750gt Engine Rebuild

Discussion in 'Vintage' started by 68c, Mar 25, 2021.

  1. Hello all, I am a newbie who posted in the introductory part, was suggested I raise my thread here, sorry for making things untidy.


    Hi all, I am rebuilding a 1973 750GT motor for my brother. He bought it new from Italy whilst serving in the army in Malta. The motor has been in bits for some years, now down to me to rebuild. Do not know much about these bikes but am impressed with the engineering and quality. Currently sitting with a parts catalogue auditing what I have, at the moment I am trying to source the 14 tooth gear that drives the 28 tooth gear on the contact breaker shaft part 075546910. Seems to be unobtainable, thinking may have to have one made. Any advice, are Ducati likely to give me the drawing and spec?

    Things have now moved on, Bevel Heaven put me in contact with Vee Two who kindly parted a gear out for me, has arrived so that is one problem fixed.

    Have sent the cases off to the welder for repairs, the lug on top of the LH case for the clutch adjuster has snapped off and the alternater cover has the support for the kickstart shaft also broken off.

    I am keen to get this motor built up, my brother was a twenty year old Royal Marine based in Malta, I sent him a photo of the new Norton Commando 750 I had bought, he replied with a photo of his new 750GT he had bought direct from Italy. We both still have our bikes, somehow I feel his investment has worked out better than mine. Over the years his bike suffered from a lack of funds and being used as a daily workhorse, one accident punched the LH footrest through the Clutch cover - he had it welded up by the local welder in Malta, sadly corrosion is now showing so need to get that fixed somehow.
     
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  2. I would recommend Ducati Technical Services in Wakefield or Ray Petty Meccanica in London as well-known Bevel experts, with a good expectation of them being able to put their hands on information / parts for you.
     
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  3. You may find that any parts you need are going to be stratospherically expensive due to the rocketing value of the 'roundcase' models, with asking prices for GTs lately ranging from £18,000-£27,000, depending on just how many drugs Johnny Hopeful had taken before posting! A roundcase 750 Sport Replica! (not an actual Sport, but a GT in Sport clothes) was offered on a leading/respected dealer's site recently for £37,000!

    Similarly, spares that do appear on eBay seem to be a reflection of those asking prices; a scruffy petrol tank for £1750, a kickstart shaft for £2,000, or a seat for £650 https://www.ebay.co.uk/itm/265100739070?ul_noapp=true and so the lunacy goes.

    However, as you've discovered, you need to deal with people like Bevel Heaven to stand a chance of rebuilding a project as you're doing, that is, keeping within reason. There are other people out there that are worth seeking out, like Retroducati in Texas (no website, but contactable through Instagram, or Facebook)

    Other sources, https://www.ducativintagestore.com/two_cylinders (€1100 for a GT headlight) , https://www.oldracingspareparts.com/750-gt-s-ss-1974-2/ in Italy but you're wading into treasure trove pricing with these people.

    Super useful reference site http://www.ducatimeccanica.com/#parts

    Good luck and best wishes with your rebuild
     
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  4. I don't know what your mechanic skill levels are, or whether you have a full ducati worksop manual, and necessary tools/measurement equipment, but these motors are technically difficult to put together from a collection of parts (getting the right shims in the right places for example) is challenging for the uninitiated and small details are crucial to building a good engine.

    Given the high value of the bike when complete, have you considered getting an expert in round case bevels to assemble it right. Won't be cheap, but may pay in the long run. (And this is coming from a guy who likes to do as much on his bikes as possible.)
     
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  5. Sadly we will not be able to build an original bike, too many bodywork parts missing over the years. I think my brother is thinking of getting it up as a running bike, use it for a couple of years then sell on. However I will suggest to him he considers a professional rebuild.
    I was an aircraft maintenance engineer and am fairly handy with tools. I appreciate the need for getting the shimming right, thankfully there is a lot of support on the web from people like Bevel Heaven. Annoyingly the 750GT has a bit of a design fault in that the bevel gear driving the vertical shafts to the cans is mounted directly on the camshaft. This means any main bearing sideplay due to wear causes the bevel gears to lose their adjustment. This has happened to this engine damaging a few teeth. Luckily, and at a cost, Ricambi Desmo had a set of matched gears in stock. The 860 has a better although more complex design as the crank gear is a conventional spur gear driving a lay shaft with the bevel gear. This enables the spur gear to move with any crank sideplay keeping the bevel gear in adjustment.
     
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  6. Hopefully here are a few pictures of work in progress.

