One test drive & it’s all in bits again
Discussion
Well it was very nice to get it out on the road again, if only for a few minutes. All seemed fine initially, good oil pressure right from the start & running very smoothly. But first slight bit of revs under load & there was a nasty tappet noise, luckily the noise starting coincided with getting back to the entrance to the drive so I killed it & rolled back in.
I took the cam covers off & couldn’t see anything wrong initially, so started checking the valve clearances quite a few had already closed up by up to 4 thou which I was starting to get bothered by till I noticed one one of the exhaust valve buckets wasn’t coming back up as the cam lobe moved away. As the cam came further away some marking on the bucket became visible. The bucket did go down & come back up little, but nowhere near all the way. I took the spark plug out & could see a very slight witness mark from the valve from the piston. So took the cam out & tried to use some strong magnets to pull the bucket out but it wouldn’t budge, then I ran my finger over the marks on it, it felt like something had been trying to break through it – that can’t be good!
For those that don’t remember I took the heads off during the rebuild to fit some new inlet valves as the old ones had stretched too much to get even the thinnest shim in. With the heads off I found the exhaust valve guides were badly burned, so sent the heads off to have new exhaust valve guides fitted. They came back with the seats all recut to match the new guides. I refitted the old exhaust valves as they seemed to be in perfect condition, as was everything else.
So really appreciating the loveable Cerb quirk (as many other have before) of having to remove the engine to get the heads off I very quickly undid all the work I’ve spent the last few months doing.
I’ve not quite got as far as getting the head off again yet, but with the exhaust manifold off I can see the guide is cracked:
All I can think is that with the first real sign of any heat the exhaust valve stuck in the new guide, then got knocked back up buy the piston, back down by the cam & so on till something in the shim area gave way. The valve is stuck in the open position at the moment with no cam in there.
Really really need to change my user name...
I took the cam covers off & couldn’t see anything wrong initially, so started checking the valve clearances quite a few had already closed up by up to 4 thou which I was starting to get bothered by till I noticed one one of the exhaust valve buckets wasn’t coming back up as the cam lobe moved away. As the cam came further away some marking on the bucket became visible. The bucket did go down & come back up little, but nowhere near all the way. I took the spark plug out & could see a very slight witness mark from the valve from the piston. So took the cam out & tried to use some strong magnets to pull the bucket out but it wouldn’t budge, then I ran my finger over the marks on it, it felt like something had been trying to break through it – that can’t be good!
For those that don’t remember I took the heads off during the rebuild to fit some new inlet valves as the old ones had stretched too much to get even the thinnest shim in. With the heads off I found the exhaust valve guides were badly burned, so sent the heads off to have new exhaust valve guides fitted. They came back with the seats all recut to match the new guides. I refitted the old exhaust valves as they seemed to be in perfect condition, as was everything else.
So really appreciating the loveable Cerb quirk (as many other have before) of having to remove the engine to get the heads off I very quickly undid all the work I’ve spent the last few months doing.
I’ve not quite got as far as getting the head off again yet, but with the exhaust manifold off I can see the guide is cracked:
All I can think is that with the first real sign of any heat the exhaust valve stuck in the new guide, then got knocked back up buy the piston, back down by the cam & so on till something in the shim area gave way. The valve is stuck in the open position at the moment with no cam in there.
Really really need to change my user name...
I just read your post on my thread about my first drive, sorry to hear you are having problems. After all your hard work it must be really frustrating to have a problem which means undoing most of what you have done. And to think - I was frustrated that my brakes were spongy, your post puts thing into perspective!
If it helps it is nearly 12 o'clock and time to have a beer.
If it helps it is nearly 12 o'clock and time to have a beer.
Thanks for all the kind comments.
They are the original exhaust valves in the "latest uprated guides" I’ve got a crap memory & can’t remember what he said the new guides were made of, but now you say it they do look very much like cast iron.
I’ve no idea what the original valves were made of either, but I did make a point of telling the machine shop (who know their TVR engines inside out) I was intending to use the old valves. I was just told to check the size of the stem as it was likely to be slightly different, they weren’t so we agreed it would be ok to use the original exhaust valves.
Tarmacshredder said:
I'm sorry to hear your miss fortune, are the valves stainless and cast guides? Looking at your pictures it looks like the guides are cast and I cannot see any chrome on the valve stems, if they are stainless and cast they are not compatible and will seize in the guides.
