Commando flathead....well, somehow

Morning, Ladies and Gents,

I've just taken the head off of my newly acquired '70 Roadster and was a bit surprised by my findings: The head is ground flat on the bottom with now flange left at all with the surface really being flush to the lowest rib.

The head looks almost unused internally - as the rest of the engine, I strongly suspect that thing has never ever powered a bike, the pistons are std and the honing still looks untouched, the cam and tappets don't show any kind of wear, not even a polished marking. The ports have been slightly enlarged but not more than 30mm.

Question now is: How flat can the head be milled/ground? There was a copper head gasket and appart from the outer appearance the head looks like it could go straight on the bike again....

I think I'll strip the engine down completely, clean all the parts, check (and eventually change) for Superblends, replace the slotted pistons with propper ones and check the compression ratio - or do I miss something and the head is dead?


Tim

Hi Tim,

The combat head with "C" stamped on top is milled 10mm more than standard, but has some metal remaining. There are 10mm thick aluminum spacers for the cylinder to head joint that you might try...maybe even stack two.

Hi Tim,
Sounds like a a great find. Combat motors appeared in 71, but its plausible that yours has been decked sometime in its life to achieve the 10:1 compression ratio. The factory applied the C stamp above the right hand exhaust rocker cover to denote this. In addition, the pushrods should be shorter, not by much I think it was .039"(1mm) to maintain rocker geometry. The inlet ports should be 32mm at the manifold with the appropriately fierce 2S cam. I'm no Combat expert but having ridden some, I can say they are the quickest of all the Commandos. Assuming yours has all the related specifications it would be a good project as a performance bike, otherwise just having the higher compression may not neccesarily be much different to the standard 9:1 in performance which is a good all round rider.

Mick

It may very well turn out to be to high compression for any kind of modern petrol besides racing fuel. I put a compression plate at the at the base sandwiched between base gaskets. Yours, being freshly honed and no ridge to fret about, I would also go with a base type compression plate. This is a much tighter and secure solution.

Sounds like the PO was really trying to get power out of this engine with the head work including the ports.
Possibly a hot cam in there also. Marked 2S it would be a Combat cam.
Since it is shaved more than the Combat and it had 10/1 yours may be seriously into the detonation range with todays fuel.

You may want to detune it a bit if you don't want to be handling a hot rod.

Like pvisserii suggested a base of barrel spacer plate (I got mine from Old Britts), Standard Cam and 21 tooth transmission sprocket (I'm betting you've got a 19 tooth) plus a single 34MM Mikuni Carb and electronic ignition (I'm happy with my Pazon).
Since your compression may be sky high you may need to stack a couple of barrel spacers to get it back to 9/1 or so.

Any idea why after all that work the PO didn't appear to run it? Transmission ok?

If you want to keep it as a hot rod replacing the problematic transmission layshaft bearing a good idea if it hasn't already been upgraded to a superblend roller bearing.

Good Luck
Bob

I have 2 combat heads off here and one looks like that but if you measure from the deck to the bottom of the squish band they are both .085" or so (they aren't too clean to be taking accurate measurements). Make sure the valve pockets are cut deep enough into the pistons upon re-assembly but it could be a stock combat head.

illf8ed said:

The combat head with "C" stamped on top is milled 10mm more than standard, but has some metal remaining. There are 10mm thick aluminum spacers for the cylinder to head joint that you might try...maybe even stack two.

Click to expand...

I'm sure you meant one mm (1.0mm) not ten mm?

rx7171 said:

Any idea why after all that work the PO didn't appear to run it? Transmission ok?

Click to expand...

Thanks for all the answers, guys!

I'd guess that the PO bought a 1968, killed the engine and replaced it by a replacement one in 1971 or later. But not much later as the pistons are of this horrible slotted type and AFAK this problem was known in the mid-70ies. And than it looks like the bike never was run again. There is a lot of parts missing, side covers, oil tank etc.

It is definetly not a Combat, the engine no. is starting with 147 or so (can't check as the bike is a few klicks away) and there is no C stamping anywhere. Although the face has been skimmed more agressively than on a Combat the ports are just 30mm (but definetly machined). I've no idea about the cam spec as I need to take the cases apart anyway.

What I'll do is as follows: I'assemble the engine dry (no pushrods and no base gasket or compression plate) and measure the combustion chamber with the good old burette method. I'm really curious what kind of CR that'LL give.

Best regards,


Tim