[carthorse]

Evening all. I’m looking for some advise on a repair I’m thinking of making on a case off of a 1974 Triumph motorcycle I’m rebuilding. The rear engine mount was fargled by the previous owner. They placed the engine in the frame without a spacer on the rear mount and just cranked on the mounting bolt until the frame tabs bent in and touched the case. Consequently the rear mount chewed into the case, removing about 3/8 of material. So... I was thinking of either a) trying to build the case back up with weld. Or b) machining it back to flat, building a spacer and going with with that. I’m not looking to restore this thing to perfect condition. I just want to get it going again. My brain tells me to go with the spacer, but that devil on my shoulder says “break out the TIG welder”.

Anybody out there with any experience of this or anything close. I’ve done some build ups before on less critical parts successfully, but I really am worried about cracking these cases.


Thanks for any help.


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[cj737]

Having welded up dozens of vintage cast aluminum cases I will say this; welding those sucks. The cases are impregnated with oil, dirt, grease and grime. The second you hit them with a TIG arc, they boil out that grime, you get heaps of porosity around that surface, and your weld pool looks like the La Brea tar pits.

But it can be done...

Degrease the case inside and out. Heat in oven to 300* for about 20-30 minutes. Wipe with Acetone inside and out. Then grind the area with a carbide bit, and wipe with Acetone. If you are only planning to build up a surface thickness, I use 5356 wire, 50Hz, and a 65% cleaning action. The first surface pass I don’t use filler, just “heat and puddle” to bring up the goop. Scrub with stainless brush. Then a surface pass with filler, then I stop and peen the surface. Wipe down and a gentle scrub with stainless brush. Repeat. Let the cases cool after peening. It has helped me avoid the porosity in the build up.

No matter what filler you use, the material difference will be noticeable afterwards. If after machining or filing, you discover pinholes, touch it up and peen it again.

What a fun way to spend 3 hours, huh?

[carthorse]

Thanks very much for your information. It does sound like a lot of grief, but might be worth it in the long run. Maybe I’ll go the machining/spacer route to start and if that doesn’t hold together try a build up with weld. I’m just trying to figure out the lesser of two evils. Introducing filler metal , running into impurity’s and localized heating have got me a little scared. Especially a motor mount on an engine that vibrates worse than a breeze block in a tumble drier.

I also just discovered a crack in another case on the same bike around a threaded hole. Previous owner was obviously prodigiously strong. This hole is used as access to an adjuster for the primary chain, so as such, is not under any additional stress that the motor mount would have to put up with. I’m definitely going to use your info to weld that up.

Thanks again for your reply. I appreciate you’ve had to take time out of your day and given up hard earned information. Hopefully I can help you or someone else down along the line. (Just not with welding - I kinda suck at welding!).


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[cj737]

Thanks very much for your information. It does sound like a lot of grief, but might be worth it in the long run. Maybe I’ll go the machining/spacer route to start and if that doesn’t hold together try a build up with weld. I’m just trying to figure out the lesser of two evils. Introducing filler metal , running into impurity’s and localized heating have got me a little scared. Especially a motor mount on an engine that vibrates worse than a breeze block in a tumble drier.

I also just discovered a crack in another case on the same bike around a threaded hole. Previous owner was obviously prodigiously strong. This hole is used as access to an adjuster for the primary chain, so as such, is not under any additional stress that the motor mount would have to put up with. I’m definitely going to use your info to weld that up.

Thanks again for your reply. I appreciate you’ve had to take time out of your day and given up hard earned information. Hopefully I can help you or someone else down along the line. (Just not with welding - I kinda suck at welding!).


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Repairing areas around threaded holes are worse, I’m afraid. “Best” method I’ve found is to ream the existing hole larger for a timesert, then you’re back to the original hole diameter with a much stronger thread.

Don’t let me scare you off the repair. Your weld will probably be stronger than the original cases are vintage cast ally cases are of unknown metallurgy, and pretty crappy really. If you ever get a chance, bust one. Then look at the cross section. They look like Kit Kit bars internally as there’s so much porosity in the casting. And that’s where the welding issue comes up- when your torch heat hits those trapped air bubbles, they expand! And they billow up and create warts all over the darn place.

I probably safely estimate 50% more area of weld repair than the actual damage from the “incidental” hiccups. So, if your repair is a crack 2” long, expect another 1/2” or better on either end and adjacent to the crack by the time you grind out the repair. Just saying...

Besides, you gotta learn sometime

[MinnesotaDave]

I did a couple triumph engine cases a few years ago.

I cut and die ground bad sections, built up, drilled and filed where needed.

It went pretty smoothly really - used my 1963 Airco tig/stick (miller 330abp)

[cj737]

I did a couple triumph engine cases a few years ago.
It went pretty smoothly really - used my 1963 Airco tig/stick (miller 330abp)

Not surprising really. Triumphs are terrible at holding oil, hence the cases never get saturated

[MinnesotaDave]

I did a couple triumph engine cases a few years ago.
It went pretty smoothly really - used my 1963 Airco tig/stick (miller 330abp)

Not surprising really. Triumphs are terrible at holding oil, hence the cases never get saturated

Haha!!!