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Cylinder Head Identification

John Hudson and various authors from NOC-L Identification codes for the several types of Commando cylinder head

The Commando cylinder head can be distinguished from the earlier Heavy Twin head by its larger hemisphere which is offset to the cylinder bore and its 1½" inlet valves instead of the 1.406" valves on the 500cc and 650cc machines.

The rocker oil feeds go in at the sides of the rocker boxes, midway between the rocker shaft retaining plates, as the holes on top of the boxes are used to attach the head steady instead of the oil feeds.

The Commando head also has slightly different centres for the four bolts adjacent to the spark plugs, so that it is necessary to 'pull' the holes to fit on a 500cc or 650cc barrel. I have seen this done once, but it can only be described as a bodge.

Commando cylinder heads were not stamped RH1 from the beginning, and it would have been after 1972 when they started to make so many variants, which were as follows:-

Capacity Compression
Inlet Port Remarks
RH1 060988 750cc 9.0 : 1             30mm  Standard up to 1972
RH2 061427 750cc 10.25 : 1             32mm  AMA racer
RH3 063327 750cc 10 : 1             32mm  1972 Combat
RH4 064038 850cc 8.5 : 1             32mm  1973
RH5 064048 750cc 8.9 : 1             32mm  1973 low compression
RH6 064097 750cc 9.3 : 1             32mm  1973
RH7 064845 750cc 10 : 1             32mm  1973 short stroke
RH8 064884 750cc -                32mm  1973 short stroke race kit
RH9 065013 850cc 10:5 : 1             32mm  1974 high performance
RH10 065062 850cc 8.5 : 1             30mm  1974
A suffix S, e.g. RH6S, indicates a 750cc head fitted with 850cc guides


Will a 750 head fit onto an 850 motor? Has anybody ever tried it? Also, the identifier code over the exhaust rocker is absent, but I believe the head in question is a 1972 Combat. Has anybody else run into heads with no identifier before?

750 and 850 heads are different

The 750 and 850 heads are not easily interchangeable; they have different bolt patterns.

Eric Lamberts ( on NOC-L 22nd. Apr 1998

Fitting a 750 head to an 850

The 750s and 850s have a different bolt pattern, the four outer ones being spread more on the 850. Dwaine Williams says "you can just drill the holes over on a 750 to fit an 850". Others have filled the holes with 'dowels' and redrilled the holes, but then there's the problem of head gasket and squish bands to deal with.

I, and other racers (Todd Henning #454 for one), like small port heads for their flexibility and flow speed. Perhaps the easiest way is to port the 850 head and manifolds to 32mm, making a nice smoooth taper from the manifold surface to the valve guide area, similar to a Combat design.

A Combat head is marked with a large 'C' on the top in the middle near the top engine steady (on the two I have) and has a milled 0.040" surface. Earlier 750s have very small ports and fairly rough casting. Both my 750 S and 850 Mk.lla (with small port heads) could turn 12.8 second ¼ miles; the S handicapped by small S head pipes, and the 850 by 8.5:1 compression.

Carl Hokanson ( on NOC-L 23rd. Apr 1998

Combat head with no identifier

My Combat head has no RH identifier at all, yet all the evidence points to it being original to the machine.

Ben English ( on NOC-L 23rd. Apr 1998

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