again....
They also had some 'stepped' keys made to allow for the difference in slot sizes so now my pulley is rock solid.
Expensive way to find out about this though!
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