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Discussion Starter #1
I’ve not been on here for ages, the 360 is running great and i’ve been on other hobbies!

There’s an oil weep from both sides of the cylinder barrel joint to the head, so am contemplating having the bore increased and bigger pistons whilst its in bits :)

I’m competent with a spanner but havent rebuilt an engine before, so whilst the 378cc upgrade sounds great i think it may be too much for me.

So.... whats the best way to economically increase the capacity/power of the 360 motor whilst i have the head/barrels off?

Thanks in advance !

Steve
 

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GS850 pistons are a drop in upgrade (with boring, of course). IIRC, up to 1mm oversized GS850 pistons will work; you’ll need copper gaskets (head and base) to account for the bigger bore (the stock gasket bores won’t be wide enough on the head). Lani at Copper Gaskets Unlimited did mine and did a killer job, just let him know it’s for an overbored 360 and what the new bore size is.

Have you already done the clutch cover/oil pressure release plate mod?

EDIT: See your note about the 378 mod. To my mind, it’s honestly the same as any top end rebuild; the thing literally slots together the exact same way as if you hadn’t had it bored, you just have to find a competent machinist to do the overbore. I can’t think of another mod that would give you even half the boost for even double the cost. If you’re replacing the head gasket for that leak, all you have to do is pop the jugs out, remove the pistons, install pistons, install bored jugs, do the rest of the head stuff. The hard part’s done by the time you get there.
 

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Discussion Starter #3
Thats what inwas hoping to hear!

So the question is what size of gs850gs pistons to go for, as i didnt realise +1mm was possible??

Is there a limit for overboring the std barrels?

Steve
 

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I ended up with 69.5mm pistons because that's what was cheap at the time, but I think CrazyPJ did 70mm pistons, which is 4 sizes over for the GS850 (stock pistons are 69mm). I wouldn't go bigger than 70, personally - you start getting close to too thin on the iron sleeve inside the jugs.

In terms of limits, it depends on the bike, but most I've seen only allow for 12 sizes of overboring (so ~3mm) before you need to have a machinist pull the sleeves and replace them. I'm seeing GS850 pistons for $150 on ebay here (for four, admittedly :D), but that's with wrist pins, rings, and circlips, so a pretty good deal.
 

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Discussion Starter #5
Thanks, i’ll look out for 69-70mm piston kits, guess if i buy 4 i’ll easily sell 2 on here ��

Think i’ll strip off the head/barrels this weekend and then order some pistons ��

Look out for updates......
 

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In terms of limits, it depends on the bike, but most I've seen only allow for 12 sizes of overboring (so ~3mm) before you need to have a machinist pull the sleeves and replace them
My 450 build is 4mm over (16 over, in other words) and its long-term reliable - you just have to watch overheating more closely, don't idle for long periods, etc. I ran my 450 drag bike regularly and hard for 2 full seasons with no issues back in the day with the same size overbore (497cc) and plan to do the same with my current build once my cams are straightened out, so 3mm over shouldn't be a big deal at all unless Honda skimped on sleeve thickness on the 360 for some reason (hard to imagine)
 

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Absolutely, could be fine. I just recall PJ had it 3 over and I haven't heard anyone go higher :D someone wanna guinea pig it?

I've got a set of 360 jugs in my scrap metal bin - I'll be there later today and stick em in some calipers to see the sleeve thickness, just for fun. What do you think is the thinnest you could take a sleeve to safely?
 
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