GT380B top end

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yeadon_m
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Joined: Sat Jan 02, 2010 7:10 am
Location: Canterbury Kent

GT380B top end

Post by yeadon_m »

Folks,

I have a venerable GT380B which I've owned since 1978. Its on 2nd oversize (stock is 54mm, so I'm on 55mm bore). It runs very well and when hot its quiet. But it has started to tick loudly on the right when cold. It does not need urgent work but I'm planning ahead. I'm assuming ticking cold is clearances widening? could there be any other explanation?

I'm considering +1.5mm pistons but Suzuki didn't make them for the 380. Some use X7 pistons but others say they are different (skirt length) and kill what little power the 380 has low down (mine is quite nice there so am not willing to spoil it!). Others use PJME aftermarket +1.5 pistons, as here: http://www.pjme.co.uk/acatalog/Suzuki_G ... arts_.html
They would be the most conservative choice, if the dimensions are very similar to Suzuki. Has anyone used these?

Or, GTReiner has Italian market pistons which are genuine. For reasons to do with local taxation laws, the GT380 sold there was actually bigger and stock size pistons are 56mm. So if I go this way, I have to go +2mm on the UK bore. Reiner reckons thats safe in the liner and has never had a complaint. Anyone here done that, gone +2mm on a GT380? I might try this eventually anyway, as I do have a Plan B.

Finally, and I mentioned this at the end of 2016, I've obtained a NOS set of stock bore cylinders from a guy in Malaysia at a good price. Defo A&B model as the exhaust studs are M10, not M8 as in earlier GT380s. But, some fins on the centre pot are bent. I've still not taken the time to get them to someone who knows what they're doing, but whats the verdict here...can fins be straightened? if yes, I'd use these like a shot, either with stock or +0.5mm pistons (despite grease, there's a little corrosion in the bores, and honing might make them a little over stock. I'm sure 1st oversize will outlive my use!).

Cheers for any thoughts.

Mike
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Gary66
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Joined: Sun Jan 05, 2014 10:53 am

Re: GT380B top end

Post by Gary66 »

Mike i think i may have an old x7 piston which you could maybe look at with a view to using as a sizing guide(ie length etc), if not i certainly have a brand new one but would need it back as it matches the barrel i had rebored ready for a rainy day :lol: . Let me know if this is a route you want to take and i will look for the old one and take it from there
Kettletimes3
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Location: Sunny Wales

Re: GT380B top end

Post by Kettletimes3 »

I now nothing about the pistons,
But to straighten the fins can be done easily with a blowlamp.
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PaulD738
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Location: Warrington Cheshire

Re: GT380B top end

Post by PaulD738 »

Are you sure the ticking noise is bore related Mike? I usually relate ticking noises with the small ends or possibly a broken ring? What are your small end bearings like and more importantly what is the small end eye in the conrod like? I would measure up the “noisy” cylinder and compare it with the others. If the bike isn’t misbehaving I would probably leave well alone. As you know air cooled two strokes ain’t the quietest of mechanical things.
They're rubbish them Jap bikes lad they won't last five minutes! you want to get yourself a nice Royal Enfield!
A quote from my old dad 8-)

I started out with nothing and I’ve got most of it left!
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Gary66
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Re: GT380B top end

Post by Gary66 »

Found an old one mike if you want it for sizing
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yeadon_m
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Joined: Sat Jan 02, 2010 7:10 am
Location: Canterbury Kent

Re: GT380B top end

Post by yeadon_m »

Gary - thanks for the kind offer, but I'll pass for now (for a start, I don't have a 380 piston to compare it with).
Paul - no, I'm not certain its piston clearance and its so noisy when cold and it never was before that maybe it is a ring? but it goes super quiet and smooth when hot. Could a broken ring do that? Ditto a worn small end rod eye (though it was all renewed less than 3000 miles ago).
KT3 - thanks for that - a relief. I will contact my local 'metal god'. Old skool type who will have seen everything.
All, as its all working, I don't plan to take it apart for now. But if & when I ever get around to the mythical 'light refresh' (frame paint, headlamp & fork bracket replacement, new looms & a few stainless parts is about it) then the motor will be out and it would be a good time to at least inspect the top end. I had promised I'd do this in retirement, which has arrived a bit early.
Cheers guys!
Mike
ps: if anyone knows about the pistons, do shout. Its always good to learn from others and to pass it on ;)
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Gary66
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Re: GT380B top end

Post by Gary66 »

No problem mike. I had a broken ring on my gt185 which rattled like a good un, how long it had been like that i dont know as it was just after i bought it that i found it, the bike still ran but was just rattly
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mizzytheman
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Location: Knowsley, Liverpool

Re: GT380B top end

Post by mizzytheman »

Here you go Mike. Hope this helps.

Grampian motors.


http://www.mitaka.co.uk/ROAD_BIKE_PISTO ... KI/X7P.htm

They also do 2.00 mm oversize. http://www.mitaka.co.uk/STOCK_LISTS/ROAD_PISTONS.htm


01.3301.200 P/KIT SUZ GT250X7 2.00 (t)
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