thoughts on running different size bores

[A100man]

On my GT250 one at 54.5mm and one a 55mm.

Clearly not ideal, but people often talk of compression figures only having to be within 10% of each other...

Cheers

J
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Now: A100, GT250A, XJ598, FZ750, SL1000

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600, Skorpion

[redeem ouzzer]

Is it ideal? No.

Was it commonplace back in the day?

Yes.
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[A100man]

Hmm my thinking too. My brother would say..

'Well, it's not a race engine is it?'
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Now: A100, GT250A, XJ598, FZ750, SL1000

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600, Skorpion

[stinkwheel]

0.5mm is a full piston size.

So standard bore on a GT250 is 54.0mm? (according to google) So you have one at +0.5 and one at +1.0 already. That'll slap about like a pencil in a tin can if you use standard pistons

You probably want to take them both out to 55 and fit some +1.0 pistons?

Or more probably both out to +1.5 because if it's +1.0 through wear, it's probably not round either.

Piston kit is £60, probably about the same again for the rebore?

EDIT: I'd be less worried about using different sized pistons and rings if they fit the bore.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[A100man]

[stinkwheel]

I wonder how much difference in weight there is between the two pistons and if it could be balanced out by a little judicious sanding off of casting marks etc on the inside of the heavier one.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[A100man]

Can't be much can it? In any case most (ok a lot) of the weight will be in the rod pin and bearing. It's not high revving either - 8k red line I think.

I'll dig out my Anthony Worrall-Thompson electronic scales later and see what were talking about
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Now: A100, GT250A, XJ598, FZ750, SL1000

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600, Skorpion

[redeem ouzzer]

Was quite common back in the day with hamfisted servicing (or lack thereof) for people to burn a hole in one piston and just rebore that side (and likely just use a hose pipe to flush the crankcases out - be honest, we've all done it, especially when planning to sell the bike). You could arse around weighing pistons, or you could just sling it together and ride it.
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[Easy-X]

Get 'em both out to +1.0 by all means but I doubt the liner would like any more than that taken off
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[stinkwheel]

I was just interested.

I've sent a twin engine for a rebore before and the (brand new) pistons have come back with R and L written on them in marker pen because they were slightly different diameters.
____________________
“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[redeem ouzzer]

[Pete.]

A GT250 probably suffers far more power imbalance from the wanky siamese throttle cable setup than half a mm difference in the bore size.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[A100man]

[A100man]

[Pete.]

Quick fag-packet maths says your swept volumes are 2.3cc different.

That's 2.8%, blow-by will probably vary more.
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a.k.a 'Geri'

132.9mph off and walked away. Gear is good, gear is good, gear is very very good

[Riejufixing]

[Riejufixing]

+Look at it this way. There's no harm in doing what you propose on this oldish machine, and you're actually conserving the thing by not taking metal off where you don't need to.

Win, win.

[A100man]

Thanks chaps.. my instinct was to give it a try.

Its a survivor when all said and done and there are a few escape routes i can take if needed
____________________
Now: A100, GT250A, XJ598, FZ750, SL1000

Then: Fizz, RS200, KL250, XJ550, Laverda Alpina, XJ600, FZS600, Skorpion

[Shaft]

I'm with stink, pete and forced, bore size will probably make eff all difference so long as everything fits, but I would check some weights, just to be sure you aren't loading one side of the crank with too much moving mass.

Just belt and braces, as you say, it's not exactly a high revving race motor, some of them probably left the factory with higher tolerances.
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[jeffyjeff]

Hone the smaller bore out .5mm, and knurl the piston skirts to fit.
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