Cracked head

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Bison
Posts: 132
Joined: 27 Mar 2013 10:49
Location: West Lothian

Cracked head

Post by Bison »

Folks,
Finally started the rebuild on my 1979 3 1/2 Sport, all looking good so far apart from a barely visible hairline crack between the valve seats on one of the heads. What to do?. I'm fairly sure I read somewhere that the engine will run for a long time like this?, although that doesn't seem quite right to me?. I do have access to a TIG, and also a very competent aluminium welder. Can it be dotted with the welder then faced off, or will the valve seats distort?. Does it have to have the valve seats removed, welded then new seats let in?, or should I leave it alone for the moment?. It will only get worse surely?
Any suggestions gratefully received.
Thanks,
Alan.
smotorboy
Posts: 254
Joined: 04 Oct 2011 17:54
Location: costa mesa, ca usa

Re: Cracked head

Post by smotorboy »

To properly repair a cracked alloy head,,, it is always best to grind out the crack completely then fill the area with weld rod that is compatible with the head material...

The area then has to be remachined to bring it back into proper tolerances for the valve seat pockets...

I would find a serviceable head rather that repair the cracked one if possible.....

Save the head for repair at a later time as there are no new ones being made...

Others may know how long before the crack becomes very serious on these heads.... Worst case is that a valve seat comes loose and wrecks the piston, cylinder, head and valve.

Good luck with the repair,

Robert in California
MickeyMoto
Posts: 2415
Joined: 22 Nov 2008 17:41
Location: Even further oop North

Re: Cracked head

Post by MickeyMoto »

Thought this topic was related to some non-Morini narcotics subject...
George 350
Posts: 450
Joined: 16 Jun 2007 09:43
Location: Northampton

Re: Cracked head

Post by George 350 »

Hi,

How many miles per year do you ride it?
I had a 'cracked head' many years ago. I ran the bike for 3 years and 20,000+ miles before the inlet valve seat began to recess into the head. At the time I put it down to the fact I was running the bike at speed in very hot weather (42°C in the shade!).
First noticed it went quiet as the valve clearance closed up, then it would misfire because of the lack of compression.
Back then, the cost/aggro of repairing it was way greater than simply buying another (2nd hand) head, so that is the route I chose.
As said, I did over 20,000 miles from when I knew there was a crack (how long it had been cracked before I found out I have no idea, since it had been 25,000 miles since I had previously removed the head) , but the failure when it came was quite dramatic - the valve seat had pushed back more than 4mm into the head in less than 500 miles.
If you do choose to buy another head, don't forget to examine the exhaust port and rocker stud threads carefully - you don't want to buy a head only to have to get those repaired! The NLM price list shows them at around £145+VAT.

Hope that this helps,
George
George
350 sport 1978, 350 Strada 1978
650 Norton 1967, 650 Kawasaki 1977 and 650 Enfield 2019
Bison
Posts: 132
Joined: 27 Mar 2013 10:49
Location: West Lothian

Re: Cracked head

Post by Bison »

Folks,
I took the advice and purchased two used heads, with nice threads and studs. Hopefully all will be well now.
Thanks once again for the information and opinions.
Alan.
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