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Commando engine query(s)


Hi Guys,

Hankering after a serious motorcycle project, I am now the proud owner of a frame (with a V5c), a pair of wheels, and half a dozen boxes of assorted Commando parts. Probably 90-95% of a complete bike!

From the engine number and various observations and measurements, it would appear to be a 1972 non combat version. The barrels have been rebored to +0.030" but I have no pistons. Also short of a L.H. inlet rocker, both inlet valves and a primary drive cover. Maybe someone has some of the spares I need? Be good to find pistons and the rocker??

Also, a bit of an oddity - one exhaust push-rod measures 186mm (7.323") and the other 187.5mm (7.382") Could anyone enlighten me please?

Thanks very much for any help you may have.

Cheers, Andy



Hi Andy

I measured both my O.E. 850 exhaust pushrods which are the correct 7.30" or 185.42mm. Pushrod vertical, ball flat on the surface plate with a 12" straight edge next to it using the metric and decimal inch scales.

Metric measurement came out by eye at 185.5mm or a difference of 0.08mm/0.002" which shows how close you can get with a good steel rule and in my case +1.5 diopter glasses.

Your 187.5mm/ 7.382" pushrod is 1.5mm/0.59" too long so adjustment will be different and with a 1.13:1 rocker ratio an unknown geometry change.

I would replace both with new to match them up and maintain the rocker geometry assuming the head hasn't been skimmed. Yours are correct for a non Combat 750.

Ashley, you need to junk your old wooden ruler or zip down to Specsavers, no way is that tolerance acceptable.




My Mk3 workshop manual has the exhaust pushrods assembled length @ 7.321" to 7.285"

No need to replace any of them. If your bothered about it Andy, you can shorten the longest one by removing one end, filing the alloy rod end and refit the end cap.

As i have to tell clients, 'they weren't made very well'   


Don't need to junk anything or go to specsavers - I have access to the drawing and they are in tolerance. 7.3'' was not the correct length, it is just a useful approximation. 



Just been looking again at the pushrods, with the suggestion to adjust the length in mind, when I had an engineering 'moment' !

Sitting the cup end on a nicely fitting ball bearing, I re-measured the lengths. Lo and behold - only a few tenths of a millimeter difference.

Thanks guys for the comments and suggestions.

I am still in the market for +0.030 pistons, primary drive cover, inlet valves and LH (drive side) rocker.

Cheers, Andy


Good call Ashley

Question, does the drawing show the length measured from spherical surface to spherical surface (ball surface to socket surface?) or/as well as the overall length?  


Great result with the engineering moment of clarity Andy. I've sent you a message re used Kibblewhite USA inlet valves.




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