Arse that sounds rather too plausible for comfort! They are the original exhaust valves in the "latest uprated guides" I’ve got a crap memory & can’t remember what he said the new guides were made of, but now you say it they do look very much like cast iron.
I’ve no idea what the original valves were made of either, but I did make a point of telling the machine shop (who know their TVR engines inside out) I was intending to use the old valves. I was just told to check the size of the stem as it was likely to be slightly different, they weren’t so we agreed it would be ok to use the original exhaust valves.
longbow said:
I'm pretty sure the valve guides in mine are Bronze - I had to replace one a few years ago and the machine shop definitely put another Bronze one in.
Mine are phosphor bronze, I got a peak when I had the exhaust manifolds off.Really feel for you OP, cast steams have been used for years in engines but as said the 2 materials might have created a great deal of friction. I hope the damage is minimal.
My old exhaust guides were bronze, but the new ones look very much like cast iron. I’ve got it all stripped down now. A magnet sticks well to the new guides (obviously not to the original bronze inlets that are still there) but the magnet has no interest in any of the valves, the original exhaust valves, or the new inlets. The odd thing is that I’m sure the cast guides were sold for use with the new valves. The magnet not sticking can’t tell what they are made of, but can point to what they aren’t made from. The new inlets are black so maybe have some kind of coating on them.
I’ll post the pics tomorrow from work (PC problems at home) but the only failed part is the shim. Cause or effect? I’m sure the bucket would have been smashed to bits seconds later had I not killed it there & then. That bit sounds like Jammy’s problem, I’ll look for his thread tomorrow.
But with two votes for cast guides & stainless valves not being good mix, maybe failed shim is an effect.
I’ll post the pics tomorrow from work (PC problems at home) but the only failed part is the shim. Cause or effect? I’m sure the bucket would have been smashed to bits seconds later had I not killed it there & then. That bit sounds like Jammy’s problem, I’ll look for his thread tomorrow.
But with two votes for cast guides & stainless valves not being good mix, maybe failed shim is an effect.
FarmyardPants said:
What a b
h 
You have my sympathies but from the sound of it it could've been a lot worse damage-wise. Every cloud etc
Indeed!h You have my sympathies but from the sound of it it could've been a lot worse damage-wise. Every cloud etc
So I spoke to the machine shop, they were very helpful & good about it. After looking at these pics they are sure it was shim failure that caused everything.
I was very glad to find all the bits of the shim in the head, here they are:
The rest of the damage:
The piston cut out is very accurate!
It seems the shim failed, moved off the top of the valve & got stuck between the valve retainer & bucket. That then pushed the valve too far down so it contacted the piston, that bent the valve stem slightly causing it to crack the cast guide & stuck in it.
I really should have RTFM, particularly as I had it
(Well I had read it some time ago but forgot!) So I dug out my Four-Stroke Performance Tuning book. Sure enough in there it says austenitic stainless steel racing valves (best identified with a magnet) are very good for the exhaust but have very poor scuff resistance so must be used with guides made from silicon aluminium bronze.However that book is a few years old now & these new cast guides are self-lubricating so should be ok with the stainless valves, however I don’t think anyone else has actually tried it because everyone else has fitted the latest valves that have a special coating on when fitting these new guides. Mine will be going back together with all new valves!
But to the main problem, it seems there could be some faulty shims out there, so watch out!
Here is Jammy’s thread, seems mine was just the same, but I was lucky to be somewhere that suited killing the engine instantly (outside my drive). Also I’d already seen that thread so had a bit of a bad feeling about the shims. I’ve driven other engines to death because I didn’t want to stop for various reasons before so I know it’s easily done.
If you check my contributions half way down page 4 of that thread you’ll see the problems I found with my shims. However the failed shim doesn’t look to have any of my machining marks inside it, so could it well have been one that did fit ok, but was actaully just a bit too tight & the first bit of good heat up the exhaust valve expanded it enough to crack the shim
If you check my contributions half way down page 4 of that thread you’ll see the problems I found with my shims. However the failed shim doesn’t look to have any of my machining marks inside it, so could it well have been one that did fit ok, but was actaully just a bit too tight & the first bit of good heat up the exhaust valve expanded it enough to crack the shim
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