Combat to standard conversion

batrider said:

I really don't know why people do this. It's a crime. There is nothing wrong with a combat as it is once the bearings have been changed. There is no loss of torque. Perfectly rideable and reliable.

Just buy a stock bike if that's what you want.

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I agree.
I am wayting the time when my Combat will explode.
It will be great!
But i fear that with Superblend it will never happen!
Ciao
Piero

Hi Tri-man @ All,

If you go for the "Gaskets Solution", have a look to Jim Schmidt's (jsmotorsport.com) web site. His .061" head gasket + his .021 base gasket result in a standard CR with a Combat cylinder head.

That's what I use and am very pleased with.

Laurent

I've ordered a .061" head gasket from Jim Schmidt and will use a standard cam, be it a "crime" or not. The big end journals cleaned up at .030", but only just. There are red flags everywhere in this engine, if we had decided to go for a Combat tune, I wouldn't have agreed to build it unless the owner had invested in a better crank AND cases.
As it is, the owner is aware of state of things, but I feel it will hold together if used sensibly, given a proper warm up and kept below 7000 rpm, that sort of thing.

Thanks to all who's contributed, much appreciated!

I think you are wise, but then I have witnessed destruction of a new original Combat Commando. My cousin bought a new Combat in 1973. After a restrained 1000 mile break-in we had some good fun with for a few days until it blew its crankcase and pretty well everything else motor related into pieces large and small at about 1500 miles.
The Combat catastrophic destruction problems are very real and the potential for injury is part of it.

I wouldn't count on Superblends alone to solve the problem. For those who are opposed to detuning, note that after the Combat ,Norton detuned and strengthened( late 750 and 850 to MK3) , then strengthened cases and crank yet again with the MK3.

Glen

with a thicker copper head gasket will that result in more frequent head tensioning ?
Don

Hi Don,

I retorqued twice my cylinder head (at about 300 and 1000 miles) after installing the .061" copper gasket and it's been really leak-proof until now (about 18000 miles).

Gasket needs to be annealed, and it probably helps a lot to use a copper wire around the pushrods and oil return tunnels (what I did) as per Jim's recommendations.

Laurent

worntorn said:

I think you are wise, but then I have witnessed destruction of a new original Combat Commando. My cousin bought a new Combat in 1973. After a restrained 1000 mile break-in we had some good fun with for a few days until it blew its crankcase and pretty well everything else motor related into pieces large and small at about 1500 miles.
The Combat catastrophic destruction problems are very real and the potential for injury is part of it.
Glen

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Just don't rev the crap out of a Combat without Superblends. I'm sure the first 1000 miles were as restrained as some, what 20 yrs olds, could manage?

swooshdave said:

worntorn said:

I think you are wise, but then I have witnessed destruction of a new original Combat Commando. My cousin bought a new Combat in 1973. After a restrained 1000 mile break-in we had some good fun with for a few days until it blew its crankcase and pretty well everything else motor related into pieces large and small at about 1500 miles.
The Combat catastrophic destruction problems are very real and the potential for injury is part of it.
Glen

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Just don't rev the crap out of a Combat without Superblends. I'm sure the first 1000 miles were as restrained as some, what 20 yrs olds, could manage?

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I was 20 with my first combat. Not much restraint on my part and it was still doing fine at 5000 miles when I sold it to an MD in Riverside, Ca. Needed a car.

The Combat saga is a well documented part of Norton's history .Plenty of bikes blew up, enough that Norton scrambled to make major changes in order to salvage their reputation.
As far as not revving a bike to it's designed red line in order to avoid destruction, what's the point of the extra top end performance of a Combat if one is forever afraid to use it?
I don't see the point in preserving a design that didn't work and caused a lot of grief. Far better to do as the OP is doing, detune slightly to give the thing a good chance of holding together.


