The GT is dead....and some dumb questions. :(

[bonester]

Yup the mighty GT550 holed a piston (#3) last weekend. Was losing compression in that cylinder for ages. I guess I asked for it:
33 degree day
towing the GN250 and loaded with bike gear etc behind it @ 100km/h
hit 'reserve' which I'm guessing caused the leaness which killed it.
It spewed oil out everywhere- weirdly from the exhaust flange of #3 it seems. Musta not been sealing there very well. I used that cement-like gasket stuff for exhausts to seal the headers- all seemed to crumble. Next time silicone. Got freakin hot too.
It still managed to do another 60km like that and pulled 500kg (230kg GT, 120kg me and 180 or so GN and trailer) up the Toowoomba range which is a 10% grade.
I hope I haven't caused too much further damage. I did a compression test on the engine of my '83 wreck and it was all over the place. I don't want to pull apart a complete running bike (my '87) just to get an engine for the '86, so have decided to fix the current engine. Last night I pulled the '83 engine down and she has a beautiful set of pistons. So am going to use those and the barrells to get her going.
How do you tell if the pistons are oversize? They have no numbers on them. I'm guessing you measure them with callipers, which I'm not too good at. Gotta buy some new rings.
How do you measure the bores to see if they need boring? I hope they don't! Going to be an expensive fix otherwise.
The bit of the piston that blew out musta got jammed between piston and head- there is a witness mark on the outside of the piston- actually bent the top ring land down to hold the top ring tight. It has some small marks on the head, but the valves seem alright.
Would you worry about the marks in the head? How do you check the valves for bend? How do you seat or grind the valves? I guess I should since I'm in there. Would you be concerned about the bits of piston going through the engine and causing further damage? There is a little scoring on the camshaft bearings, but not too much. Thanks for any advice. Reckon this one is gonna keep me busy for a while. Rough guess on cost is $500. Last time I did rings, gaskets and o-rings for the topend cost me $430 but I broke two sets of rings assembling it. ($40 a throw.)

I started it up the next morning and it blew the oil sightglass 20 metres. Musta had some fuel leak into the sump. Hehehe!

[Gosling1]

Yup the mighty GT550 holed a piston (#3) last weekend. Was losing compression in that cylinder for ages. I guess I asked for it:
33 degree day
towing the GN250 and loaded with bike gear etc behind it @ 100km/h
hit 'reserve' which I'm guessing caused the leaness which killed it.
It spewed oil out everywhere- weirdly from the exhaust flange of #3 it seems. Musta not been sealing there very well

Mate I would say it wasn't sealing there at all...... I used that cement-like gasket stuff for exhausts to seal the headers- all seemed to crumble. Next time silicone.

NO - Next time use *genuine exhaust gaskets* - leaking exhausts will burn out your exhaust valves eventually.....using cheap stuff in important areas is false economy.....

Got freakin hot too.
It still managed to do another 60km like that and pulled 500kg (230kg GT, 120kg me and 180 or so GN and trailer) up the Toowoomba range which is a 10% grade.
I hope I haven't caused too much further damage.

I did a compression test on the engine of my '83 wreck and it was all over the place. I don't want to pull apart a complete running bike (my '87) just to get an engine for the '86, so have decided to fix the current engine. Last night I pulled the '83 engine down and she has a beautiful set of pistons. So am going to use those and the barrells to get her going.
How do you tell if the pistons are oversize? You will need to measure them with a micrometer - individually, so when the re-bore is done to the block, the correct pistons are fitted to each cylinder...

They have no numbers on them. I'm guessing you measure them with callipers, which I'm not too good at. Gotta buy some new rings.
How do you measure the bores to see if they need boring? You need to use an internal micrometer to measure the bores, take about 3 readings from the bottom to the top, and turn the micrometer 90deg between bottom, centre and top reading. Whoever does your cyclinder boring will know how to bore the cylinders correctly (if required)...

I hope they don't! Going to be an expensive fix otherwise.
The bit of the piston that blew out musta got jammed between piston and head- there is a witness mark on the outside of the piston- actually bent the top ring land down to hold the top ring tight. It has some small marks on the head, but the valves seem alright. You will need to pull all the valves out and check them for straightness...

Would you worry about the marks in the head? Not unless they extended into the gasket area, or valve seat area....

How do you check the valves for bend? Easiest way is to roll them on a glass pane, with a light on the other side....or you can put them into a drill press, rotate slow with a dial indicator to check any run-out on the valve-stem...

How do you seat or grind the valves? Valve-grinding paste, and one of those old suction-style valve holders. Its time-consuming, and you have to give the head 2 or 3 good cleans later, to ensure every tiny trace of paste is removed from the head....

I guess I should since I'm in there. Would you be concerned about the bits of piston going through the engine and causing further damage? Not unless you can see any visible damage....make sure you clean out all the oil galleries with compressed air before you kick 'er in the guts !! I would also take the sump off, and do an inspection from underneath in the gearbox, you don't want any bits left down there....


There is a little scoring on the camshaft bearings, but not too much. Thanks for any advice. Reckon this one is gonna keep me busy for a while. Rough guess on cost is $500. Last time I did rings, gaskets and o-rings for the topend cost me $430 but I broke two sets of rings assembling it. ($40 a throw.)

I started it up the next morning and it blew the oil sightglass 20 metres. Musta had some fuel leak into the sump. Hehehe!

Good luck mate. With all the engines you have, keeping the costs <$500 should be easy....

[bonester]

Thanks Gos! Ordered parts today- $350- gudgeon circlips, 4X sets rings, base, head and valve cover gaskets, and all o-rings in there. Thought might as well do it properly. Piston to cylinder clearance is about 3 thou. Tight enough I'm told.

[Gosling1]

3 thou sounds OK, its a rule-of-thumb for 1thou per inch of cylinder bore (approximately).

Hope it all goes well, $350 is a bargain mate !!!

New plugs !! Maybe a range colder ? while the weathers hot and you are lugging big loads around anyway....

[bonester]

Colder plugs- good call! Never thought of that. I was blaming leaness from hitting reserve. Hmmmmmmm.