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sparkieb

348 crankshaft/conrod gap

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My conrod had sideways movement of over 3mm, is this normal. Clymer manual says .4 to .5 of a mm but the book does stop at 75 and i have  76 mrr. Is it spaced into position by little end ?  I did run/ride it a short distance before dismantle and it ran smooth and quiet so unsure whether it need replacing or not. Just seems very odd to have such movement but nothing seems amiss apart from this 3mm sideways movement. Has anyone any ideas at to whats amiss. Engine didnt seem to have recently if ever been apart, bore+piston in excellent condition just wear on one gear, barrel nuts were very difficult to remove giving me the impression it had not been apart. Hope someone has an answer for me. Cheers Mark

20190722_155438.jpg

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On 7/23/2019 at 11:58 AM, sparkieb said:

My conrod had sideways movement of over 3mm, is this normal. Clymer manual says .4 to .5 of a mm but the book does stop at 75 and i have  76 mrr. Is it spaced into position by little end ?  I did run/ride it a short distance before dismantle and it ran smooth and quiet so unsure whether it need replacing or not. Just seems very odd to have such movement but nothing seems amiss apart from this 3mm sideways movement. Has anyone any ideas at to whats amiss. Engine didnt seem to have recently if ever been apart, bore+piston in excellent condition just wear on one gear, barrel nuts were very difficult to remove giving me the impression it had not been apart. Hope someone has an answer for me. Cheers Mark

20190722_155438.jpg

I’d be a little concerned about the a) rust evident on the crankshaft wheels, there might be rusting on the big end crank pin b) the obvious welts on the right hand side wheel, what caused those? On the other hand if you hadn’t taken the engine apart it would probably gone on for years without any trouble what so ever....but now you know??

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The rust was a little worrying but its light and there was quite a coating of oil with it, there was water in the gearbox and clutch so yes i have quite a rebuild on my hands (first montesa too) the marks on the crank make me think it has been apart but many many years ago as it truly was difficult to dismantle (the barrel studs were quite a quest - and the base gasket was an original) but theres virtually no bore wear and the piston is like new(std too) So possibly it was either rebuilt years ago or its never been apart and the marks are a mystery. Searching for fork yokes too as one is very badly corroded which takes me back to the water issue, but wheel rims/frame etc have no corrosion. Cheers Mark

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17 hours ago, sparkieb said:

The rust was a little worrying but its light and there was quite a coating of oil with it, there was water in the gearbox and clutch so yes i have quite a rebuild on my hands (first montesa too) the marks on the crank make me think it has been apart but many many years ago as it truly was difficult to dismantle (the barrel studs were quite a quest - and the base gasket was an original) but theres virtually no bore wear and the piston is like new(std too) So possibly it was either rebuilt years ago or its never been apart and the marks are a mystery. Searching for fork yokes too as one is very badly corroded which takes me back to the water issue, but wheel rims/frame etc have no corrosion. Cheers Mark

Corroded yokes is a common thing with 348s but there are usually some being broken down for parts in the UK. Be aware that the yokes from a 247 look very similar and will fit, but have slightly different geometry. Both 247 and 348 yokes were made from rolled aluminium plate which lends them to that sort of failure. I saw some NOS 348 yokes advertised a while back in the US. Sorry not allowed to mention suppliers on here.

As for water getting inside things, one of my 348s came with collapsed forks and I found they were full of water. Lots of corrosion on the insides of the tubes.

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Yes i have one front fork with corrosion ☹ have considered getting some yokes made but will price them up first. Its been a strange dismantle as some parts have never been apart but as its a 76 and has so little wear or damage its amazing, will enjoy the rebuild process once i get a bit more organised and save up a bit.  Cheers Mark

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On 7/26/2019 at 8:02 PM, sparkieb said:

Yes i have one front fork with corrosion ☹ have considered getting some yokes made but will price them up first. Its been a strange dismantle as some parts have never been apart but as its a 76 and has so little wear or damage its amazing, will enjoy the rebuild process once i get a bit more organised and save up a bit.  Cheers Mark

One 348 I bought in about 2000 still had visible cross-hatching on the fork tube chrome and in the cylinder bore and it still had the original sprockets, handlebars, levers, cables etc. It had been ridden in trials but very little. I have a suspicion that some people bought them new and found that they were not as easy to ride as they had thought and just parked them up

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It has been apart as those braying marks on the flywheel are not factory.

I would say the crank has been apart in a previous life and a copper or dead blow was not the hammer of choice.

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