Glen

My 1972 Combat lasted 10,000 very hard miles before the bottom end went south. Heck, I was 19 years old. Redline was a goal to be challenged.. I toured, road raced, commuted, and street raced that purple beast. When the bearings did go it was gradual. A subtle knocking on the drive side. Pokes Cycle in Seattle was surprised that my bike lasted as long as it did. Poke did charge me for labor but the parts cost was picked up by Norton. Their mechanic screwed up during the reassembly and I only made it about 8 miles before the whole lower end froze up. No oil to the bottom end. Back to the shop for a complete bottom end rebuild on the house. A week later I picked it up and it went another 60,000 miles before the cam lost it's hardness. I did do a couple of top end rebuilds in that time span. I never really changed how hard that I twisted the throttle or abused that bike. I sold it in 1976 to get money to buy a house but bought a nice 850 as soon as I could afford it.

worntorn said:

The Combat saga is a well documented part of Norton's history .Plenty of bikes blew up, enough that Norton scrambled to make major changes in order to salvage their reputation.
As far as not revving a bike to it's designed red line in order to avoid destruction, what's the point of the extra top end performance of a Combat if one is forever afraid to use it?
I don't see the point in preserving a design that didn't work and caused a lot of grief. Far better to do as the OP is doing, detune slightly to give the thing a good chance of holding together.


Glen

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Preserve them because those of us that hang on to them really like them. When they work a combat is sooo nice.

worntorn said:

The Combat saga is a well documented part of Norton's history .Plenty of bikes blew up, enough that Norton scrambled to make major changes in order to salvage their reputation.
As far as not revving a bike to it's designed red line in order to avoid destruction, what's the point of the extra top end performance of a Combat if one is forever afraid to use it?
I don't see the point in preserving a design that didn't work and caused a lot of grief. Far better to do as the OP is doing, detune slightly to give the thing a good chance of holding together.


Glen

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if it's been Superblended rev it all you want.

It would be nice if it was that easy but it sure didn't work out that way for Norton. They made the bearing change, plus strengthened cases plus detuned. Even with all of that there were still too many failures hence the further strengthening of cases with the MK 3. The MK3 also got a stronger crank, yet tuning was left at the lower post Combat level, while competitor bikes were adding more power every year.
I'm sure with their financial situation they would have preferred just a bearing change and to maintain the high Combat engine output for better sales against competition. Reality forced them to both make the engine stronger and to detune.

Glen

worntorn said:

It would be nice if it was that easy but it sure didn't work out that way for Norton. They made the bearing change, plus strengthened cases plus detuned. Even with all of that there were still too many failures hence the further strengthening of cases with the MK 3. The MK3 also got a stronger crank, yet tuning was left at the lower post Combat level, while competitor bikes were adding more power every year.
I'm sure with their financial situation they would have preferred just a bearing change and to maintain the high Combat engine output for better sales against competition. Reality forced them to both make the engine stronger and to detune.

Glen

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Commercial considerations and reliability as related to factory success for the Commando are in the past. Preservation is all that keeps them going today. The motivation for that comes from what the owner perceives as desirable attributes. I like my combat in full tune and I'm sticking with it.

That's a personal choice and I have no argument there whatsoever. It's your bike to do with as you wish, whether you want to keep it stock, detune or modify it for more power. Some even say the earlier bearings are fine. I guess if one is absolutely adamant about keeping things stock, then run with those.


My point is only that the OP , who had good knowledge of the Combat issues, asked here for confirmation and got at least some condemnation. This was off base since his ideas were sound and based on real problems that occurred.

Glen

Why over rev them anyway,a Combat cam will give heaps more power and rev faster between 4,000 rpms to well into redline but why would you want to go past redline, doing that to any motor will kill it in the long run, use the power where its needed mid range, the bikes do have gears so no need to run them into redline all the time, the hot cams will rev freely way past redline if you let them, but I can't see any resond to do so, my hot Norton revs very fast and have surprised a lot of moben bikes with its power range but I know when to change gears, I never look at the tacko I just know when to up shift, to get long life out of my motor is whats its all about, on my Triumph the rev limiter will kick in before the big bang, its all about saving your motor, where not running on a race track, on the road the fun police will get you, so just back off before the big bangis all that is needed.

