HELP ! GT550 Barrel dilemma

[hutchie63]

Can anyone advise me on how to identify the correct barrels i have fitted to my 76 GT550. the original bores on this model were the nikasil lined ones. when i stripped the bike down i noticed that the pistons have 0.50 stamped on them. This has now led me to the conclusion that the barrels have been changed from another donor engine at some point to allow the rebore.
On checking with robinsons foundry there are 2 piston kits available as below. what i need to do before i order is identify which barrels i have fitted can anyone assist please

PISTON KIT (0.5 OVERSIZE) (M MODEL) (12103-34810-050)
Old Part Number 12110-34702

PISTON KIT (0.5 OVERSIZE) ( J K L MODEL) (12103-34810-050)
Old Part Number 12110-34700

[markush]

Hi!

Use a magnet. If it is attracted by the cylinder wall, there is a gray cast iron bushing.

Normally GT550-SCEM(Nikasil)-cylinder have two additional scavenging ports and therefore longer rear cylinder base bolts.

Markus

[yeadon_m]

+1 on what markush said.
Furthermore, it is possible that an idiot re-bored nikasil cylinders. Yep, I've not only seen it but bought a bike where this had been done. A GT550A with high rear cylinder studs and 7 ports, but carrying +0.5 pistons. Which means they were running in alloy, which wouldn't last. I sold the thing on sharpish
Caveat emptor.
Finding a decent set of nikasil cylinders is not easy, so if yours are wrecked, a number of choices exist:
1 - buy oversized pistons and entrust your cylinders to one of the very few companies in UK that can apply the special coating and get the clearances correct. I considered and rejected this.
2 - get a set of proper cylinders from a scrap motor and get these replated. I did this.
3 - get an iron liner motor or at least cylinders and go with conventional rebores. I am not sure if the porting in the crankcase of the nikasil, 7-port cylinder motors matches the 5 port iron liner cylinders. If they don't, it might still be OK, but others will know. If unsure and so long as you are not bothered by having a hybrid bike, you could recondition and install a whole iron liner motor. I considered this. The plus of iron liner motors is that dozens of reboring companies exist who know what they're doing and many piston sizes exist.
I consider the nikasil overkill and I would have found life easier if the 550s in Europe had shared the same motors as in the USA. But we are where we are.
Good luck!
Mike

[markush]

Some years ago in the german GT-Forum someone wrote about GT550 SCEM cylinders, they were drilled and fitted with gray cast iron bushings.
Unfortunately, the work at the ports wasn't executed very reasonable, so that the engine wasn't running well.