Cracked Crankcases !

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Jan 10, 2006

I really search through the forum before posting anything, so I hope I'm not asking something that's been asked before, but I've broken both crankcases in a very short time I've thought maybe I havent searched very good:

The timing side crankcase cracked behind the tacho drive loosing oil like hell. I cured it with Nural 50 and worked amazingly!

The bike was going great this last months and I made an oil change this last weekend in order to take it for a bussiness trip to Lérida (up north east of Spain) which is about 200 Miles from where I live.
I thought: well maybe I'll manage to do it on my Norton so this could be a nice journey instead of a boring work trip.

The engine was still into the break in period (1000 Miles from rebuilt), so I rode it at between 65-80 mph and it seemed quite nice.
When I had ridden for about 150 Miles, an oil fountain was coming out through the front of the primary side by a crack where I could insert my fingernail. This crack starts at one of the alternator bolts and finishes at the center gasket between both crankcases.

Both crankcases broken in less than 1200 Miles...

Have I done anything wrong during the rebuild?

Is there a real way to repair and check the crankcases before going for another rebuild?

I have another complete engine which looks awesome, so I'll try to make one engine with the best parts of each one of the two engines and I'll rebuild a spare engine from the worst ones afterwards.

Any advices about the cases before starting again from zero?

Thanks a lot for the help!
Hello sparkplug,

Are those the same crankcases that were fitted when your engine blew up before?
Maybe they suffered some damage that wasn't spotted, or were over-stressed in some way when it blew? Cracks on the primary side can happen, but I've not heard of the cases cracking on the timing side before, maybe other members have? I would not bother trying to repair them.
Hi, Les:

Yes they're the same old ones. Last time the engine blew up the timing side con rod snapped and kept turning into the cases, so I thought this side crack could have happened because of this, but the primary side crack is horizontal, long, deep, wide, scary and from outside there's no signs of previous hits or damage.

I have the other cranckcases which look very good so I'll use them for this next rebuild, but now I'm afraid they could crack too after all the work.
If you say that's not common, and could be just because of the last break down, then I'll throw them away and do it all over again with the "new" crankcases.

By the way, this complete engine I've bought plus another spare cylinders and and RH10 head have cost me 680€!! :D
I got them from a man called Juan who has told me he tuned Commandos back in the 70's in Barcelona (when Norton won the 24 hours of Montjuic) and who met Gus Kuhn, Barry Sheene and other legends. He told me amazing stories (don't really know if all of them true, lol)
I re-read your description of what happened when the engine originally blew:

sparkplug said:
After dismantling the engine, what I've found is that a con rod was broken into peaces, whipping the whole thing inside the crankcases and breaking part of the barrels, valve heads, etc, etc... before the rear wheel stopped turning.

I can only guess that the left-hand case suffered some sort of stress damage due to the rod breaking which has resulted in the crack, but it's difficult to say for sure? And it's normally the 750 cases that are more prone to cracking around the drive side bearing area as the 850 cases are supposed to be stronger.
That's bad luck Sparkplug. When I got my bike in 98 the timing case had been repaired close to where you describe, after a right had rod had broken. That was 72 Combat motor which later blew when a 'new' rod bolt broke. The drive side damage is unusual. Most damage here is horizontal.
As Les says you cannot successfully repair these and looking at your original failure description there would be enormous forces from the broken parts trying to escape the cases. Usually that does happen and you find a nice big hole at the front, rear or bottom of the cases like I did!
The crack must have been there when you rebuilt.
I would check the 'new' cases for any cracks, easy with Magnaflux on steel and iron but I'm not sure what's available for aluminum. What year are they?
Buena suerte.......
Yipes! time for new cases for sure after a snapped connecting rod whomped them like that.
As you, mates say I'll check for any cracks on the "new" ones (Jason's recipee seems quite logical) even if they look gorgeous.
Definitely I'll throw the old ones away.
What about the bearings and bushes? Should I replace all of them for new ones? Will the Superblend bearings stand all the heat in the oven to take them off and still be reusable?
How can I tell if I should rebore the crankshaft?
How about the cilynders?

As I said before they really look great, but that's what it seems to me... someone who has rebuild the engine with cracked crankcases :?
Main bearings can be removed and replaced (How hot was your oven ?) but there is the additional complication here that they have not been well supported by your failing crankcases.

If it was me, I'd replace the main bearings. I once did it with a bearing that I dropped in crooked (burned my hand, let go of the bearing and it jammed. It wouldn't shock out so I had to hit it :oops: ) I didn't feel that I could 100% trust it and it's a lot of work (as you know too well).

Dependent on useage, cam bushes should probably be OK.

You will need to measure the crank journals with a micrometer, paying attention to ovality as well. If you're not confident then any crankshaft reconditioner can do this for you. If you need a grind, pay VERY careful atttention to the warning and drawings in the workshop manual regarding radius. They didn't write it for fun !

Cylinders should be measured with an internal micrometer but you can insert a ring and measure the gap at various points. To be honest, if there isn't too much of a wear lip they should be OK unless you're intending to delete the base gasket. Again, if you take the cylinder with the Workshop manual to a reborer, they should be able to tell you if it is within spec.
In May the timing side main started to turn in the case, so with the poor weather I decided to check the drive side main and loctite them both in.

Everything went smoothly until I've tried to fit the drive side case over the crank. The outer race seems to have shrank as it has to be softly tapped over the inner. It rotates freely but will not slide. It can only be the loctite filling the gap between the bearing and the expanded case bore. I must has let it soak at too high a temp.

The question is, should I run it as when the case gets hot the bearing should expand relaxing its grip. Or bite the bullet and some how get it out and start again ?

