GT 750 MOTOR REBUILD ?

[aussie gt]

When looking to strip down bottom end to clean and check engine, is it better to spend the money on all crank seals when the motor is this far apart??. Also how bad do the pistons have to be before replacing them??

[oldjapanesebikes]

When looking to strip down bottom end to clean and check engine, is it better to spend the money on all crank seals when the motor is this far apart??.

That is a bit like asking 'how long is a piece of string ?' If they have never been done then I would, but it isn't cheap.

Also how bad do the pistons have to be before replacing them??

Its not the pistons you normally need to worry about - its the bore that wears. Have a mechanic measure the bores - with that information in hand you should be able to make a decision.

[aussie gt]

Thanks, I will check the bores to see if they are to spec. I thought that maybe being a two stroke, the pistons may have a used by date(ever played with two strokes before). By the way, i am a mechanic (heavy vehicle )

[oldjapanesebikes]

I thought that maybe being a two stroke, the pistons may have a used by date(ever played with two strokes before)

No worries ! Its a broad generalisation, so take it with a sack of salt, but because of the ports you tend to see higher rates of wear in 2 stroke bores than in 4 stroke engines with similar mileage.

[aussie gt]

Starting to get a picture of two strokes. So some times you get pistons with marks down the skirts, is this from the ports wearing pistons or heat from wrong fuel/oil mixtures. cheers

[ConnerVT]

Marks?

If it is scoring -- Piston is seizing in the bore. Lean mixture, or improper warmup. Aluminum piston expands faster than the steel liner.

It it is dark blowby -- worn rings, improper ring gap, or bore too big for piston.

[oldjapanesebikes]

Some scuffing isn't an issue - its normal, and likewise some sign of blow by is also not unusual. Abnormal would be blobs of smeared metal, signs of tearing due to partial siezure or the whole side of the piston coked up with carbon from blow by. Its more a question of degree - and if the bore measures to spec, then re-use of the pistons is an option. Take a look through the service manual - piston section starts at this link.

[aussie gt]

Thanks for the info conner vt and Ian, I will go away with this wealth of knowledge and try and put it into practice. The only problem is the heat (its bloody hot this time of year, in this part of the world) . Iam shore i will be asking many many questions to get my (basket case) gt750 back going again. cheers