Short Stoke Big Bore

[OX88]

Hello.

I have an '88 750J.

Want to keep the original engine and gearbox, but I want some more low-mid range torque.

Anyone on here got any experience in doing a big-bore kit on one of these short strokers?
 

Finding a good bit of info on the long stroke engines, but not on the short stroker.

All info appreciated.

-Cheers

Edited November 13, 2017 by OX88

[clivegto]

Not done this my self but from what I have read it is possble to fit the 1052 crank & the top end. But why bother when it is so much easier just to do the 7/11 thang.

[R1guy]

1 minute ago, clivegto said:

Not done this my self but from what I have read it is possble to fit the 1052 crank & the top end. But why bother when it is so much easier just to do the 7/11 thang.

Only advantage I can see of this is keeping the engine number matching the frame....

[OX88]

Reason to keep the origial engine is I'll have a really hard time getting a 7/11 build legal over here (Norway) and it would be even worse to get it insured. So keeping the impression of a stock engine would save me alot of paperwork, money and time. 

Any increase in HP is a bonus, just looking for a bit more power in the lower rev range.

[colinworth79]

1 hour ago, OX88 said:

Reason to keep the origial engine is I'll have a really hard time getting a 7/11 build legal over here (Norway) and it would be even worse to get it insured. So keeping the impression of a stock engine would save me alot of paperwork, money and time. 

Any increase in HP is a bonus, just looking for a bit more power in the lower rev range.

if it was me I would purchase an 11 motor and frame with the paperwork then swap over all the running gear, bodywork ect onto the 11 frame and then its a gsxr 1100. The j/ k 750 frame is slightly different but not much to notice on the road . me and mate were building a couple of slingshots he had an 11 frame and i had a 750L both frames side by side on the bench and neither of us could see any difference. I know years ago there was an 884 big bore kit available for the 750 motor but not sure if its still listed and fits the j/k engine

[zookrick]

There used to be +3mm overbore pistons available for the short stroke 750, also 1mm overbore ones, by wiseco.

There are still overbore pistons available for the long stroke version, made by JE.

[OX88]

3 hours ago, zookrick said:

There used to be +3mm overbore pistons available for the short stroke 750, also 1mm overbore ones, by wiseco.

There are still overbore pistons available for the long stroke version, made by JE.

Google came up with parts# for both wiseco kits. Might have a lead on the +1mm kit. Thanks alot.

[MeanBean49]

I had a big bore short stroke motor in my race bike. Was 837cc iirc. Mad it an ace motor. Was about 115bhp with ported and flowed head. 

Not sure if you can still get pistons though. Well worth the upgrade if you can.

[canamant]

Big bore 750s are a lot of fun.  Doesn't have to be GSXR specific piston as long as it has an 18mm little end and a reasonable deck height.  You can skim or shim small corrections.  Overly short or long pistons will run you into timing chain issues.

 

Edited November 19, 2017 by canamant

[OX88]

The short stroke pistons/cranks are 19mm in the small end if memory serves me right. But I'm aware of the possibility of pistons from/intended for other engines.

[Cheeky4648]

You could use gsxr1000 pistons and bush the little end down to 16mm. 1000 piston is available in a few oversizes. Stock it's 73.4mm. Deck height dosnt look far off either. 

[TiZiK]

Not sure what's available but I do know that JE (and maybe other companies) will make you a piston in any configuration you want. A friend had a set made tomhis specs and they weren't much more than an off the shelf set. Might be worth emailing a couple companies to see what they'd charge.

[canamant]

15 hours ago, OX88 said:

The short stroke pistons/cranks are 19mm in the small end if memory serves me right. But I'm aware of the possibility of pistons from/intended for other engines.

I only have long stroke engines so my default setting is 18mm !

 

[Rene EFE]

On 13-11-2017 at 8:03 PM, OX88 said:

Reason to keep the origial engine is I'll have a really hard time getting a 7/11 build legal over here (Norway) and it would be even worse to get it insured.

Just don't tell? That's how we do it over here.

[Crass]

On 11/13/2017 at 9:07 PM, colinworth79 said:

if it was me I would purchase an 11 motor and frame with the paperwork then swap over all the running gear, bodywork ect onto the 11 frame and then its a gsxr 1100. The j/ k 750 frame is slightly different but not much to notice on the road . me and mate were building a couple of slingshots he had an 11 frame and i had a 750L both frames side by side on the bench and neither of us could see any difference. I know years ago there was an 884 big bore kit available for the 750 motor but not sure if its still listed and fits the j/k engine

There must be a reasonable amount of difference though surely, as I thought that an 1100 motor in a 750 frame meant you couldn't get the rocker cover off, valve clearance time you need to take the motor out.  Surely with an 1100 motor in an 1100 frame you don't need to do that so the frame must be different.

Wasn't the steering geometry not different on the 11, suggesting the headstock angle is different?

Personally I would think you would notice the 1100 frame on the road, hence why the popular mod is 7/11 to retain the better handling 750 frame - no?

[MeanBean49]

Pretty sure its just the slabbies and the short stroke slingshots that have different frames to the 1100. After that the frames are the same dimensions

[dupersunc]

The 750j/k  are very tight for getting the rocker cover off, and are the smallest frames.   L/m 750, and 1100 are closer, only very small difference in the height of the frames.

steering geometry differences are down to the swing arm length and ride heights.