Matt's Combat Commando overhaul by GrandPaul

hudson29 said:

When I bought my '72 the previous owner said something about it having been a Combat at one time.

What are the telltale marks of a combat? Were they serialized?

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The cylinder head (& carbs), pistons and camshaft were different between "standard" specification and Combat engines (and I believe the later '72 standard engines also got the stronger Combat pistons) and Combat engines usually had black cylinder barrels.

Any Commando engine within the approximate serial number range 200000 (officially from 200976) - 211110 could potentially have been built to Combat spec. originally-as the factory apparently produced more Combats than standard tune engines during that period.

My Commando is 204687, made shortly before the Superblends according to the Norton Owners Club book of Commando tips. It was a bitsa when purchased 20 years ago with the glass Roadster tank and the little headlight. The PO was a BSA guy who knew little about Nortons and said it had been a Combat. I installed 9 to 1 pistons to ease running on the poor quality modern fuel. I'm not sure about the head, cam & carbs. Phil Radford reworked the breather when he installed the Superblends and the motor now breathes out of the timing chest with a brake valve. How do I check the head? Is that C GrandPaul photo'd visible on an assembled engine?

Vintage Paul, curious what the beast started life as . . .

The "C" stamp is partially covered by some types of head steadies, and they aren't always stamped perfectly, so some are less visible than others.

One (not flawless) indicator of a Combat engine is the tighter fin spacing between the head & cylinders due to the skimming to raise compression.

Coming along nicely Paul, will you be drilling any extra holes in the Crankcase for breathing.
After machining the case for the oil pick up I now have a lovely stream, of oil.
I fitted your reed valve horizontal, but I think it may be better vertical, as far as hose location.



Hudson29,... When you say that you fitted 9-1 pistons, you may have upped the compression ratio
instead of lowering as you believe. The combat engines had a Planed head (.040), not high compression pistons.
Or did you remove some off the top of the pistons to lower the comp ratio.
If you do have a Combat head, and higher comp pistons, plus the added compression of the rebore,
you might be a little too high comp for an old 72 motor.
Just a thought.. AC.

grandpaul said:

As far as I know, Combats are 1972 serial numbers only, and fall within a certain range.

The lipped oil wier at the back of the crankcase sump is something that should be removed. I believe that was the only year for that lip, and not sure what it's intended purpose was.

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All norton twins 1949-1975 have the "weir".
IMO this removal mod is one of the most inadvised mods being popularized by norton web groupies...this old brit/INOA tech digest mod still has a very fatal flaw....I would NEVER do this to my customers 72 or 73 "combat breather" engine.
yes combat "state of tune" was only in 72 but NOT all were combat.

dave
52 M7
63 atlas
68 dunstall atlas
202206 with my own oil return mod....
75 MKIII

The rear breather will be blanked off, and it will breathe very adequately from a timing chest mounted reed valve. I've found that combo to work quite well with no drawbacks whatsoever.

I will admit, my combo has not been used in racing applications, only on the street.

Dyno Dave - What do you recommend for the '72 crankcases then? Is the fatal flaw with the mod that the oil pump scavenge can pick up big pieces of swarf floating around the engine?

All Cdo's can suck up metal parts into the sump size of pump and Combats are most famous for this because they used cam thrust washers with fracture prone fracturing just the right size to be inhaled into pump. Combat's suck up oil near front of sump so when held at high rpms as they are eager to do the oil piles up in rear to wet sump until the secondary breather return hose kicks in to return way more oil volume that the pump can. Its not the inside flange that is an issue just the oil drain hole is often moved to rear.

Unless the 9:1 pistons stick up above barrel .050" then likely you did decrease CR. CHO is stamped on early and maybe all Combats.
Combats also killed Norton so they quite making them instead of fixing them like we can/do.

My Trixie Combat is all around real one and so far no detonation in hi summer temps loaded and climbing steeps in 4th or taking off with some spunk on non ethanol no lead 87 octane, and she is definitely miss tuned on the lean side so much will not pull the ton before running out of main jet and also surges strongly on throttle releases as mixture richens up some. But I'm not holding WOT very much or long to press my luck further. E-gas has a bit higher octane and cooling combustion so will be safe to run that as long as not stored in tank long.

The big mistake people make in detuning a Combat is to switch to standard cam and not lower the CR. Bing Bang Boom, as the 2S cam has enough over lap to lower effective CR until the rev's build past most detonation build up temp/pressure. My Combat's did not really kick in the 2S cam until upper 6000s then felt like another piston added but also nearing destructive red line, ugh. They held their own against the low end grunt of well tuned 850's in 2nd grunting out of apexes in twisted Mt roads but left them when the road opened up and speeds over 80 mph.

Part of the problem for me is my poor memory. I built this Norton in the mid nineties, a project that wound up taking 8 years of off and on work as other projects took priority at times. Happily, the motor ran very sweetly on the premium pump gas that was available in 1998 when the bike was being run. It ran 700 miles without issues before it was laid up for the rear drum/hub issues detailed on other threads. If it was really a Combat, I'm not sure what Combat bits are on it now. Now I'd like to figure out what is there and what damage I may have done following the Norton Tech recommendations and other advice.