Ashley

One way to help preserve a Combat engined bike, is to fit a modern ignition system that will keep proper timing rather than the original points that lost timing due to vibration and wear.

Mine ran through the bearings at around 5000 miles, then dropped a valve when I was on a trip in Israel, fixing that was fun.

Re: Combat to standard-update

After all the good advice and input freely given to me by you all, I felt it was about time I reverted with an update on what I did with the engine, and the reasoning behind it.
Due to the badly worn crank, ground to -.030", the owner and I decided that a standard cam to replace the worn out 2S cam was the way to go. Revs are what kills these engines IMHO. The con rods needed a lot of work to polish out all nicks and dings too, which underlined the decision. They are strong items but no need to stress them more than necessary when you don't know what abuse they have been subjected to before.
The cam bushing in the TS case was replaced with the twin Mk3 items, along with a new standard cam and thrust washer. The original system was rather strange I thought, no wonder they re-designed it.
I struggled to get a .060" seat width on one of the exhausts, due to the inside angle being close to 90 deg, I had to settle for around .056" but it should work OK I guess. I've used less on Triumph engines before, but these days I like to use .080" exhaust and .060" inlet seats for longevity. I'm sure opinions vary...
I bought a .060" copper head gasket from Jim Schmidt to reduce CR to a theoretical 9.3:1
There were a number of other issues with the engine too, indicating much abuse and poor mechanics, so no need to stress it more than strictly necessary.

The reason for opening this engine in the first place was the crank case failing to return its oil at higher revs. The seller had claimed the rest of the engine was in perfect condition, the only problem being the scavenge issue. Oh well...
On opening the cases I found the cast in weir having been ground away, but the oil pick up hole had not been re-located. I wouldn't have removed the weir if it had been there I think, but I re-located the return hole to the rear as per common practice as I understand it.
The DPO had blocked the original breather and installed a new one in the timing case, which is or was a common practice it seems. I suggested buying a CNW reed valve breather but the owner thought the cost (add Exchange rate, P&P, 25% VAT...) was excessive, so I made up a disc type valve held in place by a light spring to fit inside the original breather housing. The timing case breather is sealed off. It works really well, and helps clear the sump when it's been sitting for a week. Testing with a finger on the timing case breather it puffs hard at first then less and less until you actually feel a negative pressure.
It may not be as effective as the CNW breather but it seems to work well enough to keep the engine exterior dry, and the sumping problem seems to be gone. Mission accomplished.

As I reported earlier, the inlet ports tapers from 32mm down to 29 at the guides, and it seems to suit the standard cam well, with good response from low revs. The mid range is where it shines though, very impressive, and it revs out pretty good too. The Mk2 850 (single Mikuni 34mm) in my shop pulls harder from low revs but by 4000 rpm the 750 has overtaken it and from there it's no contest. The 750 delivery is much more akin to the Triumphs I'm used to and I much prefer it over the 850. A dual Concentric set up would probably wake it up a bit, but the owner (same guy) is happy with the Mik he says.
Incidentally, my wife's Triumph Street Twin develops its power in a very similar way to the 850, and again I preferred the power delivery of the (tuned by me) 865 Bonnie SE she had before. Horses for courses, like the 850 Commando, with the ST you short shift and revel in the abundant torque. If you're old, but I'm only 60.

Sounds well done. Congrats on getting another one back on the road.

Ken

Some good choices there.

Re the 850/750 thing. The small single carb kills the 850 after 4 k. With twin carbs the 850 will rev right thru the redline very quickly if you aren't on it. Try a twin carbed 850 with open exhausts and a little compression bump, stock cam. They are quite healthy and strong right thru the Rev range .

Glen