Combat head and base gaskets

I stripped the 1972 commando yesterday and found out my motor is combat C head 32mm ports no base gasket and thick copper head gasket. The pistons where sitting proud and facing the wrong way around with no valve contact : ) Anyway 10.1 on AU pump gas is into ping territory if you get bad fuel so thinking of running the Cometic MLS base and head gaskets to lower the the compression . Questing for the brains trust what's your call on the ideal comp ratio for the 2S cam to still work on slightly detuned combat engine.

The RH1 head as used on 71 and earlier is 9.0:1 The RH3 Combat head has 0.042" off the face for 10.0:1 The missing base gasket is about .020" (squish some when torqued) Never heard anything about Combat engines stock without a base gasket? Might be over 10.0:1 by then.

Peavey Jeltz said:

The RH3 Combat head has 0.042" off the face for 10.0:1 The missing base gasket is about .020" (squish some when torqued) Never heard anything about Combat engines stock without a base gasket? Might be over 10.0:1 by then.

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According to Service Release N3/23, the Combat RH3 compression ratio was only the full 10:1 with the standard 0.030" head gasket and -no base gasket.
9.7:1 with a base gasket (9.8:1 according to Service Release N3/21).

If you go with the Cometic base gasket, they offer a few thickness choices. Allow about 5 thou for compression of the gasket on snugging things down.

Glen

When a Combat is set up at 10:1, does it use the squish band area?

If so, I would imagine there’s less chance of knock etc at 10:1 with squish than 9:1 without.

But only I’m hypothesising, I’ve never owned a Combat.

Have a look at Jim Schmidt's jsmotorsport web site. He sells a wide range of head and base copper gasket thickness allowing to adjust the CR.

Who ever built my Combat engine fitted the pistons around backwards so the valve pockets where opposite the valves with no contact so the squish zone is quite a lot with the 40 copper head and no base gasket. My guess is for tighter squish zone you would need domed pistons.

ozcruiser said:

Who ever built my Combat engine fitted the pistons around backwards so the valve pockets where opposite the valves with no contact so the squish zone is quite a lot with the 40 copper head and no base gasket. My guess is for tighter squish zone you would need domed pistons.

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Why would the pistons be put in backwards am i missing something here. There are a lot of ways to tighten the squish, have a look at the JS MOTORSPORT web site Jim will enlighten you to the world of commando building/tuning for power and reliability.

Whoever built the engine fitted the pistons backwards by mistake exhaust facing inlet and inlet facing exhaust the valve cut outs in the pistons where no where near the valves.

Weren't some pistons marked the wrong way round leading to some people fitting them incorrectly? (these aren't mine btw).

Mine measured 8.25 :1 , but run custom improved manifolds rather than the stock kinkey ones . ( Top passage length is twice bottom. Ish Std, )
& 32 Mk IIs .

Syd Vicious . Dont take for granted . Hit the cam at 3200. Pulled rocket thrust by 3500 if were not being pedantic . But Head wasnt your spec .
I reckon a 45 DCOE weber or suchlike , 32 pumper Dellorts of a BM or Guzzi etc . Or 40s if you really are keen .

ANYWAY , was no lack of grunt and likely woulda been more misserable to ride at an ' optimum ' Comp Ratio .
The OPTIMUM ADVANCE being 3/8 btdc , which is 30 degrees . You loose power backing it off at ratios used when there was ' 5 Star ' 100 octane pump gas .
Incidently I ran W & S ( Norvil ) Valve springs . Which are the old Johnson Motors Triumph set up , used from the 1950s !

These are progressive rate centerless ground aircraft spec. stuff . seats collars collets & springs . Told was 3S so ran 16 thou . clearance .

Quite schwang noise . Think the inirtia in the heavy lifters flung them through . Inertia Transfer . So low actual running loads .
Had the BSA Advance ( 24 degrees ? ) and would idle 500 rpm steady . Lower rpm idle gets better engine braking control & effect .
INCIDENTLY the T 100R 1955 CLASS C ran 7.25 measured . Regs were to get optimum side vale comp. ratio for racing . More and theyed start warping. More .
Still ran better on 100 octane av gas , & pulled 8.000 . Which is true comp. spec redline .

18 year olds on new Combats ran 8.000 . But not for long .

Real bitch to push start on pump gas with the 100 octane advance on a cool day . Spit back at the rear wheel . Adv cable gone & switched fuel .
A Commando on fixed advance can be cruel too .

Bedding in , cam . rpm etc etc . Cam Lube Id think would be vital . To much pressure and the cam bends of if iron , snaps . So top end set up / cleaancing is vital .

As they learned to their ( enormous ) cost on release of the Mighty Commbat . The cam was the early P R cam . So its not slow . Over 9:1 and Id think youd loose .
unless its a track bike or youve a friendly Av Gas pump .

P. S. , theres a lot off '.040 's in there , when your clearancing top ends . Nothings Closer .

Alloy rod stretch at racing rpms. Any wear on a road bike the fling closes things up . So read up on ' all that ' .