GR72A (-83 GS750) pondering about big bore, sleeves etc.

[Guest]

Hi there!

It's been a few years since I took a look at my GS, and it seems that I have lost all the paperwork I had for the engine I was planning to build. The idea was to build a "budget" big bore engine, with maybe bigger valves and possibly a performance cam. And an endless list of other mods.

If I remember correctly the cylinder sleeves from a stock GS1000 were the ones I was looking at. Does anyone have the measurements of the sleeves? This is what the stock sleeves on my engine look like:







By clicking the pictures they appear larger, you know. So the length of the sleeve is not really that important, but the OD of the sleeve and the OD of the lip are the limiting factors. The lip seems to be either 78 or 79mm, but how big can you go? Maybe 6mm? And what about the OD of the sleeve? It's 72.6mm at the moment, so how big is too big? I'm a bit concerned with heat issues if I go too big.

My memory tells me I got somewhere around 930cc (perhaps?) with my calculations.

BTW, what ARP studs have you used, if you have, with these engines? The studs are 10mmx170mm, or close, and the only studs I found with those measurements were for a Kwak ATV.

The bike will be a street(tracker/fighter) bike, and forced induction might be an option. The exhaust manifold is pretty much ready, but I'm not sure if I should try it N/A first. I do know this is not exactly the best engine for this kind of stuff but I'll just have to live with it.

So, if any of you kind folks have any information you might want to share with me, I'd be more than happy. I'll post more plans and silly ideas as I remember what I was about to do...

[blowerbike]

just my opinion but i'd slide an oil cooler GSXR/KAT/bandit 1200 engine in your chassis and call it good with less spent and more HP.

[stetracer]

Opps I double posted the same post

[stetracer]

just my opinion but i'd slide an oil cooler GSXR/KAT/bandit 1200 engine in your chassis and call it good with less spent and more HP.

+1 on that I would use the Bandit or GSXR motor. The only reason I shy away from the Katana motors is that it has a heli cut crank gear. The Bandit motor has the biggest bore. I just picked up another Bandit motor from a wreaked bike for $400 so that can be had for cheep

[Samikoo]

Yes, as I said this is not really a smart project. The fact that I live in Finland makes things a bit complicated if I want to keep the bike legal. Engine change is a possibility, if the output increases by 20% and the displacement by 25%. The same rules apply if you mod the engine, but "don't ask, don't tell" is a lot easier if the engine still looks the way it did from the factory.

The prices of engines are a bit steeper here too, and the fact that I would still have a stock engine is something that would be too easy and reasonable.

I'm sort of trying to see what could be done with this engine. I've had the bike since 2002, and the engine is pretty much the only place left unmodified.

This is what the frame looks like at the moment, so no bolt-on for me, thank you very much.



The head might be a bit difficult, or expensive. Stock seats limit the use of bigger valves and a head from a different engine won't be fitted overnight either. If other heads even fit...

As for pistons, 18mm wrist pin is a must. I haven't really looked at rods if there are any stronger ones available. I'm quessing the stock rods will be fine since this will not be a power monster no matter what I do with it.

[Samikoo]

It looks like the max piston size would be 74mm (or even 75mm), either stock or Wiseco. Stock GS 1100 sleeves are 72mm ID, and they just might fit in my cylinder block. I'll try to find the OD of other sleeves.

[Samikoo]

Opened her up today, I'll post some pics of the bearings that might need to be renewed. This is what it looks like, she's done more than a 100k kilometers. The reason for opening the bottom end was a slight whine from 5th gear, but everything looks mighty fine. There were also some minor oil leaks. I think I'll get new bearings for the crank ends, and new seals. And maybe some new needle bearings while I'm at it. I'll take a look at the rod bearings later this evening.



What might be the dimensions of GS 1150 studs? Spending money on my motorcycle is always a good idea, and APE studs seems like a good place to start.

[Samikoo]

Since there was no need to rebuild the bottom end, I'll try to find some spare cylinders or even a whole engine for the big bore project. This engine will be used to test cheap tricks to (try to) improve power output.

The first thing I did today was a 30 degree back cut on two valves just to see what it looked like.



I don't think I'll try to fit bigger valves on this head, but I did measure the seats. A 27mm intake and a 23mm exhaust would be possible with the original seats. I just don't know how much +1mm intake and +2mm exhaust valves would help this engine breath.

Future plans include doing some shy porting on the head, mainly to smooth things a little bit. I'll also do some bowl work and possibly see if the valves need unshrouding.

Was it the GS1150 a.k.a. GSX 1100 EFE that had the "best" cams out of the GS family? Since I try to keep this under a strict budget I won't look into aftermarket cams, not at this point. Or were there other stock cams that would work with the 16V 750 GS?