Engine Rebuild

[es2GT550]

Good evening all, been a while I know but too many bikes !!
Anyway eventually decided to have a proper look at the GT550 - did a complression test - 75psi on all cyliners- pretty sure thats not right lol !!
Took the top end off this week- piston crowns look good , spark plugs look ok, no major scoring on the pistons or bores that I can see. There is defintely some blow by the gudeon pins - will get some pics up. I measured the piston ring to groove clearance and it exceeds the 0.006" in the Haynes manual - so I guess that means new pistons - yet apart form this the pistons and bores look really good ??Confused.
Apart from a lack of power there was no really bad noises from engine before I dismanted. The big ends seem good in terms of movement up and down but how much "end float" or side to side movement is acceptable ? Also the small ends seem a loose fit in the con-rods when the poistin and poin are removed - is this right ??

Thanks
Eamonn

[es2GT550]

[es2GT550]

[barney01]

Hi
How did the bike run before you stripped it down? I ask because 75 psi is very low it should be 100 -120 ish... As everything looks okay it may be a gauge fault. Of course the only way to be sure is to measure the bores and piston rings to see if they exceed limits. There is quite a bit of lateral movement of the conrods and the little end bearings are not a tight fit.

[Gary66]

I sm currently in the process of getting some barrels and pistons ready for my 380. That too seams low on compression at 100psi so I bought 2x 0.5 bored ones off eBay, luckily they look like they were bored but never used! I had a.right side which I am having bored over the next few days. Have ordered up new pistons and small end bearings which will arrive soon. Then over the winter I shall switch over the lot and see what's going on with the originals :-/
Keep us posted on progress and I will do the same

[es2GT550]

Thanks all - the bike started well and ran ok but always seemed to lack power- it struggled to pull top gear on anything than a level road. I need to get it all measured up but apart from the compression problem and excessive ring groove clearance the rest looks ok. Might try a new set of rings and see what it’s like - would rather leave rebuild till winter !

[teazer]

Those pistons appear to be badly worn and in need of replacement. New rings won't do a lot. Since the top end is off anyway, get it bored and fit a set pf Cruisinimage pistons and enjoy the bike.

[Gary66]

Plus one on the above, not worth just putting new rings in to find it's all to come undone again, the time and effort alone makes a rebore a much better proposition as it's all new and good to go

[es2GT550]

Can anyone recommend someone in Lancashire that can rebore the cylinders if I bring the pistons ? Thanks

[barney01]

http://www.grampianmotors.co.uk/
These are in Liverpool and come highly recommended I will be using them soon!