LCGB Forums

I have a Italian GP 200 engine all standard spec. except for 12v Varitronic.
Total rebuild original GP200 cylinder, stub no6. rebored to 66.6
N.o.s. 3 ring Asso piston from Rimini.

Started running in, at 340 miles riding at 40 mph she locked up.
Cylinder off piston had nipped up by exhaust port.I had fitted new high tensile cylinder studs which i thought had caused the heat seize,
so i had the shaft reduced to 7mm.
Continued running in and at 970 miles riding at 40mph up a 1/4 mile long hill locked up again.
Cylinder off piston had nipped as before, but the underside of piston is black with heavy build up of carbon. the conrod small end and bearing are showing overheating colour.
It looks like i have been riding for the last 600 miles at the point of heating seizing.

My question is would it be safe to use 2 piston rings (less friction less heat)

Spec. Piston to bore .003
Ring gap .012
Squish 1.1
Scootopia sh2/22 carb
choke 50
pilot 45
main 123
jets are Delorto.

regards phil.

I hate to say it but I don't expect the 2 or 3 rings used to be the issue that is causing you problems. I would look towards this potentially being a timing or carburation issue (including such issues as fuel starvation). Standard models used 3 ring pistons without issue and I've done thousands of trouble-free miles on 3-ring pistons.

Good luck getting this resolved.

Adam

I had a similar problem with my silver special, 150 iron cylinder. I had it re-bored in a M/C shop in Lincoln, I measured the clearances and ring gap, all fine. Ran it in carefully for around 400 miles and opened it up a bit and it seized. Took it apart clean it up and ran it in for a couple of hundred miles more and it locked up again. So I took it to Chiselspeed and Martin measured it up and was surprised to find the bore was fine at the top and bottom but too tight in the middle! How they managed it we did not know but it goes well now.

some cylinders run with one ring....

Give the bore another thou clearance, 4 thou would be better, My preference would be to run with 3 rings fitted, I know some people run with 2,but I have seen ring lands collapse, also the cylinder stud diameter is not so much of an issue on cast cylinders as they not enclosed and are cooled via the flywheel and have more room to expand, hope you get sorted, Good Luck

I think that there are many misconceptions of how best to run in a fresh bore. There are a lot stated on this Forum that would have you increase the oil ratio thus making the fuel/air ratio leaner!

My belief is that the engine should have a constantly varied speed, that is, be prepared to rev it, rather than labour it. After all, particularly with a fan cooled engine, how is a "quarter throttle" going to blast enough cooling air over the cylinder up a hill, where it will tend to be loaded & overheat? Far better to change down to third, or even second gear to keep the load varied & the cylinder cooled.

I doubt the rings have caused any issue, though the clearance of only three thou' is a little on the tight side IMHO.

With luck, the piston will clean up & the bore will benefit from a light hone.

If you fit just two rings, I think it better to leave out the lower ring to act as an oil groove.

If I were you I would take the barrel to a reborer that can accurately measure the bore and have it bored to 3.5 thou.

I'm still not sure that the bore or the piston has anything to do with this issue.

Adam_Winstone wrote:I'm still not sure that the bore or the piston has anything to do with this issue.

No,neither am I,but at least having it accurately measured and then rebored to the required clearance precludes that as being the culprit.

^... indeed

Thankyou all for your replys.
Update:
The piston and bore have cleaned up well and a light hone,
As i was hoping to keep as original as possible have re-fitted with the 3 rings.have put new small end bearing in.(will try and post pics)
Will continue running in for about 100 miles,then open up and check piston for overheating,if it has will clean up and refit with 2 rings.run another 100 miles and check.
Have measured piston to bore in various positions and all check at 0.003".with reference to Stickys manual (0.002-0.0025) for original cylinders and 0.003 for tuned cylinders,it shouldnt need a rebore.
I have changed main jet to 125 and it is now running quite rich. will try and get a dyno when available.
The timing has been checked and strobed all as Varitronic spec.
There are no air leaks. The Cam Lam fast flow tap checked.
Starts usualy 2nd kick
Will post any news.
regards phil

IMG_20180618_154147 by [url=https://flic.kr/p/28nsnHU]IMG_20180618_153806_BURST001_COVER by [url=https://www.flickr.com/photos/163013955@N05/]

phil99 wrote:Thankyou all for your replys.
Update:
The piston and bore have cleaned up well and a light hone,
As i was hoping to keep as original as possible have re-fitted with the 3 rings.have put new small end bearing in.(will try and post pics)

The timing has been checked and strobed all as Varitronic spec.

It wouldn't do for us all to be the same in what we strive for, as far as 'originality' is concerned.

However, as far as what is obvious to anybody looking over our beloved Italian shopping machines, one modification easy to spot will be the ignition, rather than the piston.

(Don't get me wrong, or think I am criticising, because I have been using retarding ignitions &'even single ring pistons since the 80's, so realise the advantages of both)

But when it comes to a 'period' looking machine, my preference is to resign those old SX & GT barrels & particularly the three cast iron ring pistons to the back of the garage & take advantage of the increased performance of a bored barrel/Kawasaki H2 piston (or even a Rapido/Suzuki TS piston) set up that gives a huge grin factor, whilst persevering with the so called 'unreliable' original points system, admittedly with a Vega flywheel.....