Valve stem cap

[Jens]

Could someone explain t me the function of the valve stem cap ? Is it just there to prevent wear on the valve stem ?
In what must be one of my more brilliant moments, I somehow managed to assemble Thumper with one of the little buggers missing and this was a while ago and I have no way of retracing my steps. I even managed to adjust valve clearance without noticing It's been running fine without it but I am sure there most be a reason for it's existence. I could just turn a cap on the lathe but it wouldn't be hardened. Is there anything critical about it ?

[Crofter]

Jens, it does contribute to more even wear and likely presents a harder target than the end of the valve stem. It is free to rotate so not likely to wear a rut on one plane. Possibly it allows some rotary motion to the valve that may help a bit with seat sealing but nothing like positive rotators would do. It should not be too hard to find a bit of something with enough carbon in it to turn one out of. Just quite a performance to make and heat treat all for such a dinky little item.

I remember seeing discussion about the geometry of the system being able to optimize the amount of top scuffing and valve side thrust. Undoubtedly the missing item changes the angularity somewhat but I wouldnt hazard a guess about how much of the original lister engineering is still there anyway after the Indian re birthing!

[bschwartz]

Jens,  the inside of mine are FAR from smooth.  I think anything you machine will be better than what comes out of India.  I don't think these are critical parts, as you are currently running without one. 
I'd recommend putting one back (home made, or Indian) just for geometry, but as far as hardening, I wouldn't bother.  These are slow moving, low pressure valvetrains.

[billswan]

jens

http://www.generatorsales.com/order/Valve_Cup.asp?page=Valve_Cup_Collet

Billswan

[bschwartz]

Oh c'mon Bill!!!!  That's too easy!!! 

[billswan]

2 thumbs up

Billswan

[XYZER]

It is good you found it! Running without it could also swell the valve stem making it not so easy to remove.....