GS1000 top end on a gs750. Will it work?

[terry]

Hey Guys, I was going to leave my 1979 GS750 (2 valve) stock, but I keep wondering if I can fit a set of pistons, cylinders and head off a 1979 GS1000? (2 valve)

In my yoshimura book they quote their biggest GS750 big bore kit as 69mm, and as the stock gs1000 barrels are only 70mm, providing the cylinders fit into the crank cases and the stud pattern is the same (it is) then I assume everything would work?

I don't want to do anything radical on this bike, but I thought a bit more displacement might be nice, and if I top it off with a set of 28mm slide carbs off my 1979 GS1000S parts bike, it might be nice?

This all started when I finally noticed that there are a couple of chipped fins on my 750 cylinders, so if I have to go to the trouble of taking them off to repair them, why not do the top end swap while I'm at it? Has anyone done this before? Any hints? :twisted:

[humhead]

Here's some things to keep in mind when contemplating this swap. The bore spacing is the same, so the 1000 block will fit.....until the last inch or so. I tried this with a 750 case & 1000 block & it looks like some machining is in order. I think the 1000 wristpins are larger, so The 1000 pistons won't fit. 850 pistons should work. I've read about people using 1mm over (70mm) 850 pistons to get ~ 870cc. You might want to compare the deck height (depth of cylinder blocks) because of the longer stroke of 1000. The 1000 head will lower your compression, you'd have to mill it. You should be able to use the 750 head & cams. Compression will increase, but if you use premium it should be ok. I hope this helps.

Terry Loftus

[earlfor]

Seems to me the easiest solution for more power would be to just drop the 1000 engine into the 750 frame.

Earl

Here's some things to keep in mind when contemplating this swap. The bore spacing is the same, so the 1000 block will fit.....until the last inch or so. I tried this with a 750 case & 1000 block & it looks like some machining is in order. I think the 1000 wristpins are larger, so The 1000 pistons won't fit. 850 pistons should work. I've read about people using 1mm over (70mm) 850 pistons to get ~ 870cc. You might want to compare the deck height (depth of cylinder blocks) because of the longer stroke of 1000. The 1000 head will lower your compression, you'd have to mill it. You should be able to use the 750 head & cams. Compression will increase, but if you use premium it should be ok. I hope this helps.

Terry Loftus

[terry]

Seems to me the easiest solution for more power would be to just drop the 1000 engine into the 750 frame.

Earl

Here's some things to keep in mind when contemplating this swap. The bore spacing is the same, so the 1000 block will fit.....until the last inch or so. I tried this with a 750 case & 1000 block & it looks like some machining is in order. I think the 1000 wristpins are larger, so The 1000 pistons won't fit. 850 pistons should work. I've read about people using 1mm over (70mm) 850 pistons to get ~ 870cc. You might want to compare the deck height (depth of cylinder blocks) because of the longer stroke of 1000. The 1000 head will lower your compression, you'd have to mill it. You should be able to use the 750 head & cams. Compression will increase, but if you use premium it should be ok. I hope this helps.

Terry Loftus

Thanks Terry, and I agree with you too Earl, but sadly I don't have a complete GS1000 engine to drop into it, and if I did I'd use it to rebuild my 1979 GS1000S parts bike, plus my intention was to keep my 750 looking like a 750, kick start and all.

Here's another question though, does anyone know will my GS1000 crankshaft fit in the 750's cases? I guess in theory that would resolve some of the problems highlighted by Terry, and would also up the capacity to a full 1000cc. (or close enough for you "train spotters", ha ha)

If the cases have to be split to machine them out for the cylinders anyway, the crank swap shouldn't be all that difficult? In fact rather than tearing down my good low mileage 750 engine, I think I know where I can get hold of another 750 engine that isn't as good as mine. What do you think guys, can we do it?

[Anonymous]

Seems to me you're the man to try it!

Me, I'd be inclined to keep the 750 a 750. Maybe fit the 850 top end, or the 844 kit.

[humhead]

Me, I'd be inclined to keep the 750 a 750. Maybe fit the 850 top end, or the 844 kit.

I agree. The 844 kit is the easiest & cheapest way to go.

I think I know where I can get hold of another 750 engine that isn't as good as mine.

Good idea. As long as the bottom end is in good shape, send this one off to the machinist for enlargement. The only question is: 844 or 870?
The 1000 crank has helical cut primary gears. The 750 crank has straight cut gears. The primary ratios are different as well. That rules out a crank swap.

[terry]

Me, I'd be inclined to keep the 750 a 750. Maybe fit the 850 top end, or the 844 kit.

I agree. The 844 kit is the easiest & cheapest way to go.

I think I know where I can get hold of another 750 engine that isn't as good as mine.

Good idea. As long as the bottom end is in good shape, send this one off to the machinist for enlargement. The only question is: 844 or 870?
The 1000 crank has helical cut primary gears. The 750 crank has straight cut gears. The primary ratios are different as well. That rules out a crank swap.

Hey not so fast Terry, I've got a quite reasonable stash of bits for both engines, so if I use the helical gears from a GS1000 crank all the way to a GS1000 clutch etc, won't it still work? I thought you could convert the primary drive gears on a GS1000 using GS750 items?

I know that it'd be easier to go the big bore option or do an engine swap, but I like to tinker with my bikes, and have an excellent home workshop with my own lathe, mill, welding and painting equipment, and here in Victoria, Australia we are heading into winter, so I've got "opportunity and motive" to do this before summer.

So my intent is to make a "mock up" with the remains of 2 engines, and if I can get it all to work then I'll strip the hybrid engine and rebuild it as one of my "pre-summer" projects, along with the GS1000E/S special I'm mid way through. I'll take some digital pics along the way, and once I can find a free URL I'll post them if anyone is interested?:twisted: