cracked head

Is it possible to repair an 850 commando head that is cracked by an oversize valve guide?

Yes as it happened to me when I first built my Norton to go in the Featherbed frame, put my trust in a so called head expert but with only jap bike experance and had never worked on a Norton head, cracked around the 2 exhaust guides, he payed for it to get repaired, that was 33 years ago now and I am still using the same head.

Ashley

Anything can be fi$ked.

I had an 850 head converted to big valves by a reputable vendor and it came back with a crack starting at the oversized intake guide and back up the port a bit.

You could thoroughly clean it with say (MEK) and try push JB Weld into the crack if it is minor.

If the crack is minor and mechanically sounds then the goal is keeping the oil from migrating from the spring pocket down into the port.

If you don't seal it you will have..............

Crack head smoke.

Exhaust guide yes, there is enough metal there to weld and access is OK.

Intake guide I would not recommend. There is very little metal there as it is and even if it is welded successfully it is likely to crack again as the weld will not be as strong as the original casting.

I have seen it done successfully by welding in a sleeve and then boring the sleeve for a 750 sized guide. There was quite a little head induced softening of the head. He did it without replacing the valve seats and one fell out shortly thereafter. Then he had to find another head. Jim

i only ask as i have an 850 big valve head that has been at the back of my shed for 20 years it belonged to a mate of mine,it has been bored out on the right hand inlet valve guide hole and machined deeper where the valve spring cup sits and an alluminium top hat bush was made by a reputable norton dealer to take the 850 large type guide,this didnt work it came loose and the crack was worse so i started to weld it up to have another go at it then i decided to fit a complete 750 top end to his bike he still has it to this day!, having dug the head out of my shed i can see the over side of the head is also cracked by the inlet guide i really dont want to throw this head out when it may be repairable cheers for the help on this forum baz

baz said:

machined deeper where the valve spring cup sits

Click to expand...

Therein lies the problem. Forget my comment about sealing with JB Weld. There is very little metal between the bottom of the inlet valve spring cup area and the roof of the port on a stock head. The machinist has made the margin even smaller.

Welding to build it up seems like the only viable option.