The ambitious GT750 build!

[dude99]

Well ambitious for me that is.

Just wanted to start off by thanking Suzsmokeyallan for all his time spent troubleshooting my registration issue so I could begin posting here. So without further ado, here it is:

I've been looking for a GT750 ever since I sold my favourite bike, my 1981 RD350LC. I replaced it with a 1978 GS750, which I still have and love, but have always liked the look of the engine on the GT, and I miss the two stroke fun. So I managed to track down a fellow selling this one, and he has all the bits I need to make it complete. So here we go! I've built bikes from spare parts before, but usually I start with something a little nicer than this one. Still I'm very excited to finally be doing a GT....

So here it is, as I brought her home








Seems its been off the road for a while.....


The seat, and a spare crank and set of cases. The top half on the one in the bike is damaged from it throwing a chain at some point.




He also through in some fenders that were in better shape and told me to send him a list of anything it was missing and he'd dig the bits I need out of his stash

[dude99]

So after the rain stopped, the next weekend I sank my teeth into it and began to tear it down





They always look sad when they're just a frame


The engine lump awaiting my teardown


I'm planning to tear it all the way down and put in new crank shaft seals. My RD350LC that I had before grenaded (to put it mildly) shortly after I got it due to a crank seal that failed. It leaned out and blew up before I could get it home. I'd rather not blow this one up so new seals will be a must. While I'm in there I'll throw in some new rings and have the cylinders honed as well.

Look! its a GT750 jigsaw puzzle!




So after I got it all apart I noticed a few interesting things:
The MFG date says 10\74 so I had initially thought that it was a 1974 model. However the VIN# is GT750-58362 which from what I've read means its a 1975 model. Is it a 1974 or 1975???

The engine that came in the frame is stamped gt750-63704, which I think means that the engine is from a 1976 bike. The bike threw a chain at some point and smashed part of the upper case and the the chain cover. So the fellow threw in a set of engine cases with a crank in them, they are stamped t750-46297, which I think means they are 1974. I know that the engine was revised in 1975 for a higher power output. Was this all done with revised porting? From what I've gathered the 1974 and up models have the same cases, so using the good top half shouldn't be an issue.

I'm sure many more questions will follow... since this is only the beginning...

[tz375]

10/74 manufacture is for the 75 season and the engine and frame numbers are both 75 model year (M).

76 (A) started at engine number 67558, so it's all 75.

The spare set of cases is a 74(L) and are identical to the 75s. Pipes look like they do not have cross over tubes, so that also points to an all 75(M) model.

Do you need an oil tank by any chance? I probably have one here.

[Alan H]

Looks like plenty of work to do. But a good sterter project. Lots of spares available.
When you refit the exhausts, all the hanger brackets go on the INSIDE of the footpeg hanger, so the exhausts are closer into the bike.
I don't think you'll be bothered about that for a bit though!

[dude99]

I will be needing an oil tank, however, part of the deal I struck with the previous owner, is that he will supply all the parts I need to make it whole again. He's a collector\restorer with literally hundreds of bikes. He had several GT750's in various shape (mine was the best one he had left) and many rubber made bins of parts for them. So after I tore it down I fired him off a list of all the missing\broken parts.

That all being said, if I end up needing one, I'll definitely send you a message.

Thanks for the info on the years. All the serial number info I found must have been wrong then, or just too generic.

[Alan H]

Just noticed on your earlier post - you can't use a top half case from one engine and a bottom half case from another as the cases are made and bored together and so, manufactured as a pair.
Alloy welding can make it look like new though.

[dude99]

Looks like plenty of work to do. But a good sterter project. Lots of spares available.
When you refit the exhausts, all the hanger brackets go on the INSIDE of the footpeg hanger, so the exhausts are closer into the bike.
I don't think you'll be bothered about that for a bit though!

Good tip! Yes, it might be a little while... but hopefully not too long...

[dude99]

Just noticed on your earlier post - you can't use a top half case from one engine and a bottom half case from another as the cases are made and bored together and so, manufactured as a pair.

Ahh! More good info that will probably save be hours of grief. I have both the top and bottom '74 cases so I'll just use those. Thanks!

[dude99]

So here goes Day 2, I decided to tear down the engine to see what lurks within....

It's shiny on the outside, so the inside must be too right?


First off came the head, it looks decent. No signs of previous catastrophic failure.


The bores don't look too bad at this point


Amazingly the cylinders just lifted right off with only the wiggle of a screwdriver. I knew it had been apart before though, mostly because of all the orange prematex all over it. Then things went downhill...

Ahh crap... I guess its going to need a rebore






The wrist pins all have what I would consider to be a lot of wear on them a well


One of the pistons had a really odd wear spot on it


I pulled all of the case bolts out, as well as the starter, oil pump, and oil spider. Decided to leave it there for tonight.


This is always the part of the build, no matter how many I do, where I wonder if I'll ever get it all back together again

[Warehouse1001]

And that's what makes it so much fun. Starting a build myself. Keep the pics coming!

[dude99]

Oh, don't worry, more pics will come. I'm currently surprised at how cheap piston sets are for this bike. The price for all 3 is less than half of what I paid for 2 pistons on my RD350LC.... The beast will definitely live again... at some point...

[Drewski]

Nice project
You're cylinder sleeves are about as off-centre as mine, also a '75....something to bear in mind at rebore time.

[dyrberg123]

Looks like a really great project, and lots of spares and lots of loads of info too...

[dude99]

Nice project
You're cylinder sleeves are about as off-centre as mine, also a '75....something to bear in mind at rebore time.

I took a close look after you said that. The sleeves are not actually off center. The head gasket was slightly so it left darkened rings. The sleeve itself appears to be centered on all the bores.

Looks like a really great project, and lots of spares and lots of loads of info too...

It's a lot to take on, more than I usually do, but I've always wanted a GT750 and with their prices going up this seemed to be the only way I was going to get one.

I've got the cases apart now and have been cleaning them up, I'll post some pictures of that later, the crank seems good, as does my spare one so I'm confident I can get one good crank out of the two of them. Hopefully I'll drop it off at my machinists on Thursday and at the same time have him confirm how many sizes over stock I'll have to bore the cylinders out to.

[dude99]

Dropped the cranks and jugs off at my machinists yesterday. Hopefully I'll be able to start the engine rebuild soon.

The bores (according to him) look like they might be serviceable at the stock 70mm. So he's going to hone and measure them and then call me to let me know if I have to go over size or not. SO in the mean time I suppose I'll turn my attention to cleaning up the frame and all other odds and ends.