Crack in crankcase

Got a '71 Commando that has a faint crack line at rear of crankcase where the g/box cradle bolts on (top bolt). Runs vertical alongside the hole and horizontal for about an inch along the top of the boss. Took engine to a recommended aluminium welder who said he could weld it but prefer to have engine stripped. Rest of bike is still in bits so am reluctant to do this in case it never gets finished. Guy said he'd drill holes 1/2 way into case and then weld. So was wondering if this is a bad idea, am worried about crankcase distorting.

Strip her down. The crack will only grow at that high load point. A good welder will jig the part prior to welding to prevent warping. If you decide not to proceed, I'll buy it from you. 8) BC

Ugh, that's the area hi rpm out of their minds racers crack their old 750 cases. Its structural so no JBW there. Can have just the crack welded and behave your self below much redline use or have it beefed up with a plate that connects the two upper rear bolt flanges. Of course this will distort cases more than just a weld fill. Fix is beating welds with pointed slag hammer back in in shape to seal. If 2 plates put on, one on each case half then there's room for an 11th clamp bolt right where all cases are leak prone. I use a pancake heat tank top propane heater to get the whole case and mounted barrel to oil smoking hot. Heavy lineman leather gloves are handy for helping.

All 750s crack here. My 72 did and I left it for over a year without mishap. I had it welded when the crankshaft snapped and simply had to strip engine down. Crack is mentioned in the NOC service notes. I had a local Norton specialist do the weld repair and he welded ribs up both sides from the top bolt and said that's what the sidecar motocrossers did. It also cured that leak from the crankcase joint at the back near the base flange that 750s are prone to.

This is the sort of rib reinforcement mentioned by hobot and myjota. In this particular case, a race engine with belt drive and no alternator, a plate has also been welded to reinforce the main bearing area. There was considerable distortion of the cases from this amount of welding, requiring some machining afterwards. For street use, i'd recommend just having the crack welded, with no reinforcing, to keep the distortion to a minimum. I would definitely tear the engine down and clean the cases thorouoghly before welding. When I had cases welded like this, I took them to the welder with both halves bolted together solidly, but still had some small distortion at the joint between case halves, near the welded area. Using Locktite's Master Gasket sealer on the joint worked just fine, with no leaks or other issues.



Ken

I just had the top fillets done and when ground back looked like they came that way. Pre heating cases helps prevent distortion, but any good welder should know this. I would find someone used to doing this type of repair on British castings as they sometimes bubble out engine oil to contaminate welds.

ITs takes lots of acetylene and a long time to try to get a hunk of Al the size of both cases hot enough to matter to welding temps. Give some thought to that, like maybe 3-4 little propane torches or remember my tank heaters remark. I did mine to handle over redline rates and worked a treat. On factory Combat I'd just fix enough to get about normally.

myjota said:

I just had the top fillets done and when ground back looked like they came that way. Pre heating cases helps prevent distortion, but any good welder should know this. I would find someone used to doing this type of repair on British castings as they sometimes bubble out engine oil to contaminate welds.

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"Sometimes bubble out engine oil"? Are you sure you don't mean always bubble out engine oil? The welder I took my cases to was very experienced in this sort of repair. He was the owner of a large welding business that did a lot of work for the local oil fields, as well as repairing aluminum blocks and heads for local racers. He was also the welding instructor at our local community college. He used a large industrial oven for preheat, but when the welding was as extensive as we did for the race cases, some distortion was unavoidable. If you just need to weld a small crack and aren't doing any reinforcing, you shouldn't have much trouble with it.

Ken

There is Only One Way to get oil out of Al pore and that is to heat Al over the oil's flash point which generally means a good bit over 500'F so smokes like crazy for while.

note this version has hoses to allow dismounting to hand aim all around.

When my 750 crankcase cracked just by the rear mounting point, I stripped it down and gave it to Richard Peckett ( the P of P&M Motorcycles).
His cure for the oil bubbling out of a casting that's been immersed in oil for 30 years was to take it to the local chroming works and leave it in their trichroloethylene bath for two weeks.

He had it bolted together while he welded it. There's no distortion that I can tell.