I could just scream!

2.1mm with a .021" base and .062" head gasket.

More babble:
Something I'll mention for anyone that has not used Jim's head gaskets is Jim's head gaskets for the 750 are punched with a very tight diameter hole around all the head mounting holes. If you use his Piolbond sealing method which is equivalent to welding the gasket on (exaggeration but moving them once down is near impossible) the hole alignment if the gasket is put on the barrels first is critical or those studs may not drop in easily. Nor will the through bolts thread in smoothly. Nothing like the old big hole copper gaskets I've used in the past. Thoughts: Either adhere the head gasket to the head first fitting it over the 3/8" studs so everything lines up, or use some spare studs in the barrels to help with gasket alignment if putting the gasket on the barrels first. The two little studs in the barrels are not enough to get all the holes aligned perfectly. Not for me anyway.

Did the Combat head use 3/8" through bolts next to the spark plugs? If not drilling them out and getting the barrels threaded for 3/8" obviously helps seal the head.

My only suggestion is to start drinking heavily! Hope things go better!

mean gene said:

My only suggestion is to start drinking heavily! Hope things go better!

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Good advice, but I quit drinking in 2013 on the 20th anniversary of quitting smoking. I keep trying to get doctors to focus on what's actually wrong instead of smoking and/or drinking. Now they're on being fat. I'm sure if I lost weight, it would be "you're old".

Still waiting on the thread repair kit and while making a drilling guide I realized that this head has also been skimmed. It was done well by Jim Comstock. So, I got to measuring and the exhaust valves will touch or almost touch - I read somewhere that the valve to piston clearance is supposed to be 0.050". Yes, I could put in a thicker head gasket but a Combat to be 10:1 is no base gasket and a thinner head gasket. Since I have a base gasket and a 1mm head gasket, I think the problem is elsewhere. Right now I'm guessing that the valve timing is wrong. I know it is setup correctly - I know how to doit and I've double-checked myself - I think the key slot may be in the wrong place in the new cam. So, I'll work on checking he valve timing once I have the head back on unless someone has another idea.

BTW, the engine built by "a famous engine builder" that I took the head from has the pistons installed backwards! Sure glad I didn't try to start it - new bore, pistons, and a $1000 head job in 2010 by JC.

Your comment on cheese quality head material can also be applied to the Mk1 version of the Fullauto heads.
I know, I have one. :-(
Those studs pulled RTF out. I was not amused.

The rest of the story:

When I measured the head, the 0.005" difference was determined with a caliper since I didn't have a depth micrometer. I bought a depth micrometer and measure the other head that Jim Comstock (JC) did and realized that the first head matched the JC head on one side and was only 0.0025" (not 0.005") lower on the other. I wrote JC and he promptly wrote back and said that he would not have machines it, but only lapped it as there is no room to machine a Combat head. So, I lapped the original head - not hard to do. So, what now? Had to be pistons or valve timing. I already rechecked the valve timing, but I had not used a degree wheel just made sure it was setup normally. I was thinking that the key slot might be in the wrong place. Well, the valve timing checked out. Then I had the terrible feeling that I had put the pistons in backwards - they are not marked "L" / "R" and no Ex marking. They were certainly opposite the one in the bike I took the head from and those were marked "L" / "R" but again no Ex marking. According to the workshop manual, I had them right, and comparing to original pistons I took form my last Combat build they were right. By then, I was doubting my self so another email to JC and his answer made me sure I was right - the other engine has the pistons backwards - not the one I'm building.

So, I put it back together. The only differences are: I used the steel pushrods from the other bike (they are slightly shorter), and I put it back together without anyone helping. If you're waiting for me to stay what was wrong, I can only say it started on the first kick, idled, and made no noise. I have no idea!!!! I have a silly theory that the drive side exhaust rocker was not seated in the pushrod but that makes little to no sense since the gap was set and I can't imagine the engine running with rocker ball sitting on the edge of the cup. It could be that it was stilling there when the gap was adjusted and then fell into the cup and the noise was the rock banging from too large a gap - I didn't check the pushrods and gap myself - I wish I had thought to check before removing the head but I didn't.

Many thanks to Jim Comstock. People call me for help almost everyday so I usually don't need help, but his answers to my silly questions helped me greatly - often knowing what is not the problem is more than half the battle!

Glad you got there Greg!

I recall JC once explaining how easy it was to bend a valve, very slightly, upon installation. Perhaps this is what happened, causing the vavle to stick open slightly and contact the piston until use straightened it ?

Or maybe the gaskets you’ve used this time are slightly thicker?

Or maybe …

Why do we doubt ourselves so much? I doubt myself to the point I take pics of everything! Age? One of my better spent dollars was for a bore scope, these were very expensive until computers. Now you can get on on line that plugs into computer for a larger image. Looks into bore and piston thru plug hole. check valve conditions, Many uses $60

Fast Eddie said:

Glad you got there Greg!

I recall JC once explaining how easy it was to bend a valve, very slightly, upon installation. Perhaps this is what happened, causing the vavle to stick open slightly and contact the piston until use straightened it ?

