Gearbox problem

crossflow
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Scott Flying Squirrel Racer
Location: Devon

Re: Gearbox problem

Post by crossflow » Mon May 15, 2017 8:13 pm

ActionManMelb wrote:Forget the bent shaft theory!

Look for a piece of debris in or near the hairpin spring of the selector pawls (the claws that grab the pins on the drum).

Could also be a piece junk temporarily stuck in the track of the selector drum (where the fork rides).
I will be looking at the selector mechanism but I'm starting to think that I'm going to have to get the engine apart to inspect the box properly. If there's any where behind the clutch where I can poke my cheap laptop-linked borescope then I will have have a look...just in case there's something obvious .

crossflow
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Re: Gearbox problem

Post by crossflow » Wed May 17, 2017 4:00 pm

Bought a stainless clutch locking tool off Ebay ( very nicely made in U.K. and sold for £18.49 delivered by 'turbosuzukis') which came yesterday, so last night removed clutch basket to see if I could identify problem. Cannot find any trace of a problem whilst shifting so I've pulled the motor out and will start stripping it this evening. I've done roughly 12 miles on this bike since purchase. I really thought I would get away with just the carburettor rebuild and the wiring but i guess at least this way I get to know it. Starter motor put up a hell of a fight and I'm not looking forward to refitting the exhaust as the headers have been welded to the down pipes so the whole front section is one. A lot of heaving to spring them enough to get past the frame tube.
I really hope there's some obvious damage in the gearbox...

crossflow
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Re: Gearbox problem

Post by crossflow » Thu May 18, 2017 2:22 pm

Ok, so engine stripped and trouble found! I was really dreading there not being an obvious problem!
It is obvious though!
The damage to the case hardening on the gear teeth for 5th is probably incidental to the problem, but it might be that some debris from here was part of the reason for the damage.

Image

Image

The selector to engage fifth is bent away from 5th engagement and blued showing that it's been under significant load.

Image

Image

You can also see blueing on the edge of the selector groove on the sliding gear for 5th engagement.

Image

Also what is not easy to see on the photos but very easy to see on the gear is that the dogs on the sliding third gear which engages with 5th are also damaged, rounded off on the leading edges. It may be that it has damaged the dogs because a problem meant that they couldn't engage far enough and the corners simply couldn't handle the torque.

Whatever happened, at least I know where I stand.

I plan to do no more than necessary to sort this problem and rebuild as it hasn't yet contributed toward this relationship, but I will obviously go through and make sure I'm not rebuilding anything that might give short term issues. One thing I wondered about is the cam chain...do these go in the end? I have no experience of them. Otherwise, I have an impacted key on the crank so I'll renew that.
I'll start ordering then...

jibber
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Re: Gearbox problem

Post by jibber » Thu May 18, 2017 9:32 pm


crossflow
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Re: Gearbox problem

Post by crossflow » Thu May 18, 2017 10:25 pm

Yes, I've already got the gear set guy to clarify his listing...he said the shafts and splines were good in the listing, when actually he says that the whole set is good, gears and all. It's not bad and in general it seems like the spares for these are really quite decently priced but it is still £180ish plus gasket set (around £65 for pattern set) even without any bearings, etc. That's why I was interested in knowing what the parameters were for the cam chain, as it rather quickly could become expensive. I am thinking about the sense of rebuild vs new engine, though I d like to stick to rebuild if I can justify it. There are another couple of engines on Ebay too ( that one is great obviously but I can't justify the price)... for about £100 more than its likely to cost me to rebuild this one. Decisions, decisions.

crossflow
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Re: Gearbox problem

Post by crossflow » Thu May 18, 2017 11:03 pm

I might have an option on most of a bottom end for less than the gearset and the selector forks from the breaker. It's a risk ( again ) as I can't see the gearset, but this was apparently a working engine until a top end problem necessitated its replacement. I do believe the guy but at the same time I have little faith in my own good judgement. :rolleyes:
It would then mean that I would replace the entire lot: gearset,selector drum, shafts, change mechanism...just in case the problem the bent the selector fork caused any slight issues elsewhere. Again, assuming that it's ok.

crossflow
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Re: Gearbox problem

Post by crossflow » Sun May 21, 2017 9:47 pm

Ok, i had to split the cases to check but the gearbox in my new bottom end looks really good, with no apparent damage to the teeth, selectors, bearings at all.
Piston / bore / cam lobe dimensions are still within spec on mine so I'll stick my top end back on this bottom end and hopefully it will all be fine.
Image
All good.
Image
So relieved. I don't like relying on luck but it worked out this time...

crossflow
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Re: Gearbox problem

Post by crossflow » Sat Feb 10, 2018 7:10 pm

So this week, almost a year since I found that the ‘project’ XTZ that I bought needed a new gearbox, I finally got around to fitting the rebuilt engine. Today was to be the post-rebuild, maiden (hopefully 5 geared) voyage.

I live on the edge of Dartmoor in Devon and I rode up to Princetown in constant rain. There are a few niggles to sort ( carb leaks on the side stand, neutral switch not working, low speed handling a bit vague) but it’s now pretty much what I imagined it would be... really happy and looking forward to some fun now!

jibber
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Re: Gearbox problem

Post by jibber » Sat Feb 10, 2018 9:56 pm

:goodjob:

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