Cracked crankcase.

Just got in from the workshop & am consoling myself with a pot of tea. I went out for another good thrash yesterday. 235 miles of bliss on my lovely 850 mk 2a. Just checked in the primary chain case for any signs of a leaking crank seal and found about 200ml of engine oil. Very disappointing as I only changed the seal a week ago. Upon removing the belt & engine pulley the reason became clear. A hairline crack has appeared at approx. 8 o'clock from the bearing right over to the chaincase.
Question 1.
Am I asking too much of this engine? I get onto the motorway & cruise at 5000 to 5500rpm, approx. 90 mph on current gearing, for between 50 & 80 miles. This gets me to some fantastic & fairly traffic free roads. The bike then gets thrashed up & down the gearbox but hardly ever revved beyond 6500.
Question 2.
Do I buy a new pair of Norton cases or a pair of Maney cases. It seems to me that which ever I could do with the drive side not being machined away for the primary chaincase.
Question 3.
As I love the look of the primary cases & most definately want to keep them, has anyone done this? Regards, Martyn.

Ugh, yes you've run the 850 into metal fatique w/o prepping engine like a racer inside and out. Others will soon fill you in on how to deal with cases but I alert ya that just seeing fracture on outside don't mean its the only one lurking below surfaces so if repairing this one like racers do, better fix the stress risers on the cradle mount flanges and hi recommend going 7/16" bolts. Expect distortions galore if welding very much to beat back to sealing shape, ugh. Next thing I'd think about is ligher smaller flywheel lighter piston kit then run it up to 6800+ often and gear up some to go even faster.

I get onto the motorway & cruise at 5000 to 5500rpm, approx. 90 mph on current gearing, for between 50 & 80 miles. This gets me to some fantastic & fairly traffic free roads. The bike then gets thrashed up & down the gearbox but hardly ever revved beyond 6500.

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Hi matchless,

I am sure when others see this post you will get some good answers as to what crankcase repair or replacement is possible.

The only comment I have is that you may have expected a bit too much from your long stroke parallel twin.

Holding the motor for some 50-80 miles continuously at over 5000rpm...that crank be whipping good

the manual says the maximum continuous rpm is some 5800, and even that would scare me to back off

since maximum hp and torque are produced well under 6500rpm why would you rev it up there?

ok, maybe I am wrong, but there is a reason why the Commando cranks flex with two bearings,
and ya gotta be close to the theoretical maximum piston speed of 4200 feet per minute doing that

Man, that is tough. You DEFINITELY want to know if there's something going stray inside those cases...

Peter Turner discovered this crack at the National last year and Lucky for him someone there sold him matched 828 cases cheap (out of pity ?). Hairline. Drive side.

From memory the 4000 feet/minute figure relates more to rjng & bore wear. On a Norton twin with 89mm stroke this figure would be reached at 6840 rpm hence the 6800 redline.

The 114 crankshaft rps would be a concern perhaps given the stress is squared with the revolution increase.

That is a fairly common place for Nortons to crack when they are being raced.

It usually only happens with high mileage and hard use on the street.

I have several sets of cracked cases from my roadracing days. Stock cases would last about a season revving to 8500 or so. Weld reinforced cases would last about the same. I have stock cases that have had the entire drive side cut out and replaced with a 3/4 inch thick aluminum and in about a year they just cracked around the outside of the thick piece. The damage from welding heat takes away about as much strength as the reinforcement adds. Adding weight to the crank will make the cases last a little longer.

The only good way to avoid cracks when racing is to use Maney cases. They are twice as thick there. If your running a belt drive you can machine the inner primary case away where it mounts to the engine. Then you have to make a new way to mount it and the alternator but it's not too tough to do.

How many miles are on your bike. Stock replacements should last as long as the originals did and longer if you get your gearing a bit higher for those 90 mph cruises.

Jim [stock cases, heavy crank, and around 100,000 miles of abuse on the cases. I expect to see cracks any day now.]

comnoz said:

Adding weight to the crank will make the cases last a little longer.

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Would you like to explain this a little further ?

