Mk2A and Mk3 Cylinder heads

Identical. When they left the factory anyway; who knows what previous owners might have done in the intervening half-century.

Will be stamped RH4.

1973 850 had RH4 head, 1974 had RH10 head & mk3 1975-77 went back to RH4 head which has 32mm inlet ports - RH10 has 30mm ports - fittings exactly the same as stated above. There are different inlet manifolds to match 32mm carbs to each head.

Some of the later Mk 3 Commando cylinder heads had 1/2" AF size Adjuster Nuts instead of the more common 3/16" Whit. However all used 9/16" BSF x 26 tpi for their threads.

The MK2/2A head is the best 850 head, more support for the inlet valve guides where the holes for the guides are prone to cracks. You just need the tapering manifolds and you can stick with the 32mm carbs.

@Alan Sessions

My 1974 850 Mk2A Interpol had an RH4 head. 32mm inlets. So I guess there was a bit of mixing & matching going on.

Your current MK3 head should be an RH4 with 32mm ports and I assume your manifold is also 32mm at the head. So if you use that same manifold on an RH10 head with 30mm inlet ports then there will be a step between the manifold and the RH10 head. Now if you read un on the Goldstar development they accidentally found a step at the head actually improves the Goldies performance and they introduced it into production. 

Best measure the current manifold first and then take it from there.

Roland Pike

One of the strangest occurrences to do with carburettors was when one day Reg Wilkes sent an apprentice from the test shop into our, main shop for a 1 3/32 GP carburettor and he misread the size and gave Reg: a 1 3/16th carburettor and they immediately got quite a jump in power. They reported this to me and I went to the shop and the test was repeated and there was no doubt it was quite a gain in power. I suggested to Reg to open the port to match, immediately we lost what we had gained plus a bit more, so we made a thin sleeve and pressed it in and the power was back. We repeated this on other engines always with the same results. A number of private owners of DBD Gold Star noticed the carb being bigger than the port and opened up the port thinking to gain power, but not having a dyno were not aware of the results. Later on we fitted a venturi behind the carb and picked up even more power. The smallest diameter of the venturi could be 80% of area of the carburettor. We tested this on several engines and it always worked and seemed to improve carburation. The venturi had to have the classic included angles of 22 degree in and 7 degree out.