Norton 850 Commando Rebore and head trouble

Steven,

I would suggest that your Commando has either covered a very high mileage or given a hard life with inadequate maintenance. I say that because my own 850 has covered over 30,000 miles and still on standard bores. But I have known many examples that have covered twice that mileage and still running well on standard bore.

1934260 + 60 certainly was available, but if I were you; I would first consult with John Dunn with your  issues.

Good luck.

My ex-police Mk2a Interpol was at +040 and worn out after 105k miles. I went to +060 and it's been running fine ever since - over 40 years now. My pistons are original Helpolite, don't know what's available today.

I know someone in Birmingham who will make bronze valve guides to order but it would be advisable to get them to look at the head as well rather than just specify a size. PM me and I'll put you in touch.

"Bronze" covers a multitude of alloys, not all of which are compatible with all types of valve materials. G&S valves web site tells more, but it seems difficult to work out what valves are made of and which guides are best. Maybe our usual suppliers know, but I suspect that replacing like materials with like is safest.

Over size guides.

I'm pretty sure RGM has them in cast iron or bronze "colsibro". Or send the head to SRM and they'll sort it out and they're not too expensive either.

Dave

It looks like your going to need a custom guide making for your head. I've seen this sort of thing many times as the workshop manual method of heating the head and knocking the guide out takes some of the alloy with it.  You need to check the bore the guide will sit in, is parallel as i bet it isnt!  I would try and stay with using cast iron for the guides, it wears better than bronze. The last cylinderhead i did guides for was a 650 triumph, replacing the bronze guides with custom made oversize cast iron ones as that had the same problem as yours, hole too big for the available oversize guides. 

Cylinder barrels should take a 60 thou oversize rebore. If done correctly and you use a decent air filter (you do use a filter don't you?) and the use of good oil and regular filter changes  you should expect quite high mileage (at least 70,000) 

My Mk3 is still on its standard bore at just shy of 76000 miles. The original AE pistons were worn out at 55,000 miles. I replaced them with the hepolite branded JE items and its been fine since then. Using Mobil One 10w60 fully synthetic.

The bloke who did the rebore on my ES2 also made the oversize guides.

Maybe you should take the barrels and head to a good motor engineer and see what he can do.