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99 Rebuild. Damaged cylinder question

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I'm rebuilding a 99 600cc (SS?) and just taken the barrels off. 

You can see the damage in the attached photos.

The pistons are stamped +020 so it's had one rebore.  From the cylinder scoring it needs another.

What are my options with regard to the damaged skirts?

Many thanks,

Derek

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In looking at the third photo, it appears that there has been a previous installation of a liner.

Did you do that or was it that way when you bought it?

I am guessing (and it is just a guess) that the existing liner could be bored out and new liners installed.  Have you discussed that with your local Norton machinist?

Mike

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Mike, thanks for your reply.

The bike was new to me last year after being laid up for 27 years.

I'm not sure what to do about the damaged skirts. Can they just be left and the cylinder rebored or resleeved or do they need to be rebuilt?

I'll need to speak to my local machinist in the next few days!

Cheers,

Derek

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Hi Derek,

I was in a very similar situation to you having bought a barn find Domi 99. When I stripped the engine I discovered the badly broken spigots (but yours look a lot worse). I did a lot of research into liners and engineering companies that fit liners. In the end I found an engineer in Slough who sounded very honest and on the ball regarding sleeving barrels. His name is Andy and he works at H T Howard (1753 525549). After talking to a lot of people, I decided to use liners made by Westwood Cylinder Liners (01905 799470) as they have a more substantial lip at the top of the liner - most seem to have a lip of only 0.25mm at the top but the Westwood ones have 1.5mm (the lip removes the risk of the liner sliding down into the crankcase). Andy at HT Howard said even though he has fitted lots of liners,  it would still be better if I could find a good set of usable barrels rather than sleeve the broken ones as the bit of broken skirt would get very thin once the barrels had been bored out and sleeved and there was a danger that the remaining thin skirt might break off in use and drop into the crankcase with devastating results.

Luckily I have found a good set of barrels so don’t need to worry about having liners any more. I got my barrels from Geo Yeomans Motorcycles (0121 453 8886) based just west of Birmingham - just off the M5. When I went to collect my barrels, he had a ‘spare’ set of +20 barrels on the shelf - I think he was asking around £220 for them.

I would just like to add that I have no links to H T Howard, Yeomans or Westwoods.

Hope you manage to get this problem resolved - I know how frustrating it is.

Regards Tony

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Tony,

Thanks for the reply.  It's horribly frustrating buying a bike that was running when it was laid up to find the damage that's out of sight! I'll be out of circulation for a while but will decide what to do finally after seeing my local machinist.

 

Cheers,

Derek

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These broken barrels appear to have been double sleeved. To sort out the mess would probably mean removing both sets and starting again with new liners. This is not really a very economical fix when you factor in the cost of the machining needed, plus new liners and pistons. My suggestion would be to search for another set and use the old barrel for spare fins.

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Philip,

I suspect you're right. I'm tied up for the next few weeks but will eventually get over to my machinist for another experienced eye.

Cheers,

Derek

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By scaling from your photo, the original spigots had 75mm outside diameter, with the original bore being 68mm.  So the wall thickness of the spigot was (75-68)/2 = 3.5mm.

The liners from RGM have an OD of 71.2.  So at 68mm bore, the liner wall is only (71.2-68)/2 = 1.6mm - or about half the spigot thickness.  The liner visible on your photo 3 is even thinner - maybe only 0.8mm, but .025mm has already been taken away from the liner by the +20 rebore.  So the liner looks pretty similar to the RGM diameters.

It looks possible that someone lined it after the original spigot was damaged, and hoped that the new liner would bridge the gap (since there is no gas pressure down there).  But sadly the liner was not up to the forces from the piston skirt, and it failed.  The rebore would not have helped.  

So a liner on regular bore will have 1.5mm wall thickness, compared with 3.5mm for the spigot on regular bore.  If the original spigot was bored out to +60, it would be .030" (0.75mm) thinner, giving a minimum thickness of 2.75mm.  So 2.75mm must be acceptable.  But that is still more than the 1.5mm that the liner would provide in the absence of part of the spigot.  Whether 1.5mm is enough seems to be the important question.

All numbers are scaled from the photo - so may not be all that accurate. 

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David,

In away at the moment. Just dropped into the forum in a rush so will need to think about your post.

I'll get the micrometer onto the liners when I'm back and try and work out how the barrels got to where they are. I know that the bike spent a lot of time on a Scottish island which means that access to engineering factors would be relatively limited compared to living in the Midlands!

Thanks for your calculations.

Cheers,

Derek

 

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