Inlet valve/valve guide seizure

[berger]

swarfy its still a mystery to me how my shooting star engine did well over 60 miles with no big end oil pressure as such and no feed to the rockers when I put the oil pipes on the wrong way round. even though I know it was dragging a bit from the return pipe in the tank which I had over filled  and on a slight left hand lean it was enough to get some down the pipe , the colisbro guides are still in the head and it didn't effect them at all. it got very hot threw a circlip and ground the left hand cylinder with wrist pin-- haha wrist pin love it--- pulled out a flat topped camshaft and slightly concave followers, rebuild with timing bush, big end shells different barrels and pistons ,cam and followers and re designed oil pipes

[Mosin]

swarfy its still a mystery to me how my shooting star engine did well over 60 miles with no big end oil pressure as such and no feed to the rockers when I put the oil pipes on the wrong way round. even though I know it was dragging a bit from the return pipe in the tank which I had over filled  and on a slight left hand lean it was enough to get some down the pipe , the colisbro guides are still in the head and it didn't effect them at all. it got very hot threw a circlip and ground the left hand cylinder with wrist pin-- haha wrist pin love it--- pulled out a flat topped camshaft and slightly concave followers, rebuild with timing bush, big end shells different barrels and pistons ,cam and followers and re designed oil pipes

I feel positively lucky in comparison!

[chaterlea25]

Hi Mosin,

The guy who did the work said that he did not want to over-ream the guides

That sounds like the person does not have much experience with old Brit iron (or alloy) 
Work to the clearances in the service sheets or manual and "All will be OK" when using standard Bronze guides
The valves stems are usually the correct clearance undersize from the nominal 0.3125 (5/16in.)
so (usually) the guides are reamed or honed to 0.3125

John

[Butch (cb)]

Sounds like there is something more fundamentally wrong with this one. Ordinarily if you have a sticky valve, you strip it and ease it - once. 

[muskrat]

I've been thinking that as well cb.
Hay Mosin, ask the mechanic to measure the valve stem on both. May be an off size valve. As John said the clearance is built into the valve stem.
Cheers

[Mosin]

Thanks for the thoughts guys. Whatever the case it is looking like the whole thing is going to have to come apart again. The strange thing is that the engineer who did the work on the head specialises in classic/vintage/veteran vehicles and runs a few classic bikes including a Vincent himself. That is why he was recommended.

It will be interesting to see if it is the same inlet valve which has seized this time or the other one. I will also have a good look at the banjo bolts. I know that there is good pressure getting to the rocker box, but I guess that if one of the banjo bolts is partially blocked this would not necessarily guarantee that the oil was actually getting to the rocker in sufficient quantity to keep things moving. If this turns out to be the case then that would explain why it was not picked up on when the head was off and in the shop.

[Swarfcut]

Mosin  The hollow rocker shafts have small metering holes which feed into an oilway running lengthwise along the outside of the rocker shaft. Well worth taking apart to check for a blockage. For the oil to evenly lubricate the top and bottom of the rocker forgings,  the rocker shafts should be fitted with this oilway uppermost.

 Swarfy.

[Mosin]

OK. I have just removed the rocker box (for the fourth time in as many days) and two things stand out immediately:

• The inlet valve rocker on which the pushrod had become disengaged this time (OS) was not the same one as disengaged last time (NS).
• Both inlet valves were now fully closed

To my mind this suggests not so much a valve sticking through heat or lack of clearance, but rather a lubrication problem. I have removed the rocker gear and checked this over. There were no obvious blockages, but I am not convinced that the oilway on the outside of the spindle was facing upwards. Is this crucial? It certainly doesn't get mentioned in Haynes.

Mosin

[Swarfcut]

Mosin... I never bothered which way it went, so one day in discussion with fellow   alcoholic  backyard mechanics, the general opinion was that  oilway on top was best.

 This is what we reasoned.

    Oilway to the bottom means very little oil to the top surface of the shaft and also less bearing surface at the underside, where you need it, but possibly more oilflow to the loaded area.

  Oilway to the top gives a wide oil film which can enter the low load side of the shaft/rocker, and the remaining oil in the oilway stays there by gravity to flow to the radial groove at each end.

  You could argue that the reduced  shaft diameter where the oil holes are forms a more than adequate oilway, and as the radial groove at each end coincides with the rocker oil hole feeding oil to the underside of the rocker arm and hence the valves, it really does not matter anyway.

 The pictures in parts books and service sheets illustrate the oilway on the side of the shaft. So, top, side or bottom, I suppose you take your choice, even then you can bet it will move round when you tighten the acorn nut.

 Swarfy.