Or maybe the gaskets you’ve used this time are slightly thicker?

Or maybe …

Click to expand...

I read somewhere that the valve to piston minimum gap is 0.050". I forgot to mention that I bought an endoscope that actually works and I was able to see plenty of gap when I put it back together. So, the drive side exhaust valve hitting was none of the things I initially thought. I'm now half convinced that the rocker ball was not in the pushrod when the gap was set. You can bet I'll check myself next time I have a helper. Also, I did check that valve and it isn't bent - the touch was so light that is barely marked the piston.

mean gene said:

Why do we doubt ourselves so much? I doubt myself to the point I take pics of everything! Age? One of my better spent dollars was for a bore scope, these were very expensive until computers. Now you can get on on line that plugs into computer for a larger image. Looks into bore and piston thru plug hole. check valve conditions, Many uses $60

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I doubt myself because I have seen myself fuck up plenty often.

Gas fill-up and shakedown ride. Clutch and gearshift not properly smooth at first but that all cleaned up on it's own. Have a minor leak at the oil pressure switch (Don Pender) - easily fixed - I reused the washers when I re-installed after checking the valve timing without annealing. Another interesting thing is that he bike came with a new set of Hagon rear shocks that are a little long. The ride is nice but the rear wheel just touches the ground on the center stand (brand new). Need to think that one over.

Forgot to put the left-side panel on and going to strobe the timing when I have a helper so the "points" cover is off. Yes, I'll install a mirror and the seat really lets the bike down but new ones are really hard to find right now, especially with silver lettering.

Need proof that a newly rebuild 1972 Combat (15 miles since build) with correct Premier carbs and a Tri-Spark ignition just statically timed can idle nicely? Look here: https://www.gregmarsh.com/MC/Norton/Images/Idle.mp4

Nice job Greg, looks fantastic.

Is the rev counter correct? Didn’t seem to match the sound when you blipped it to me ?

Fast Eddie said:

Nice job Greg, looks fantastic.

Is the rev counter correct? Didn’t seem to match the sound when you blipped it to me ?

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I think so, it is the correct part number, just rebuilt, and it seems right while riding. When first starting cold it idles at about 750 and when warm, at about 1000. I may turn that up a little (I usually set to 1100 at operating temp). The sound is funny. The peashooters are very loud but my phone makes them seem quiet. At 3000 RPM with a helmet on it is way louder than a 72 year old should like

Fast Eddie said:

Is the rev counter correct?

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marshg246 said:

I think so, it is the correct part number, just rebuilt,

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,,,But fitted on the 'wrong' side.

L.A.B. said:

,,,But fitted on the 'wrong' side.

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Yes, I know. This is a pre-sold bike and the buyer is, like me, is used to the Triumph way so he wanted it on the left. While we're at it, yes the Triple Tree is black. I detest silver against chrome so I always try to talk people into black for them. Black next to chrome looks good to me. And, I power coat black in-house. I've never found a suitable silver color in power.

Onder said:

Your comment on cheese quality head material can also be applied to the Mk1 version of the Fullauto heads.
I know, I have one. :-(
Those studs pulled RTF out. I was not amused.

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It's another weak area on these engines. Lube thread/dry thread debate aside, the Comnoz video showed how marginal these fastenings are, even after fitting helicoils.
Jim does mention that he has fastening methods which will give greater holding strength for those studs.
Especially if opting for high compression/ big bore it's good insurance. I will likely end up there eventually with the high comp. 920.

-One thing I have noticed about all of the original studs I have here- they only have 7 full threads before the shank. I made up some SS studs with more threads and managed to grab 10 threads before touching bottom in an RH 10 head, original threads, no helicoils. Whether this actually helps , I'm not sure, but I suspect it will.
One day I'll do a simple torque test to see what happens with 10 thread insertion vs 7.


Glen

marshg246 said:

Yes, I know. This is a pre-sold bike and the buyer is, like me, is used to the Triumph way so he wanted it on the left.

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Yes, I remember you mentioning it before. The new owner obviously isn't taking Norton ownership seriously enough.

Machining the head to get more compression is silly stuff. When you tune a motor, there is a balance between fuel octane ratio, compression ratio, ignition advance, and jetting. Changing any one of them can be compensated for by changing any of the others to get the same result. When you shave the cylinder head, you make it easier to distort. If you want more compression, buy new pistons with higher crowns. Most road-going Commandos are probably jetted too rich anyway. When you raise the compression ratio, it has the same effect as leaning off the jetting or advancing the ignition timing. When your jetting is as lean as possible without burning valves or pistons, that is when your motor will be at it's fastest And that is almost irrespective of which fuel you use.
When you raise the compression ratio, you usually need to jet larger, so you use more fuel to restore the balance Theoretically you should get more power. But what happens in practice is different from the theory.

New pistons are cheaper than a new cylinder head. If you want more power, do the job properly and fit fuel injection with a decent engine management system. Modern cars run much higher compression ratios than old Commandos. Jim Schmidt sells 12 to 1 comp. light pistons and long conrods.