Material for kick starter repair bushing

[bikester250]

I've been reading postings about how to repair a broken k/s boss (picture of mine attached) and I'm tending towards avoiding weld repairing the damage and just having a flanged bushing made. My concerns about weld repair are due to worries about distortion of the case (from what I've read this will depend on the skill of the welder so I'd rather not rely on that).
I'm a retired metallurgist so I have some thoughts about what to make the bushing from but I'd be grateful for any comments or other peoples experience.
My candidate materials include:
An aluminum bronze alloy such as C623 which is twice as strong as typical aluminum die castings and much more ductile plus it should have decent bushing properties (not sure that this really matters for this application since it isn't a constantly rotating item).
4340 steel or something similar-again twice as strong as the die cast case and much more ductile. Probably not as good a bushing but easier to find than the bronze.
A martensitic steel such as 17-4 PH which have a yield strength around 6 times that of the die casting and hardness around 35 Rc. Ductility isn't as high as the other materials. One disadvantage is it will be harder to machine than the others.
I'm tending to avoid stainless steels such as 304 because their yield strength isn't as high as these other alloys and they may tend to gall on the k/s shaft (but again this might not matter for this application).
Let me know if you have any experience with this topic.
Regards,
Phil

[DesmoDog]

I don't understand how a bushing fixes this? Would you machine the case to accept a new bushing, and the flange would replace the boss cast into the case half?

I had a Porsche 356 case that had a lifter bore repaired with a bushing (a rod must have let go and damaged the original). Looked like a very well done job, from what I could tell the bushing was aluminum. Different application though, the lifter moved up and down the bore, it didn't put force on the side of it like a kickstart does.

FWIW I had the boss on my 160 welded/machined by someone who's done this same repair before and had no problems with it.

[bikester250]

DesmoDog,
First of all, this case never had any bushing in it; most cases have a 18mm OD bushing with a 15mm ID that the K/S shaft fits into. So the plan (and you can see this in other postings on this site- Dew Cat Tea Bob has written in favor of this approach) is you make a bushing that has a thin flange (dimensions of the flange are on the order of 25mm diameter and .5mm thick-outside of that, the rest of the bushing is basically 18mm od, 15 mm id). This bushing is then fitted into the boss. In my case I'm thinking about making the flange a bit thicker by removing some material from the face of the boss (maybe make it 1 or 1.5 mm thick).
I've talked with Malcome Tunstall at Syd's Cycles and got an e-mail from Nigel Lacey @ Lacey Ducati about the weld repair and they both expressed concern about distortion (especially of the adjacent housing for the roller box). On the other hand another poster (Sebring Mike) has repaired a case using weld repair plus the flanged bushing and I suspect there have been quite a few cases that got the weld repair treatment (including yours).
Phil
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[Rick]

I don't think a flanged bushing will restore the support that's lost when that chunk of aluminum broke off.
I've done a weld repair that worked- didn't find any distortion, but I'm not sure what a 'roller box' is.
I heated the crankcase up in an oven, probably between 150 to 200 degrees, and then fit a 6 inch length of copper rod into the hole- the aluminum weld material didn't stick(much) to the rod. The rod absorbed lots of the heat, but I also packed some heat absorbing clay into and around nearby machined surfaces. Most guys prefer TIG welding, but in this case, a MIG welder with a spool gun will pour in lots of material in a short time- less heat will be dumped into the crankcase. I was skeptical about the heat dam material until a salesman brought some by the shop where I was working- he heated up two strips of steel with a torch but put the heat dam on one strip- it was easy to see that the steel was cooler on the other side of the heat dam, we told him he didn't need to pick it up to prove the point.
You'll need to get the welded area really clean, but it can be done.
I think the heat dam clay was called 'Heat Fence'- no idea what part of the world you live in, but a welding or plumbing supply store will probably have it- it's probably just a clay that will absorb lots of water that absorbs heat during the weld- it does dry out.
I need to do this again and have thought about fitting thin wall copper tubing into the hole with hose barbs on both ends and running cold water through the tube as I welded, but that may be a bit overboard...
Rick

[Nick]

Why I push-start whenever possible....

[DBDBrian]

Damage to the starter shaft housing and it's causes has been discussed in this and several past threads. I have the same problem with my cases.
I think I will bore and bush,(Bush materiel Phosphor Bronze) rather than weld, but I do not see a problem with weld if done properly. I do not think there is a high load at this point, generated by the action of the starter lever, if the bush in the outer cover is in good condition, keeping the shaft in line.
The damage being caused by using the shaft as a lever point, as pointed out in the Ducati manual.
Brian

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