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550 head on a 750 motor?

  • Thread starter Thread starter isaac
  • Start date Start date
I

isaac

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Heyo! I've come to the conclusion that I need a new head for the GS750 and have found a 550 head for a great deal, but don't know if it'll fit. It's an 8 valve head and this is a 16 valve 750, but they still seem to share the same components, just are different up top.

Which other heads might work on a 16 valve 750? Thanks.
 
Jim, thanks for the link. That was very helpful in showing me diagrams of the two engines in question.

I can see the difference in piston design might be a problem just because the 4 valve engine has reliefs cut and the 2 valve does not, but I'm swapping the other way.

I've noticed the differences between my 850 (8 valve) and my 750 (16 valve) are the kind I'd expect to see between the 750 and the 550. The 750 has much lower lift camshafts than the 850, probably because of the difference in overall valve area. This makes me think that maybe the 550's valves would open too far and smack the pistons, but I don't know this for sure. Aside from there being an obvious difference in valve orientation, maybe the block deck is a minutely different height in the 8 valve engines to take up the difference and allow more room for the pistons to avoid the valves.

Aside from that, they appear the same. The cam chains might be a few links different, which would be difficult to tell, but the cams line up identically, in that their marks are 20 pins apart from each other on both engines.

The bolt patterns also appear the same. The only really obstructive difference (aside from maybe the piston/valve interference) is the combustion chamber diameter. I'm guessing the 550 is probably a smaller bore, so a slight change in the head's CC might be an issue. Of course you could always machine away at the edges of the CCs to clean up what would be a messed up squish area otherwise.

Has anybody matched the two up before?
 
The diagrams don't tell the whole story. The cylinder bores are radically different. 70mm for a 16v 750, 56mm for a 8v 550. The 14mm difference is way too much - the change in combustion chamber size is just one of the issues. A 550 head will not fit on the studs of your 750. That's what jimcor meant when he said
Two totally distinct engines.
Click to expand...
16v 750 engines & parts pop up on ebay from time to time - or you could make yourself a 7/11.............

Terry
 
I can post a pic of an '83 GS550 head next to the cases and block from an '83 750 engine if you want but what Humhead said is right on, similar looking but incompatible engines. The 550 is like a 750 someone tossed in the dryer...you know these things are dry clean only!

/\/\ac
 
Well that definitely makes things more certain as far as compatibility goes. If I'd known that finding parts for a 16 valve 750 would be so hard, there's no way I'd have bought this bike. Of course I bought it thinking it wasn't going to eat its own head and intake cam 2,500 miles later too.

It'd be interesting to see production numbers on these bikes. Just based on what I see on ebay (90% of where I look for parts), it seems like the smaller bikes in the 450-550 range were by far the most numerous, followed by 8 valve 750s, then all the rest, my 16 valve 750 and the 1150 being at the bottom of the heap.

In my 8 months of owning this bike and having numerous problems, I've seen the parts I've needed for it approximately twice. Meanwhile entire GS550s worth of parts keep flying out the door from that place. Hell, just about every time I go on there I see an entire GS450 motor on there for $50 bucks if you're lucky enough to live in Oregon or Wisconsin or some other place thats a few thousand miles from my broken bike.

I've even broadened my search to the oil cooled motors in hope of finding one of those before I find one for my own bike, then just adapting it. Nothing. and believe you me, I've definitely been looking at a 1000 or 1100 swap if a motor becomes available. I'd do that in a heartbeat.
 
Couldn't tell you anything about production #s, but I think a lot of what you're seeing is folks willingness to break up the smaller bikes into parts in an attempt to make more $ than selling the bike whole.

If an electrical system fries it doesn't cost any more to fix a 750 than a 550 or a 450. Folks will fix a 750, it's a keeper, and scrap a 450 or 550. You've got to be committed to the old middleweight bikes. Man, my 550, 54 galloping horses when it was new. What's a new 600 have today? It's got 54 hp in the starter motor. So you've got to be dedicated (a nutcase). :lol:

Hang in their Isaac, you'll find one.
 
I would be on the lookout for a 1100 or 1150 motor, or GSX750R/F but it's more work to fit it properly, my '90 Gixxer 750 had 105 hp at the rear wheel btw. Also check out parts bikes :!:
 
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