Matchless said:

On a Norton twin with 89mm stroke this figure would be reached at 6840 rpm hence the 6800 redline.

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The redline is quoted as 7000 rpm - and marked on the tacho as such.
For short bursts....

The Workshop Manual also quotes that Continuous Cruise Maximum is 5800 rpm, for the 850 Models.

The Combat 750 models made max claimed power at 6800 rpm, not too far off redline.
Easy to go over, in a fit of exuberance...

Rohan said:

comnoz said:

Adding weight to the crank will make the cases last a little longer.

Click to expand...

Would you like to explain this a little further ?

Click to expand...

The larger mass dampens the out of balance forces realized at the main journals. Think extreme here to visualize, say a one ton flywheel with say 2 lbs out of balance mass, what's the net out of balance reaction force at the main journals. Now consider a fly weight flywheel with 2 lbs out of balance mass. The light flywheel would hammer the mains whereas a large mass flywheel would dampen the forces as the out of balance mass must now also accelerate the mass of the flywheel.

Dances with Shrapnel said:

Rohan said:

comnoz said:

Adding weight to the crank will make the cases last a little longer.

Click to expand...

Would you like to explain this a little further ?

Click to expand...

The larger mass dampens the out of balance forces realized at the main journals. Think extreme here to visualize, say a one ton flywheel with say 2 lbs out of balance mass, what's the net out of balance reaction force at the main journals. Now consider a fly weight flywheel with 2 lbs out of balance mass. The light flywheel would hammer the mains whereas a large mass flywheel would dampen the forces as the out of balance mass must now also accelerate the mass of the flywheel.

Click to expand...

Thanks DWS. Right on the nose. Jim

Why rev the piss out of an engine that does not produce any additional power above 5200 rpm? I guess I am trying to understand why you are riding a long stroke twin whose design dates from the 1940's like a modern Japanese multi.

Matchless said:

Just got in from the workshop & am consoling myself with a pot of tea. I went out for another good thrash yesterday. 235 miles of bliss on my lovely 850 mk 2a. Just checked in the primary chain case for any signs of a leaking crank seal and found about 200ml of engine oil. Very disappointing as I only changed the seal a week ago. Upon removing the belt & engine pulley the reason became clear. A hairline crack has appeared at approx. 8 o'clock from the bearing right over to the chaincase.
Question 1.
Am I asking too much of this engine? I get onto the motorway & cruise at 5000 to 5500rpm, approx. 90 mph on current gearing, for between 50 & 80 miles. This gets me to some fantastic & fairly traffic free roads. The bike then gets thrashed up & down the gearbox but hardly ever revved beyond 6500.
Question 2.
Do I buy a new pair of Norton cases or a pair of Maney cases. It seems to me that which ever I could do with the drive side not being machined away for the primary chaincase.
Question 3.
As I love the look of the primary cases & most definately want to keep them, has anyone done this? Regards, Martyn.

Click to expand...

Why not ride like the wind even if it uses up engines they can't feel a thing like we do then gone. 5000-5500 should be about the best zone to run in with oil surfing and ring sealing and efficient mixture velocity and tauntness in drive train for least slack slapping and good wind speed to blow off heat. Btw piston velocity does matter as stated for ring friction but its the jerk down acceleration to get to that speed that pulls pistons apart and slams ring off their lands and long strokes jerk more that short strokes so their main rpm advantage plus less travel lenght and friction. Ah if only cold weld could work here.

Ligther piston & rods sort of act like crank is heavier and also needs less counter weigh so if was me I'd try the face plate case fix with JMS piston/rod kit, make sure crank is straight and rebalanced back down to normal, up fhe CR to 10 and fit 2S cam, gear 22T then use 2nd for strafing runs into front lifting zoom to upper 80's then snick 3rd for getting to the ton then 4th to ease back and enjoy long legged 90's mph a bit below 5000 all day long everyday with some real glee time over 120 now and then.