Vintage Paul, trying to figure out where we're at . . .

hudson29 said:

Part of the problem for me is my poor memory. I built this Norton in the mid nineties, a project that wound up taking 8 years of off and on work as other projects took priority at times. Happily, the motor ran very sweetly on the premium pump gas that was available in 1998 when the bike was being run. It ran 700 miles without issues before it was laid up for the rear drum/hub issues detailed on other threads. If it was really a Combat, I'm not sure what Combat bits are on it now. Now I'd like to figure out what is there and what damage I may have done following the Norton Tech recommendations and other advice.

Vintage Paul, trying to figure out where we're at . . .

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If the front drum is original then it's not a Combat.

There is a lot of confusion about "Combat" pistons. According to the Norton service notes, pistons P/N 063338/063339 are identical to 1971 and earlier Commando pistons (P/N 062459/062462) with the exception of deeper valve cutaways. These are the dreaded "slotted" pistons, which were found in earlier Commandos as well, but survived much better with the lower compression head and milder cam. The 063348/063349 were introduced later in the model run and had the slots in the skirt, but not in the ring land. None of these pistons were exclusive to the "Combat" models, but were used for both high and low compression models. Putting "Combat" pistons in your earlier 750 will actually lower the compression slightly. (Although probably not any significant amount).

Hepolite did make 750 pistons without the slots in the skirt (like the 850 piston), but I'm not sure if they ever were given a Norton part number. And were never used in a production bike, apparently only sold as replacements.

hudson29 said:

When I bought my '72 the previous owner said something about it having been a Combat at one time. I overhauled it completely down to replacing the ball bearings with Superblends and even replacing the crankshaft which turned out to be cracked. What are the telltale marks of a combat? Were they serialized?

Vintage Paul

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Point of fact, Combat engines originally had roller main bearings on both sides. The standard Commando engine has timing side with a ball bearing and the drive side a roller. Articles on this said the increased crankshaft flex due to increase engine performance caused the edge of the rollers to dig into the races with resulting castastrophic failure. The Superblend solution provides a roller with some taper that can tolerate the flex. Not really a fix, but a good bandage.

illf8ed said:

Point of fact, Combat engines originally had roller main bearings on both sides. The standard Commando engine has timing side with a ball bearing and the drive side a roller.

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Well, according to Norton Service Release number 68, Commando engines were fitted with two roller main bearings from October 1971, so all Commando engines probably woud have had them before Combat production commenced.

"CATEGORY OF RELEASE: (as over-leaf) NO.68

NATURE OF RELEASE: Revised main bearings
MODELS AFFECTED: All years and models of Commando.
DISTRIBUTION: Worldwide (General)


EXPLANATION:
We are introducing a new roller main bearing
arrangement in production almost immediately.
The fitting of this bearing arrangement reduces
overall width between the main bearings and
increases timing side main bearing journal
diameter."

For service purposes we shall in future supply
only the crankshaft of the latest production size,
part number 063106. To fit this to any Commando
engine, the main bearings, part number 063114, must
be used in conjunction. The new bearings are 2dot
single lip roller bearings and should be assembled
with the lip into the crankcase in both cases."

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Also, only the 063114 roller bearings (2) are listed for 1972.
(The 063114 bearings were not the Superblends)

The bike was a bitsa with a Roadster tank & 5' headlight, a configuration that I can't recall seeing Norton offer. I do not know if the TLS brake was original or not. BTW, that brake worked pretty good until I "fixed" it . . .

The bearings were almost certainly rollers, I just mistyped balls. :wink:

Vintage Paul

batrider said:

Dyno Dave - What do you recommend for the '72 crankcases then? Is the fatal flaw with the mod that the oil pump scavenge can pick up big pieces of swarf floating around the engine?

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http://www.britbike.com/forums/ubbthrea ... Post262795

Nortons final 750's in 72&73 with combat breather have no return sump filter and also IIRC the very early model 7's.
Yes it will pick up trash as large as the passage leading up to the pump. This is an oil pump/engine killer and possibly try and eat combat thrust washer tabs or (in my case) a piece of distructing valve spring. With my mod it would have survived...

Thanks Dave -

Unfortunately the pictures of your mod are no longer there in the thread you listed on BritBike. Can you dig them out and repost?

GP has started a separate thread on this topic so I will exit this one.

Hope to catch up with you at the Empire rally.

Should have specified "bottom to top"! They finished the HEAD first, then the cylinders, cases still awaiting cleanup & cam bushing replacement.

Head got boiled, new guides, valve seats matched to new valves and lapped in-



Cylinders got stripped, boiled, and bored to match new pistons with .004 clearance-



Nice coat of Rust-O-Leum gloss black, now baking in the Texas sun-



Cases should be ready Monday, and assembly will start

Got the tranny torn apart today. Not too filthy on the outside-



Typical level of grunt in the outer cover; note the oversize gasket (excess inner edge) thisd can be problematic as the excess edges deteriorate and get into the oil-



Nothing overly exciting here-

Hmmmmmmm... Lots of bushing brass floating around in here-



Not good, but not the end of the world...



Just the end of a couple of bushings-

The inner bushings look okay-



Lots of fine shavings and brass chunks-



The rest of the disassembly was uneventful, parts all washed up.