Petrol Tank mounting

[StewartD]

I have modified the petrol tank mounting of my 1965 Mach 1 so that it is truly rubber mounted.

The standard setup for this model and many other Ducatis of this is era is pretty poor engineering. The front of the tank has two double thickness extensions, 'ears', with clearance holes for the M8 x 1.0 pitch threaded hex bolts. The Clymer manual (p108), says:

'Fix the front part of the tank with two antivibrators, and two dia i = 8.5 washers; then lock with TEC 8 MB screws'.

I had long lost what I think were the original 'antivibrators' - rubber washers, when I started my restoration in 2005. The problem was that the M8 bolts cannot be tightened up properly without crushing the petrol tank ears. Even if you managed to torque the M8 bolts up tight enough so that they didn't come loose after about 4 miles, and without crushing the ears (thus negating the rubber mounting benefit), there would still be metal to metal contact between the outside diameter of the bolts and the 8mm clearance holes in the tank. There is not enough annular clearance for there to be any likelihood of the rubber extruding into the gap.

Spacer sleeves are needed. The setup must provide the tank with rubber isolation in the axial and radial (in relation to bolt) direction.

I first bought some rubber wiring grommets that measure 3/8" i.d. x 11/16" o.d.(hole) x 1" o.d. x 7/16" thick. The 11/16" o.d. is the diameter that the holes in the petrol tank need to be enlarged to. The 1" o.d. of the grommet needs to be nearly matched by the washer that you use. Here's a photo of the tank with enlarged hole,

DSCF4630.jpg

(yeah I know, I should have done it before the nice paint job), grommet and sleeve. The spacers I made are 8mm i.d. x 10mm o.d. and 9mm long. Note the 9mm length will allow the 7/16" (11.1mm) thick grommet to be compressed when the bolt is tightened. This provides the tank with compressed rubber isolation in all three planes. If the rubber is not compressed then the tank ears will eventually cut through the grommets; the Mach 1 tank takes 16 litres which is about 11.5 kg. It is quite a load on these mounts.

You need a couple of 8mm nominal washers with large o.d. - 8mm mudguard washers may do the job or you may need to make them. I think at least 1mm thick would be strong enough to compress the grommets adequately without going cone shaped. The inside diameter should be not much bigger than 8mm so that it is forced to be concentric to the spacer, and will sit flat. Here's a photo of the finished setup:

DSCF4641.jpg

The back of the tank is allowed to sit on a pad of rubber that is taped to the frame. This as a factory setup leaves a lot to be desired. Especially so since on the Mach 1 the seat bracket comes very close to the back of the tank, and if the tank sits a bit too low then it can make contact with the seat bracket. The following photo shows why:

DSCF4760.jpg

Despite appearance the tank is clear of the seat bracket, but not by enough. If I am relying on a crappy bit of 1960s Italian rubber then I think with a full load of fuel, there is going to be contact when we hit a few bumps. My tank is old and has had the inevitable rust problems. I wanted to have a back mount that would ensure the force would be spread over as large an area of the tank as possible, as well as having rubber isolation but with limited movement. Note the standard setup relies on the front bolting to prevent any sideways movement at the back of the tank. An overenthusiastic rider could easily bump the tank sideways with knee contact.

I came up with the idea of supporting the tank over a greater length of its 'crotch'. If the crotch support was inverted vee shaped it would prevent any sideways movement as long as the standard back spring clip was holding it down.

This is the piece of plastic my brother machined to achieve this goal.

DSCF4637.jpg

It has two strips of rubber glued to the sloping sides. The holes are only for screwing it down during machining. Making this was a real headache. The gap between the frame and the tank is not the same going front to back. I had to adjust for a bit of taper with the router. I used plasticine to check the clearance when the tank was mounted as shown here:

DSCF4633.jpg

The final setup had to be a little to bit different to lift the back of the tank a little more. I added more rubber strips and routed grooves so that strong cable ties could hold the plastic block down, the cable ties run under the rubber strips. I have a photo but 5 is the limit here.

In hindsight, I would do this a bit differently. I think a big contact with the tank is desirable but machining the plastic was a big drama for me. I think a small plastic block shaped for the frame tube combined with a long piece of aluminium folded to an inverted vee shape with rubber strips would be a bit simpler to make and adjust for final height.

Cheers,

Stewart D

[Jordan]

Nice detailing, Stewart.

For a rubber buffer under the tank, I was shown a "rough & ready" method used by my Ducati single racing friends.
Cut some portions from an old inner tube, all of the same length - say 120mm. Use one as the outer tube, and roll up the others to fill the space inside it.
This makes a nice cushion, can be adjusted to desired shape, and taped to the frame tube.
Crude, but it's so successful, it's all I ever use now. Can't see it, anyway.

Jordan

[motoital]

Hello Stewart, I like your idea for the front mount. Did you have to spread the ears to use that grommet? Does it help stop the tank vibrating at idle?
On a side note. Is that the correct orientation for the front seat mount nut and bolt? I've seen it both ways in some books. I have the nut on the other side as I was worried it could get sucked into the carb but I always wondered.