    IMG_20210312_152636762.jpg
    Broken kickstart lug, is this common?

    IMG_20210330_184540132.jpg
    Repair, to tidied up.

    IMG_20210312_141017910.jpg
    Broken clutch adjuster lug.

    IMG_20210401_111540524.jpg
    Repaired clutch adjuster lug. The welder went a little further than I wanted and also drilled and tapped the thread. He did this without the adjuster so I hope he is right ii is 8mm.
     
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  7. You could also try Carl Harrison, who is based at Harleston in Norfolk. He has built countless Ducati bevel engines just like this one and often machines his own parts. Worth giving him a call (01379 852020). His website is: www.carlharrisonmotorcycles.co.uk
     
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  8. The large gasket that sits between the crankcase halves has a lot of spare material, is this normal, should I trim it?
     
  9. Just got the barrel back from the welder after replacing a couple of broken fins. Now when I slave fit the rear cylinder I see the new fin hits the clutch adjuster by about 1mm. As in my earlier post the welder had repaired the clutch adjuster lug. Not sure if the problem is he built and drilled the repair too high or that the original fin had a cutaway at that point.
    1/ Can anyone post a picture of this area showing the fin shape and distance to the adjuster?
    2/ How high was the original clutch adjuster thread hole.
    3/ Is it parallel to the crank or is it angled for the adjuster cable.

    Starting to pull my hair out, the welder seems a reasonable chap so do not want to go back and complain if not really a problem. To be fair the broken fin was missing so he did not know if it had any cutout or was shorter as is the next fin down. Still upset that he drilled and tapped the adjuster, he said he did not want me complaining his weld was no good if I damaged it doing the drilling etc myself. Probably a fair point.

    IMG_20210413_151628048_MP.jpg IMG_20210413_151617434.jpg IMG_20210413_151628048_MP.jpg IMG_20210413_151617434.jpg IMG_20210413_153219491_MP.jpg
     
    #9 68c, Apr 13, 2021
    Last edited: Apr 13, 2021
  10. Regardless of where the issue has arisen, surely the easiest fix is to cut away sufficient of the repaired fin. I would cut away enough to clear plus enough to get a spanner on the clutch adjusting locknut that will fit against the lug.

    If it is not original, only a concourse nerd would ever pick it up.
     
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  11. In your shoes, I would prefer to ask welder to fill his hole (fnar, fnar) and re-drill it lower, then reduce the height of his construction to solve the issue
     
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  12. I'm 3000km away from my bike but have my PC and lots of pics. There are two problems: The repaired second fin is too big; it should be the same as the first fin. Second problem is that the clutch adjuster lug is missing a little cut away to insert the cable. I'm pretty sure that the blob on the end of the cable that connects to the lever will not fit through the threaded hole. Yes, the hole is parallel to the motor top, not angled. Here's a pic:[​IMG]
     
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  13. Another that shows the fin a bit better (yes my bikes modded and it's an 81 MHR but this area is the same). The little cut away part of the adjuster hole compromises the integrity of the lug which is probably why yours broke.
    [​IMG]
     
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  14. Thank you Rick,. I will trim the second fin as your photo. I think I will leave the lug as is until I have a clutch cable in hand before cutting the slot. Thanks again.
     
  15. Before you go cutting anything away I'd suggest having a look on Made in Italy's website. They have plenty of bikes for sale with pictures allowing you to make a comparison.
    With the later square case bikes the bottom two fins were cut back but on the round case models it was only the bottom fin.
    The issue you have is that lug is too tall. Looks like a nice job but not quite right.
     
  16. Ah, good job I am a slow worker. It is beginning to look like I need to sort out the lug. Thanks for the reply.
     
  17. You're welcome.
     
  18. Just looked at the Made In Italy Motorcycles website. It does look as if the second fin is also shorter as Rick Ducati suggested. Not a very clear view though. I also wondered why they painted the outer cases black, perhaps they are aftermarket sand cast items rather than polished alloy. Could not find a definitive photo of the fin/adjuster lug clearance, found several that could be interpreted as second fin shorter or the same length. Having fitted all the new bearings to the case I am a little reluctant to give it back to the welder, time to call my brother.
     
  19. Just looked at the Made In Italy Motorcycles website. It does look as if the second fin is also shorter as Rick Ducati suggested. Not a very clear view though. I also wondered why they painted the outer cases black, perhaps they are aftermarket sand cast items rather than polished alloy, covering up damage?
     
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