Cash, I would venture to suggest that this is fairly normal with loctited and / or new main bearings.

I have noted several times on my engines and others that the end-float seems to disappear as the motor cools after assembly and have put this down to the ground finish on the tracks offering some resistance to lateral movement. The first time it happened, I stripped everything again and re-checked the end float which was still correct. The crank spun freely when grasped by the rods - no resistance at all.

As you say, with warm (not even hot) cases, the movement comes back. It sounds a bit horrible to have used up the internal clearances in the bearing (if that's what it is) but I have never seen any main bearing problems.

If you do start again, what will you do differently ? You still need the loctite (which is a recommended thing anyway) and if the cases are warm enough for the bearing to drop in, they are warm enough to cook the loctite.
The drive side didn't show any signs of spinning and normally I wouldn't have used Loctite. It just seems a little tighter than I've had before.
79x100 you're right the only real option now is to run it.

I used high temp and grip Loctite 648.

Thanks chaps,

After dismantling the engine this sunday, I've found that the primary case was cracked, but behind it, the crankcase was seriously cracked horizontaly from side to side. It seemed to begin at the main bearing and then all the rest. Could it have any other reason but the first time it blew?

Looks really scary. My feeling is that the crankshaft clould have fallen on the road if I had kept on going.

I have never dismantled a crankshaft myself, but now I'm going to...

Now I can't take off the camshaft sproket nut and the camshaft worm gear nut (left threaded) I've tried everything, but I've fallen in desperation after so many tries. Any advices about this ones? I've already prepared a cutaway timing side cover for this stuff. I've used heat, penetranting oil, strength (well, as much as I can) but nothing seems to work.

I've also found that the boyer contact breakers wires have melt. Why?
Maybe the timing wasn't right?

My pistons are NEW (well 1200miles) but they look perfect.
I'd like to change the old barrells because when I broke the con rod last year, it broke the cylinder skirt, so I put new liners on them and it's been working like that fine, but now I have the "new" cylinders.
Could I reuse the pistons? This cylinders look good but they're not perfect.
Does sound like the damage was there from the first blowup. This is what happens when a rod bolt breaks......rod, Maney aluminum barrels, piston bent 2S cam and both cases
Cracked Crankcases !
For a better pic ... Oh0=_l.jpg
That bent the left hand frame rail forward by 7mm and also bent the front ISO bolt (4000rpm and 110km/h).
For the cam just use steady counter clock pressure on this RH thread with a long (0.7M?) extension. Alternatively some people use an air impact tool to 'shock' the nut free. For the LH crank worm gear again use a long bar and steady clockwise pressure should free it. You will have to lock the flywheel with a bar through the balance holes but I guess you know that.
Pistons should be OK as long as there is no obvious damage. Just make sure there is specified clearance in the new cylinders especially front to back. Not sure exactly what this is but someone will advise.
If you are disassembling the crank use new studs and threadlock. Also, on some cranks there is supposed to be an internal sharp edge (start of potential crack) on the drive side crank web inline with the rod journal.
I have never seen that but apparently it is present on some cranks. It is worth having the crank crack tested.
Melted Boyer wires? that would affect ignition function totally. I don't understand how it would even run. The normal current through these wires would be milliamps only (0.1A with 13v and 130 ohms total boyer pickup coils resistance). Does it look like external heat or an internal short circuit. I cannot think of any condition that would melt the insulation externally other than a fire or more than 105C that the insulation should tolerate. If it was short circuit the high amps would blow the box I guess. Where did the wires melt? Any signs of contact with the cases? You mentioned a crack near the tach drive. Maybe the crack had damaged the insulation close to where the wires enter the timing cover?
What year are the new cases? Because now you have the opportunity to do any oiling and breather modifications if they are 1972 Combat cases with the rear breather. Just a thought. Best of luck with the rebuild.
Hi, Keith!

That engine looks nasty...!

Thanks for your help. The cases are 850 74' same as the rest of the bike.
They look pretty good (I guess...)

I've locked the conrods with a bar through the holes and against the cases (well protected), then tried to unlock the nuts, but... I'll try with a longer bar or impact tool as you point.

When you talk about the sharp edge on the crank as a start of potential crack, do you mean a crankshaft crack?

The contact breaker wires (the copper) is still there, but all the plastic parts of the cables were destroyed, but not the plastic on the wires going up to the coils, which seemed ok. :shock:

Thanks a bunch for the reply!
It was nasty! It had suffered a minor blowup with the previous owner an dthe cases were not good but it seemed a new rod bolt let go. Dunstall tuning notes recommend removing machining marks from the narrow diameter and mine did appear to break there.
If those wires are only overheated locally then it looks like they were touching the cases, so a short circuit is probable cause.
I cannot find the reference to the crank sharp edge. It was on the Britbike site and a search does not reveal the messages. The forum member was talking about a sharp edge (excessively small radius) inside the drive side crank web. This aligned with the left side of the rod journal and could lead to a fracture. I think he was referring to 750 cranks, it may not be on the 850 although I'd guess they are the same parts if not Mk3 which had the longer shaft. Sorry, hope I haven't worried you more!
I found the reference, seems it was on a '99' not a Commando. Apologies.
To all/ anyone thinking of having their Norton cranks hardened or anyone about to strip them..
Have the D.s. half crack tested paying particular atention to the big ends outer 90 thou radius area. You could well find cracking taking place.....thanks to Norton introducing a stress increasing raiser into the crank half roughly directly beneath the outer 90 thou stress reducing radius when they cleaned it out with a drill or something that left a sharp corner/strss raiser.
I had a 99 crank fail, at this point in the very distant past..
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