RH4 Cylinder Head-What year will it fit on??? Boyer Kickback

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Mar 1, 2007
I currently have an early 1975 850 commando of my own in need of a replacement cylinder head. It is not electric start and it has front disc only and right hand shifter. Will a head stamped RH4 work on this bike?? Are the rockers and spindles interchangeable with the original head I have if needed??---------- Thanks to all who helped with my friends bike that was kicking back. I did all the suggested repairs and it seems that there were many contributing issues. The bike seems to be running fine now. I have kickstarted it about 10 times with no problems. I am hoping my luck will hold out. It had bad wiring connections in all the usual places, bad battery, low voltage and I had the timing slightly too far advanced at 5000RPM, I replaced the Boyer, ignition switch, battery and bypasssed the kill switch. I also replace the trigger wires. I will never forget the good advise and I will use it now on the two 850s I am building up for myself. I will be sure to follow your advise before I give mine it's first kick in 17 years. Thanks guys!!
Mark, is your cylinder head actually missing or is it damaged? It would have to be very badly damaged not to be repairable and most of the usual problems are broken fins, damaged exhaust port threads, pocketed valve-seats, damaged combustion chamber from engine blow-up etc. All of these are repairable by a number of specialists within the Norton community.
*50 Cylinder Head.

I do have the original head but the exhaust threads are ruined and it has multiple broken fins around the edges of the exhaust ports. Some idiot drilled thru the edges of the exhaust into the threads in an attempt to lock the large exhaust nuts. Seems like it would be quite expensive to have the holes welded up and new fins around the edges of the exhaust replaced with a weld up??? In other words it may be cheaper just to go ahead and get another head and start from there??? Thanks
Mark, Rabers in San Jose can get the exhaust threads repaired for you no problem, it might be worth a call to them to find out how much and whether they can also trackle the fin job, It looks like a head may cost a couple of hundred dollars and even then you can't guarantee that it won't need additional work.
The head is off a Mk1 850 and will fit straight on, Its just got bigger ports.
Actually, the RH4 cylinder head was used in BOTH 1973 and 1975. The other common 850 cylinder head, RH10 was used for the 1974 model. The difference is the RH4 has 32mm intake ports while the RH10 has 30mm intake ports.

If you use the RH4 cylinder head make sure you are using the intake manifolds that are 32mm at both ends. If you are using the RH10 cylinder head use the manifolds that are 30mm at the head and 32mm at the carb.

The RH10 was introduced to provide a little better midrange (debatable) but was replaced with the older RH4 in 1975.
I must agree with those who have commented that having your own cylinder head repaired is probably to be the best option, or at least to check what any repair work would be likely to cost.
I expect many of us here have needed to get exhaust threads repaired at some time or other (I know I have) and it isn't particularly expensive? A used cylinder head in good condition that requires no work can be difficult to find these days at any sort of reasonable price.
Even if you buy another head you may find that it still needs repair work?

Cylinder head info:

(850 Rockers and spindles should all interchange)
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