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#868984 01/14/22 12:34 am
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How far can you safely bore a Triumph 650 to? I know it is possible bore to 80 over but would like to understand what some of the forum readers think. In trying to clean up some of these older bikes that have sat around a while sometimes 80 over is the only option aside from scraping the barrel. I have not heard of sleaving a 650 barrel as being an option. I would also like to know what are the issues you face if you do bore it 80 over. I have heard the lower skirt on the barrel gets too thin and that was the reason.



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I’ve had a couple of pre-unit 650’s at 60 thou over, no disasters regarding the barrels.

I think, back in the day, even +100 thou were available, though I’m not sure of the wisdom of using them.

Unit barrels, with their slightly wider headbolt spacing, should perhaps be a bit stronger?
I’m not so sure that the spigots are the weak point, it may be the lower end of the barrel above the flange, but I’ve not known either to fail in road use.

Have you located a source for +80 pistons?

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This would be a good way of using up those worn +60 barrels. A quick look online shows that there are a few odd +80 pistons around. Do any forum members have experience with +80? I have some Commando pistons lying around, they are 73mm standard so i will compare them, not expecting that they will be compatible though


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I ran my unit t120 at +100 for around 10 years.
Filing out the head gasket is important.
Use decent rings, i got hold of some Riken jap ones they are excellent.
The original rings supplied with the pistons didn't last.
Everyone said it would use oil etc but with those rings it was no problem
and used to see over a ton quite frequently.
Try MAP for them.

Last edited by NickL; 01/14/22 6:12 am.
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Originally Posted by koan58
I’ve had a couple of pre-unit 650’s at 60 thou over, no disasters regarding the barrels.

I think, back in the day, even +100 thou were available, though I’m not sure of the wisdom of using them.

Unit barrels, with their slightly wider headbolt spacing, should perhaps be a bit stronger?
I’m not so sure that the spigots are the weak point, it may be the lower end of the barrel above the flange, but I’ve not known either to fail in road use.

Have you located a source for +80 pistons?
I have an 8 bolt barrel bored +.100 and they all but break through the stud holes. It had run too but it seems but I wouldn't have trusted it to take me anywhere without a bus route. The bolt holes had distorted the bores as evident on the rings and shiny spots in the cylinders. The barrels were actually fitted to a bike from Arizona, AZ Scott. You can have them back if you want! thumbsup
I guess you could have gotten away with it on a 9-bolt cylinder, but I would measure the skirts carefully. They vary a bit in thickness there.
Cylinder liners are available (I have 4), as are new barrels.

SR

Last edited by Stein Roger; 01/14/22 7:42 am.
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Finding anywhere to fit new liners is difficult now, at least in Australia. They all say just buy new barrels as the cost is the same!! I'm not convinced though.....


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I see a +0.080” rebore as worth a try on a Unit 650. Take a bit of care over the head gasket, as said above.

It’ll give you an extra 40cc capacity, which you’ll feel.


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When you can check the rings by looking between the fins, you have bored it too far.

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You can end up with a big pile of washers...........

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Thanks for the responses, On the question on where to find .080 over pistons, my source would be JRC Engineering out of California, they have them listed. There may be other sources but that is the only one I am familiar with. I really appreciate some of the comments. We have occasionally gone to .060 over if needed to clean up a damaged barrel, but .080 over is something new.



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Have done .080 with no problems. This was 5-6 years ago, still see the bike out and about.

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What about getting the bore Nikasil plated? The process can build the bore to tolerance and they will fix cylinder gouges before plating. They will size to your provided piston. In the US I have used Chrome Bore and Millenium Tech.

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Pete, I don't know much about Nikasil Plating. Is it very expensive?



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