LCGB Forums

I stripped my RB20 motor to sort a drive side oil seal and found the crank like this. Has anyone seen little localised heat like this? I'm thinking that whatever caused this was probably the cause of the oil seal failure. I haven't had the crank checked for alignment yet but I've not felt any additional vibration. Any thoughts?

On Indian GP cranks they use heat to get the crank pins in and there always blue,but thats usually on both sides,but I dont know about your crank.

Without 'before' & 'after' images, I'd be concerned about the apparent step of the crank pin in the web. Was it not flush prior to use?

The big end of the con-rod looks like it has been rubbing excessively to cause the heat build up. It looks like it has moved.

If I were you I'd get that crankshaft pressed apart by somebody that knows how to re-build it & can access the new bearing required, shims (if necessary) & possibly new pin & con-rod.

What's the history of it? Who built it/sold it to you?

Sorry, but it doesn't look very positive. It could have been a lot worse though....

Thanks for that. I had the crank out earlier in the year and the heat was not evident then. The crank was a special build by Geoff Brittan down here in Portsmouth. He's an engineering lecturer at the university with a long scootering history. He lightly blasts the pin and Loctites it in place.

Just seen the pictures on the Scooterotica forum.Is the flywheel taper broken near the woodruff key

Post by GT Guy » Fri Dec 30, 2016 6:11 pm

Thanks for that. I had the crank out earlier in the year and the heat was not evident then. The crank was a special build by Geoff Brittan down here in Portsmouth. He's an engineering lecturer at the university with a long scootering history. He lightly blasts the pin and Loctites it in place.

I'm desperately trying not to be seen to hijack this Thread & I'm certainly not about to disparage your guy! I bet he did it for the cost of a cup of coffee & the love of scooters!

However, what I'm about to say is relevant & I'm sure most people would conclude that we could agree that the Jury is out on the subject of crank pin fitting with Loctite.

Although I have faith in Loctite products where there is a clearance to allow it to work, I have seen no proof that even the appropriate grade can be stronger than a well machined/finished, proper tolerance interference fit.

Fortunately, I am not alone if anybody cares to surf the Internet & see what Engineering Forums have to say on the subject.

It most likely won't harm the fit & may aid fitting as a lubricant. However, even Henkel's own claims are not as strong as the best interference fits, last time I checked.

IMO, a reasonable guide to how well the pin will resist movement is to see how much force is taken to assemble it.

Many years ago, I recall Malc Anderson being reassured that a crank he did for me took seven tonnes. That seems to me a reasonable benchmark....

It's interesting that you've brought that up as I've been pondering it myself. I don't know what it took to press it in but it been in there for 3 years without incident until now. What about welding the pin? Is this seen as a poor substitute? You wouldn't waste your money on a 350LC crank that hadn't been welded back in the day. Don't worry about hijacking the thread. It's all useful discussion...every day's a school day

Post by GT Guy » Fri Dec 30, 2016 10:59 pm

It's interesting that you've brought that up as I've been pondering it myself. I don't know what it took to press it in but it been in there for 3 years without incident until now. What about welding the pin? Is this seen as a poor substitute? You wouldn't waste your money on a 350LC crank that hadn't been welded back in the day. Don't worry about hijacking the thread. It's all useful discussion...every day's a school day

With respect to your guy, perhaps ask him as well.....

But, yes, unfortunately welding seems to be the way that many have to opt for.

If you look around in many two stroke applications, welding the pin has become the method by which racers achieve reliability. One missed gear with the red mist descended & the crank can twist.

I guess it's considered the option that will most likely achieve, say, a weekend's worth of racing.

Even the crank in my road scoot is welded...

Incidentally, there are/were oversize pins available from some Tuners, but the cost makes them still a risk not worth taking compared to a good crank re-builder that has experience & their own TIG kit.

I've used engineering adhesive in cranks and can tell you that it's a bugger to press the crank apart again if you inadvertently press it a bit too tight on the side float so done right it's probably better than welding.The blueing on your crank certainly looks like something is amiss but I would expect the bearing to seize first if it was overheating,maybe it was on its way but there must be some serious heat there to blue the web.

Post by Wack » Sat Dec 31, 2016 12:48 am

I've used engineering adhesive in cranks and can tell you that it's a bugger to press the crank apart again if you inadvertently press it a bit too tight on the side float so done right it's probably better than welding.The blueing on your crank certainly looks like something is amiss but I would expect the bearing to seize first if it was overheating,maybe it was on its way but there must be some serious heat there to blue the web,has the conrod been rubbing on the drive side web which would indicate it was initially too tight?

Engineering relies on production techniques involving adhesives, but, for the most part, they will be part of the Design remit. That's not to say anybody should be dismissive of the use in crankshaft assembly if it works for some.

However, looking @ the OP images, something appears to have failed on the hot web side of this particular crankshaft assembled using.....engineering adhesive.

If the pin has gone inwards, it may well have started to go the other way initially to cause the heat with the web rubbing.

All any of us can do is offer up opinions & advice.

Ultimately, it's up to the OP which advice they choose to take.

If it were a feasible option, I'd want to have a civilised(!) discussion with the crank builder before doing much else.

It's true that SIL cranks show heat marks on the crank webs, but the heat is only applied to the individual webs before assembly, not to the bearing, conrod or crankpin - the idea is to heat the webs so they expand slightly, and then push the cold crankpin into position. This process does not leave heat marks on the crankpin or the conrod, so what the image shows is overheating caused by some kind of mechanical failure or movement. If the crankpin has moved within one of the webs, as might appear to be the case, is it possible that side forces due to misalignment have pushed the side of the conrod's big end hard against the drive side web? Even worse, there's not a stepped crankpin in there, is there?