Opening up a 750 GT engine after 44 years

L

lassegeorg

New member
Hello, I have been lurking around for some time, but now I feel it is time to step forward. I have owned my 750 GT since 1974. In my youth it was my main (abused) transport. My girlfriend and I went on vacations on it. Then the kids came, and the bike went into the shed. I could afford a car. Some years ago I bought a Triumph Trophy and put a Daytona engine in it. The Triumph is though a completely different matter, and I want to get the Laverda going again.

I have posted some pictures in the (Twin) gallery showing the gear case. I would like some comments on the wear of the teeth on 5th gear, and 3rd gear.

I have sent the crank to a reputable company, but did not take notes on the alignment of the O mark, cranks, camchain sprocket, and starter wedge. I would like to double check this before putting it together again. Can anyone enlighten me?

Best regards Lars
 
Hi Lars, and welcome!

Reckon 5th gear on the layshaft is a bit shot, I'd replace that.  4th gear mainshaft has chips taken out of the teeth edges, seems its bush is excessively worn or the entire gearbox was/is not correctly spaced.  Both 3rd gears look quite ok.

Far more important are the various bushes in the gearbox, these can wear considerably and can cause block-ups and/or smashed gears!  Shouldn't be taken lightly.  Get hold of either a 750 or 1000 owners manual, a chapter in each covering various service limits and clearances, gearboxes in both are more or less identical.

Crank overhaul is quite straightforward for somebody that knows what he's doing, if your "reputable" company has any queries, consider taking the crank away from them!  Depending on future use of the bike, also consider replacing the drive side ball bearing main (if so fitted) for a roller.  Later engines all had rollers on the drive side.

piet
 
Hi Lars - The crankshaft is held together with a splined shaft - the camchain teeth are fixed to this shaft and will not move independently of the shaft - providing the webs go back in the same position they came out - you should have no problems. I usually work on one half of the crank - then replace it and work on the other side - not the way everyone would do it, but makes sure you have no mistakes and the same spline goes in the same female spline. - Regards - Phil
 
Thank you for kind and quick answer. I have uploaded four more pictures om the twin gallery. Picture 1383 shows an interference between ,I think, main shaft 3rd gear and layshaft 4th gear. Oh, the bushings seem OK, but the layshaft seems to be too much to the left. On picture 1385 can be seen layshaft 1st gear is coming very close to (and touching) the left hand main shaft bearing. On picture 1387 is missing a shim, has never been there, I can fit a shim 0.35 mm. The interference mentioned above seems to be more though. On pic.1389 there is a ring between gear and inner bearing race (left end of layshaft, and the bearing will be replaced). This ring is not in the owners manual drawing, is this a very early version? I would like to shift the layshaft to the right, but will 0.35 mm suffice? The bike has done 41000 Km's with this, amasing.

Regards Lars
 
Thought I should update the story. Today I got back my rebored cylinder, including new pistons, after they have been lost for two weeks in the post system. I was really nervous! I got the crank back two weeks ago in a beautiful state, but package no 2 (with the cylinder and pistons) was lost!
I take your advice Piet, am ordering 5th gear, bushings and shims for the gearbox. Since the engine is apart there is no reason to not doing it right throughout.

I have a question. The right end of the layshaft has a seal, but the end of the shaft just goes through, into a very harsh environment with a lot of dirt and grime. There is no reason for the shaft end to be on the "outside" of the engine. My idea is to shorten the right end of the shaft, just leaving the circlip groove intact. Then replace the seal with a "Simrit sealing cover" GA EVTOx40x0x7 75 NBR 99004. It should fit without any further machining, I think. Has anyone tried this, if so, did it work?

Second question, I cannot find a figure in the green book of the axial play required in the gear selector system. There is a nut on the left side of the selector drum that is adjusted to set this. What is correct axial play?

Regards Lars
 
Hello Lars,

From memory, the selector drum play is set by shims and not by that large nut which just fixes the drum to its shaft. Shims are of course available in various sizes.
I'm not sure that there's any recomended play, you want to test with the box in the bottom case and go through the gears making sure visually you have enough play, including in neutral. I had a box which was fine cold but when hot would make a terrible noise in neutral due to metal expansion.

Paul
 
How do you find pics in gallery seems to be rather chaotic filing system and rather limited,I ve used search for 750GT  but just get an American Eagle
 
lemonjelly said:
How do you find pics in gallery seems to be rather chaotic filing system and rather limited,I ve used search for 750GT  but just get an American Eagle
Click to expand...

In the Twin gallery, pick the pics of a gearbox.
Paul
 
Thank you Paul for your answer. I have not removed the selector drum yet, so let?s see how much wear it is. I would guess the play is 0,8-1,0 mm, haven't measured. I have had no problems, so maybe leave it as is.

Regards

Lars
 
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