Main bearings

Of all the parts used in a parallel twin, these are on the top of the list of hard to find items. Not a single bearing distributor nor manufacturer has these bearings in their standard supply, so the best (economical) decision is to make them yourself.

The design has been slightly adjusted with more lubrication groves in the flange to accommodate the axial load of the crank, and the groove in the inner diameter is now whole as compared to the original. The first test samples were made of Aluminium bronze, which was used as bearing material in heavy gas powered combustion engines for the crankshaft and camshaft and rocker arms, hens the first reason for selecting them.
The did not pass the test, as the crank locked up twice on the dyno once above 4000rpm. The cause was found to be the wrong material, and this was supported by literature on bearing designs and backed by experience from several engine tuners.

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So with more information I set out to find more suitable material and have at this moment put out quotations. While the test specimens are made, a separate engine will be put together with these new bearings and bolted into leftover GTV frame for dyno endurance testing.

Another trial is converting the crankshaft from plain bearings to ball bearings. This is parallel to the above project to manufacture new plain bearings.
Using the same approach as the Pantah, the left side will have the same steel bearing housing as the pantah (and subsequently all modern beltdrive ducs) together with the angular contact bearing (7207). This is because the angled primary drive pushes the crank to the left under accelertion and therefore almost all of its rotating life.
The right side will get a conventional ball bearing to take the radial load and the occasional axial load during engine deceleration.
The crank does need modding to get the oil into the crank (right side) and to both bigend journals. Oil supply will be from the oil gallery where the oil pressure sensor is located.
right side welded up to add material

left side welded up to add material

15 thoughts on “Main bearings

  1. am on a restoration of a 500GTV, will open crankcase. Do you have the dimensions of the main bearings? Not sure if alu bronze isn’t too hard. A softer self lubricating led bronze might be the better choice?
    Thanks for Support
    Konrad

  2. Hello Arthur,

    your efforts on the main bearing issue are extremely interesting! The original parts appear to be a weak point of design. Oviously, they are subject to easy wear and, furthermore, out of stock. Luckily, I found one pair of NOS parts some years ago (source did run dry). Did you ever thought about initiating a reproduction via a specialized manufacturer? There may be a hundred grateful guys around to buy them willingly.

    In my understanding a flaw of the design comes from the fact that the axial length of the bearing is quite large, especially the groove. I have’t seen worn main bearing yet as I haven’t opened my 500 SD, but recall some complaints of guys using PTs in classic racing. Could you publish a picture with a typical damage?

    Best regards
    Michael

    • Hi Michael,
      So far no real damage of the main bearings except for wear in the radius of the flange to the cylinder: the base material starts to touch the crank. The wear I usually do find is the scoring of the main bearing journals (ridges) and a slight diameter decrease making them a bit too loose in the bearing.

      In regards to the main bearing manufacturing: it got halted at the supplier not willing/able to supply. There are 2 bearing manufactures that I can still contact. Will revert update i have news on that front.

      The bearing length isnt that much of a problem, its more the crank that tends to bend at the weakest/thinnest point: between the web and the bigend journal (500 has most bend compared tot he 350 crank due to the longer stroke). The crank tends to push through the oil film at the edges. I wouldnt make the bearings longer as you suggest, unless the crank was supported in the middle somehow. and seeming not all crankcases have the balancer shaft bosses in both cases I dont consider it as a “universal” option.

      • Dear Arthur,

        thanks for the reply and identifying the weak point of the crankshaft. I did not think about making the bearings longer (axially) but shortening it (higher load; bearing shake, viscosity?). However, if the problem originates from a bending neat the con rod my perception was wrong.

  3. Hi Andre
    I have a similar problem with my DESMO 500 (Mototrans) main bearing and I haven´t been able to find new ones in Spain nor in Italy either. My crankshaft is in good conditions because it was the main bearing what really suffered the damage and that´s why I am so interested in your project. I even planed a change to ball bearings angular contact with a new lubrication design but is complicated without the proper means and instruments.

    • Hi,
      As it happens I have currently modifying a set of cases for ball bearing design! This is more due to the fact my main bearing project is slowing down.
      the right side of the crank gets an oil seal and the M8 hole drilled with 7 or 8mm up to the main bearing journal holes. from then 5mm hole is drilled at an angle to connect the right side bigend with the left side bigend, and the oil channel blocked (grubsrew peened and with loctite) on the right side.
      Next the inspection plug on the clutch cover is replaced with one that has a pipe and extends into the crankshaft seal and a hose connection to the oil gallery at the pressure sensor.
      will keep you all updated of course!

        • Thats why i had the cases welded up as you can see in the pictures above. Also, the 14mm wide 7007 bearing is relatively thin so the load it can handle may be not sufficient. The Pantah uses 7207 and 7307 and has much more power, so using the same bearing sizes such be sufficient, I believe.
          In the left case I want to put the same bearing ring as the pantah because of the higher radial and axial load due to the angled gearteeth pushing the crank to the alternator side. On the right side I plan to put a normal 7207 bearing.

  4. Hello Andre,

    I would like to contact you to ask you for a small help. I need to
    advise about Ducati 350 GTL engine repaire. The engine got stucked during
    race and it is needed to replace main bushings of crankshaft.
    Unfortunately original spare parts are not available on our market in CZ.
    Is that common also for your market?
    How do you solve such a issue? Do you produce the bushings completely new
    or just provide new surface treatment of existing ones?

    I am looking for your feedback. Thank you very much in advace.

    Best Regards,

    • Hi Vitek,
      Same issue here in NL: no main bearings available.
      I am in the process of engineering new main bearings but need to test them aswell, so i am making a test engine for these new bearings for dyno endurance. The only thing is it will take some time for sourcing the correct material.

      I might have some used (but in good condition) main bearings, and a 350 GTV crankshaft if you are interested? You can contact me for further details at amkvt@hotmail.com
      Regards, Arthur

      • I have a few sets of original nos main bearings. Original size. First undersize snd second undersize. I would be willing to trade for parts if anyone is interested. Parts are located in Australia. I bought what was left from an old ducati dealer here. First oversize pistons would be awesome.

        • Hi Steve,
          Thats great news!
          I have 3 engines as donor (2x 350GTV, 1x 500GTL) so if you need anything from that I will gladly trade for 2 sets of bearings (not to be gready)!
          I have a lot of 500 piston rings 78.8mm (NEW!) that can be adapted to fit standard to 4th oversize.
          Unfortunately i dont have oversize pistons…

          send me an email with the parts you need and ill reply with what i have.

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