Ok who's the Resident 750 GT Specialist.

[markfoggy]

I've been asked to give some advice/help on a rebuild project. 

It's a long time since I spent much time messing with a Stroker, this particular one could be very interesting.

Something is afoot down it's brand new bores and it's not looking pretty. 

[Fula28]

You need to speak to a good chap called James walker aka. BDK race engineering  mark he has many himself and tours on one raced one loves them with passion ! 

James@bdkraceeng.co.uk

[markfoggy]

Thank You, could be what I need.

[johnr]

theres also a guy on the kat club forum who is fitting a gt750 kettle motor into a katana frame.....

[vizman]

...here too.....probably the same dude...

[johnr]

most likely, ive not seen his post here tho, i dont go in watercooled very often, dark, scary evil place that it is.....

[markfoggy]

I've just been in the 2Stroke section of another place, been there 7 years without ever entering, :Shudders:

[markfoggy]

This is what I'm interested in

What is going on here?

[johnr]

looks like someones run a hone over a worn barrel to try and remove score marks. the gouge might be from a piston ring securing pin coming adrift, or a broken ring perhaps. only way to find out i suppose is to take a look at the barrel and the piston, but thats looking like it needs more than a hone. iirc you can go upto 2.5mm oversize on them, but the limiting factor i suppose will be if you can get the pistons from anywhere.

[markfoggy]

As it goes, this is a newly rebuilt engine that has been Bench run. From a supposed expert builder. New bore, new pistons, rings, well everything......cases up build, beautiful to look at.

The head came off as the result of a minor build error. Bad spark plug thread. This is what was revealed, interesting. 

 

[johnr]

well, it looks like a newish piston,  the marks on the bore show the limit of the pistons travel, well, the limit of the rings travel. if it had been freshly bored i would expect to not see the wear marks of the previous piston. having said that, its possible that it was built reusing the existing piston and bore. you'd have to look at the bill from the builder to see if he had been charged for rebore and pistons, or whether he just honed and rebuilt it. having said that, whats the compression like? im no expert, so dont take my word as gospel. but, if that were my engine, if i had it apart and was confronted by a bore like that, id be looking for the next oversize pistons. just my opinion. see, i told you that watercooled was an evil place......

[teltwosheds]

looks to me like the clearance is too tight. easy done on two strokes.

[markfoggy]

Yeah, Piston Gaps is all I can come up with. I'm gathering data, these pistons can go back to the builder and he can supply next size up pistons and rings. Then I'll get it bored to a dimension that I can trust and reassemble it.

I was thinking run it again and have had similar advice, but this is race heads talking, what might work for a quick Sprint race at the end of a meeting is not the same as a potentially 25k engine build.

I've got myself sat on the naughty step for even considering it.

No barrels off and away for more work and then I can build it with real care.

[Solcambs]

It's hard to tell from the photo whether the hones cross hatching is going through the scores, or vice versa.  I suspect a mixture of the two as there is one (the larger one centre stage) that looks like typical of poor (over tight) clearances.