I’m hoping to tap into the fabrication and machining experience in this community.
I have a cast aluminum motorcycle crankcase with a hairline crack that originated from an over-tightened factory tapered pipe-thread plug. The crack radiates from the threaded boss.
The engine has not been run since the crack occurred. It’s been drained for several weeks and the area has been cleaned thoroughly. At this point the crack is very fine, and my goal is to prevent it from propagating.
I understand the conventional repair is to completely disassemble the engine, clean the casting, TIG weld it, and re-machine the boss. Unfortunately, that’s financially out of reach.
I’m not looking for someone to tell me to replace the engine. I’m trying to find an experienced machinist who thinks beyond factory procedures.
Specifically, I’m looking for someone with experience in:
- repairing cracked aluminum engine castings,
- metal stitching,
- machining out damaged bosses and installing custom inserts or bungs,
- or other mechanical repair methods for cast aluminum.
The motorcycle happens to be a 2010 Harley-Davidson Twin Cam, but I’m really looking for casting repair expertise, not Harley-specific knowledge.
I’m located just north of New York City and I’m willing to tow the motorcycle anywhere in the Northeast for the right person.
If anyone knows a machinist, engine builder, or retired craftsman with this type of experience, I’d be extremely grateful for a referral.
Thank you
chris
I have a cast aluminum motorcycle crankcase with a hairline crack that originated from an over-tightened factory tapered pipe-thread plug. The crack radiates from the threaded boss.
The root issue here is the tapered thread, which will always exert radial pressure the moment it is returned to this unsupported position in the case. The only way to support that thread in that exposed position is to first reinforce the taper.
And here, I'm assuming the tapered thread is on an oil pressure sender, because you've not said what goes in the hole.
If this was my case, I'd drill out the thread to a fairly large diameter and then machine a new cylindrical part from 6061 alloy, brass or even steel. Then, after thorough cleaning, I'd use JB Weld Engine Case Repair epoxy to install the part and hold it in the case. At that point you can go back and add new tapered threads. Tapered threads are always a "Tap a bit and try the thread. Then tap a little more." type situation.
So that the joint will not leak and thus high torque on the tapered thread will NOT be needed to stop oil leaking, I'd use one of the several LocTite "gasket makers", like LocTite 510 to finish the job.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly thanks rich. The cracked area is an engine mounting boss and can be permanently plugged. I’m wondering if I cut the whole thing, what the next steps would be.
@wobbly thanks rich. The cracked area is an engine mounting boss and can be permanently plugged. I’m wondering if I cut the whole thing, what the next steps would be.
I worked on nearly every make of motorcycle (except Harley) professionally before receiving my ME degree. So I am having a very hard time picturing where this problem area is and what the function of the bolt is. You first called this a "factory tapered pipe-thread plug". The only tapered threads I'm aware of on a motorcycle are oil pressure senders and oil gallery plugs. (Tapered threads on oil pressure senders were indeed big issues on BMW Airheads and Triumph motorcycles in the early 1970's.) Now you call it an "engine mounting boss".
You've lost me. A lot more photos and details will be needed before I can understand and diagnose this issue.
The bottom line seems to be....
* Welding is probably NOT a solution. This seems to be on the bottom surface. To get the TIG welding head close enough, the part would need to be removed and cleaned, or the bike leaned WAY over. Leaning a bike always has the chance of spilled fuel... a fire hazard.
* Welded aluminum is great at stopping an oil leak (think: hole in engine case), but the alloy is never as strong as the parent casting material and if highly loaded (as in bolt near the edge) will crack again.
* This is why I suggest enlarging the hole and machining a "top hat" shaped part in a much stronger material to be inserted and epoxied in place. JB Weld Engine Case epoxy and Devcon (aluminum) both have expansion rates on par with your aluminum case and the repair will hold.
* If this is a replaceable engine cover, in the end you'll be dollars ahead to simply buy the new cover rather than pay a machinist $100/Hr to fab small parts for you.
Owning an old Airhead is easy.
Keeping an old Airhead running great is the true test.
@wobbly thanks for all the advice. The plug is at the bottom of the engine crankcase facing down, sealing one of the holes used in the manufacturing process.
I’m hoping to avoid removing the engine cases, apparently that’ll have the bike totaled out with the cost of a rebuild
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