My What Am I Getting Myself Into Combat Build Thread

Maybe on the cam.

I didn't think the factory built bikes had issues with the Combat cam, and only a small batch of Combat cams for aftermarket sale were not hardened properly.

I put a new Combat cam in my P11 motor in the late 80's, and have never had a problem with it other than the recommended lash makes the motor sound like a gallon can of bent nails on a paint shaker at idle. Sounds awesome on the throttle though.
 
Haven't gotten down to the cam yet, but i did get around to miking the barrels. On the left cylinder, I'm getting 14 thousands over on the thrust side and +10 on the non-thrust side, both constant throughout the bore (bottom did read a little larger). On the right, i'm getting +17 on the thrust side and +15 on the non-thrust side (consistent up and down). There is probably measurement error in terms of absolute values (using an outside mic to calibrate a dial bore gauge), but the right cylinder bore is definitely larger than the left by +3 on the thrust side and +5 on the non-thrust side.

If measurements are good, hone or does it need a re-bore
 
I'd go .010 or .020 over (let your machinist advise you). DO NOT BORE until you have the new pistons and rings in hand. Let the machinist fit and end gap the rings.
 
Let the machinist do the measuring and have him tell you what oversize pistons to buy. I think he would rather do it that way and it protects you also. These guys have the nice measuring tools that they can slide up and down the cylinder while reading the dial gauge. They can easily see ovality and tapering due to wear.
 
Let the machinist do the measuring and have him tell you what oversize pistons to buy. I think he would rather do it that way and it protects you also. These guys have the nice measuring tools that they can slide up and down the cylinder while reading the dial gauge. They can easily see ovality and tapering due to wear.
What is the difference in size between the piston skirt (measured low on the thrust side, versus the bores on each cylinder?
-George
 
What is the difference in size between the piston skirt (measured low on the thrust side, versus the bores on each cylinder?

I'm getting 23.3 thou on left and 24.8 on right. Assume that clearance factors into piston size
 
.024 Clearance?? I'm really concerned it is that much. That would be like putting Standard Pistons into a barrel machined for .020 over.
 
Just rechecked and it's pretty close. The bottom of the skirt has worn down ~4 thou and the cylinder wall has worn ~17 thou. Adding both nets 21 thou.

This is just to see how far out of whack the internals may be. I am not relying on these numbers for the build and will leave that to the machinist. Bottom line it looks like i may need +20 pistons
 
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Ok, new problem. The primary cover is not budging. Removed the center nut and it wont budge. I taped lightly along the seam with a rubber mallet and nothing. Any tricks? There is just the one center nut correct.
 
Take out the inspection caps and stick your fingers in and yank.

You might have to give the cover a good tap with the mallet and use a heat gun to loosen the big o ring.
 
Thoughts about JS pistons.
More than a pair of Manley SBC pistons for two of em, and you have to use his rods.

I would go all in (pistons, rods, cam kit, valves, springs, yadda yadda) if I started down the JS path. I have no doubt that motors built with all the parts he sells work well. I would also talk to somebody that knows how to port a Norton head if in your situation. The stock head is fine, but nothing like a ported head when twisting the throttle.

It's only money.
 
I would not go down the JS rabbit hole, do a good stock rebuild and I would machine .030 off the top of the pistons to help lower the compression. stay with a camshaft with the specs close to the combat or stay with the combat cam. the combat head was usualy over ported and not a good starting point for a good port job so just do a good rebuild on it. all of this is if you plan to use it as a good all round rider. if you want to build a good hot rod I would start with an 850 motor as there are improvements to help it last a lot longer and is a stronger starting point.
 
I talked myself out of the JS route beginning of May. It was tempting, but I realized I wasn't 18 years old anymore and my go fast days were long over.

I did hot rod my P11 motor for the street 30 years ago when none of the cool parts were around. The head is ported and shaved, crank lightened and balanced, +.020 flat top pistons, Boyer, and it has a Combat cam inside the Atlas cases. It's fast enough for an old guy, still runs strong, and doesn't ping. People told me it wouldn't stay together doing what I did, but it has stayed together. It's way the heck too fast for the brakes it has, so now I treat it with respect and ride age appropriately. Sort of...

Anyway, Tom do whatever makes you happy. I think the JS stuff is sweet, but hard to justify if you aren't planning to turn the wick up a lot.
 
More than a pair of Manley SBC pistons for two of em, and you have to use his rods.

I would go all in (pistons, rods, cam kit, valves, springs, yadda yadda) if I started down the JS path. I have no doubt that motors built with all the parts he sells work well. I would also talk to somebody that knows how to port a Norton head if in your situation. The stock head is fine, but nothing like a ported head when twisting the throttle.

It's only money.

I have no qualms going all in. I just want to make sure all the parts are well matched overall with good throtle response, reliability, and the distinctive Norton sound.

As for cost, this is nothing compared to my other project...

My What Am I Getting Myself Into Combat Build Thread


It's only money :)
 
I have no qualms going all in. I just want to make sure all the parts are well matched overall with good throtle response, reliability, and the distinctive Norton sound.

As for cost, this is nothing compared to my other project...

It's only money :)
I haven't read anything negative here about the JS parts, but I have not looked that hard either. Throttle response is always better with less reciprocating mass in an engine in my experience. I haven't a clue on the sound. I doubt there is anyway of changing the signature notes a lot on the naturally aspirated old Norton twin. Reliability is usually based on the builder IMO. If you do clean work and take your time, should be smooth sailing.

Enjoy the 2 cylinder project.
 
Got the primary cover off. Lots of heat and hammering harder than I wanted to

My What Am I Getting Myself Into Combat Build Thread


Now on to the clutch :)
 

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When you go back together I really like that Dupont O-ring lubricant. Keeps them slick and pliable.... or I'm just lucky. Your insides look pretty clean although on the dry side.
 
When you go back together I really like that Dupont O-ring lubricant. Keeps them slick and pliable.... or I'm just lucky. Your insides look pretty clean although on the dry side.
Actress: Bishop.... Bishop: Actress.... (Sorry!)
 
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