Btw thanks for reporting there was a reference to Combat max power @ 6800, as both mine felt like a 3rd piston kicks in then so that ain't where I want to let off, ie: just when the going starts lifting the front - but I know to only do that in rare occasions early in break in to make sure of my job, on a plain Jane factory Commando, but with reenforced cases [UGH} steel fly wheel dry friction coating and cyrogenics like Peel, I'd run in and out of red zone WOT much as I could for 60-100 miles a leg then return going 90 @ 5000's, a seeming lope, watching the temp gages return to normal low stress state.

Likely your tranny bushes are a bit long in tooth too.

Jim,
Thanks for chaincase info. I think Maney crankcases would be the best option in this case. I would like to use one of his cranks also, before mine breaks. What you are suggesting about heavier being better, does not seem to fit in with a Maney crank as these are a lot lighter than even a stock crank. But having broken two Norton cranks over the years a stronger one would make sense. What do you think?
Also, please don't think I am a mechanically unsympathetic person who thrashes the crap out of engines just for the sake of it, as I can assure you all that this is not the (crank) case. The bike is in well sorted condition i.e matched rods & pistons, dynamically balanced crank, measured c.r of 9.1 to 1 etc etc. I have a modern Triumph triple that has taken far more thrashing for 104,000 miles of continental touring but it isn't half as satisfying to ride as the Commando.

Matchless said:

Jim,
Thanks for chaincase info. I think Maney crankcases would be the best option in this case. I would like to use one of his cranks also, before mine breaks.

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Matchless,

Steve Maney can provide cases with provision for drilling for stock primary and tach drive. No problem there.


Break your crankcases, your day is done. Go out, get a beer, and work on your next move.
Break your crank, your day is done. You won't be sitting in a bar having a beer later.

What gearing are you running ?

23T gives about 110 in third at 7.000 depending on tyre . can be a bit awkward in suberbia or on wet lawns tho .

A 4.00 19 rear isnt impossable on that with a good motor , if youve flatish bars and arnt dressed like an eskimo .

The solution is to reduce the stress with lightweight pistons.

If you're going to rev it like that you should use all the lightweight JS components. Heavy cases are great but they don't address the source of the problem which is vibration. Reduce it by 1/3. If you don't - your crank will be next. see below.

Your ride will also be much smoother.



And gear it up with a 23 tooth sprocket if you're going to cruize at 90 mph (I cruize at 80 on an Atlas).

Matchless said:

Jim,
Thanks for chaincase info. I think Maney crankcases would be the best option in this case. I would like to use one of his cranks also, before mine breaks. What you are suggesting about heavier being better, does not seem to fit in with a Maney crank as these are a lot lighter than even a stock crank. But having broken two Norton cranks over the years a stronger one would make sense. What do you think?
Also, please don't think I am a mechanically unsympathetic person who thrashes the crap out of engines just for the sake of it, as I can assure you all that this is not the (crank) case. The bike is in well sorted condition i.e matched rods & pistons, dynamically balanced crank, measured c.r of 9.1 to 1 etc etc. I have a modern Triumph triple that has taken far more thrashing for 104,000 miles of continental touring but it isn't half as satisfying to ride as the Commando.

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I have been through the lightweight crank bit before. They do make the engine sound good and will increase the midrange hit but overall I have had better results with a crank that weighs near stock or a little heavier.
In the dragrace from the launch to the first corner the light crank felt faster but the heavyweight crank always won. On the track I was able to turn better lap times with a heavier crank. They will exit the corners with less drama and are less likely to leave you sliding down the track on your butt. You can use more throttle on corner exits as the nervousness of a lightweight crank is more likely to break traction. The higher the state of tune the more a heavy crank helps.

The difference between a light weight crank and a heavy weight crank was substantial as far as stock case durability was concerned. Probably not much of a concern with Maney cases.

On the street a heavier crank sure makes the motor easier to start and live with.

Dave Atherton has had some billet cranks available that were a little heavier than stock. I don't know if he keeps any on the shelf as the currant fad for lightweight cranks has made them unpopular.

I have always ridden my Commando just as hard as any other bike. Put together right with a few mods they can be surprisingly durable. Jim

So, I assume the case flex's around the bearing bore at high RPM?