E: Engine Valve split collets

oexing

Well Known and Active Forum User
VOC Member
Thanks Tim, for the photos, my guess the spring cap was the reason for this defect. They did not get collets with correct taper for the spring cap, or the stem groove did not match the collet shape - or the spring cap was too soft brass or alu. The single groove types come in several different types, see my photo. But I would not pick them as that type of collets leave a gap and squeeze the stem - unlike the three groove precision No. 22 MK8-H collets with no gap. So their perfect taper spreads all loads evenly and should go well with alu caps. Anyway, either types are produced and installed in millions of engines without problems. In your case I can imagine that the collets digged into the soft spring cap at the top edge to form a substantial ridge that prevented easy disassembly - my theory. The printed sample of collets is in a TRW book , more than 20 forms in there - but definitely no combo with a tiny wire clip.

Vic
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vibrac

Well Known and Active Forum User
VOC Member
Thanks Tim, for the photos, my guess the spring cap was the reason for this defect. They did not get collets with correct taper for the spring cap, or the stem groove did not match the collet shape - or the spring cap was too soft brass or alu. The single groove types come in several different types, see my photo. But I would not pick them as that type of collets leave a gap and squeeze the stem - unlike the three groove precision No. 22 MK8-H collets with no gap. So their perfect taper spreads all loads evenly and should go well with alu caps. Anyway, either types are produced and installed in millions of engines without problems. In your case I can imagine that the collets digged into the soft spring cap at the top edge to form a substantial ridge that prevented easy disassembly - my theory. The printed sample of collets is in a TRW book , more than 20 forms in there - but definitely no combo with a tiny wire clip.

Vic
Yes very strange -the spring cap was steel and I think I can trust one of my Hero's Freddie Dixon* to get the angles right. Perhaps it was just the years it had sat there...

"Freddie did not just ride an HRD to a TT victory (1927) and have unbroken Brooklands car records he also helped develop the post war Douglas engine after Douglas asked him to sort their initial mistakes
 

oexing

Well Known and Active Forum User
VOC Member
Anyway, even just looking at single groove stems and collets I found a few subtle details in various types so one has to check all fits very closely to avoid defects as you have experienced. A taper of 28 or 30 degrees is not difficult to handle for quick disassembly , slimmer tapers of 5 to 6 (10 to 12 deg.) degrees like on gearbox sprockets or flywheels need substantial pullers to crack them open . So really there was something quite wrong on your bike, like near enough fits but not really right - with slowly growing defects from this in that joint. So in the end I will remain faithful to the three groove non-gap collets like on most serious modern engines since decades, free for the valve to rotate for reliable good sealing on the valve seat.

Vic
 
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