[StewartD]

Hi Motoital,

The ears didn't need spreading on my tank. The original 'antivibators' (as Clymer translated it), would have needed a similar amount of space. To assemble the tank to the frame, a bit of a push is required, as the grommets need to compress a bit.

A couple of things I forgot to mention: Before starting this job, I measured the gap between, and the thickness of the tank 'ears'; and the total width of the frame bosses. The centre of the tank ears should end up central to the 9mm width of the spacers when the bolts are fully tightened up. This will ensure that there is equal force of grommet compression to the inside and the outside faces of the ears. If you think there is going to be a discrepancy, you could either bend the ears to suit or use a different thickness spacer. I would be careful about bending the tank ears though.

I forgot to note that when buying the grommets, you should check that the groove in them is wide enough for the thickness of the tank 'ears'

The tank doesn't vibrate at idle.

The books are right! On Tuesday Luigi did it one way, Wednesday, Guiseppe did it other way, on Thursday Guglielmo usually forgot to put it in.

Seriously though that seat bracket is a pain. The seat bracket on mine is about 1mm narrower than the gap in the frame plates. My frame plates have been distorted by trying to tighten the bolt up too much (not obvious in the photo).

Cheers,

Stewart D

[cooperplace]

On Tuesday Luigi did it one way, Wednesday, Guiseppe did it other way, on Thursday Guglielmo usually forgot to put it in.

exactly. When I visited the factory in 1980, workers had flasks of red wine on their work benches. There were guys assembling 900SS engines and stationary diesels at the same time. That no 900SS ever got a diesel con-rod was because they didn't fit, not any quality control process.

[graeme]

And you got what was on the shelf at the time.
A mate had a Darmah that had an imola cam in the rear cyl and a standard cam in the front.
Another mate bought a Darmah, opened the crate marked 900SD and there was a 750SS.

[motoital]

Hello Stewart, thanks for the info. Really thinking about trying this as I get an annoying deep vibe coming from the tank at idle.

I had a 4sp diana that had the same seat mount issues that you have. It had a period correct but non original Italian hump back seat. It would pivot at the front very slightly. I added some very thin machine washers to the front mount. I just painted on some epoxy to the washers. Installed waited for it to dry then just paint it black. Works really good and you can't tell on the bike.

There's a guy named Bob Budshat that owned a dealership. He told me some story's about the factory so that's not surprising to me.

[StewartD]

Hi Motoital, Graeme, Cooperplace,

Thanks for the stories; My friend Bruce visited the factory about 30 years ago. He saw the end of the production line test riders in action. There were a few of them, he said, all chain smoking. The bikes were pretty much stripped of fibreglass and each rider had their own small cylindrical petrol tank that they would put on the machine to be tested, cigarette in corner of mouth. Then it was off round the test track; flat out from the start, dead cold engine, no worries.

Sorry all you guys, who carefully ran in your engines, you wasted your time!

The 600 Pantah I had through the 90s was a quality control disaster. The insulation for the electronic ignition was dissolved by the engine oil. The nice looking, flush mount petrol cap was a big problem - If it was left in the rain, the recess would fill with water and the water would be sucked in through the cap breather hole. The tank was very rusty and eventually rusted right through.

On one occasion in cold weather, the water around the breather hole froze, and the motor kept cutting out due to fuel starvation.

The starter motor sprag clutch would fail regularly.

The brake discs wore unevenly, resulting in pulsing braking. Had the discs ground, the problem recurred. New discs, same problem.

The gearbox shaft snapped. Close examination showed that it was a fatigue fracture, and the starting point was the circlip groove that I think had not been radiused correctly to reduce stress concentration.

The side covers, (This model was the TL Pantah), were connected at the front to the rubber mounted petrol tank with M6 hand screws. The back of the covers were screwed to the frame. Naturally, the movement cracked these covers in a short time. That wasn't a big problem because I had to discard them when I put on normal petrol taps to replace the failed electric petrol taps.

Apart from all that, when it was running, it was a great bike. The previous owner had had the cam timing 'dialed in' and had installed and aftermarket exhaust system. It was very gutsy and responsive. I was always very careful to match the carburettor slide opening.

Cheers,

Stewart D

[gmlaverda]

The factory workers may not have red wine at their benches any longer but when my wife and I visited Bologna for a factory tour 2 years ago, we arrived a little early. We decided to have lunch at a pub down at the corner, where there were 3 guys in orange Ducati jumpsuits at the bar drinking their lunch.

[LaceyDucati]

Since we are on a roll here I'll share my factory story!

I visited the factory only a couple of years ago for the obligatory tour. After a tour of the production line (the fine detail I will not alarm you with) we arrived at two areas full of completed bikes. One large room with rows of bikes and a smaller area to one side with a few bikes in it. At this point our guide stopped us and pointed out that the large area was for bikes to be dispatched worldwide to dealers. Then he went on to tell us that the other smaller area was for bikes for the press and various celebrities. Anyway curious I asked the guide what the difference was between the bikes in the small area and the others for the general populous. His answer was "we check these to make sure they are perfect". I left the factory somewhat concerned about the hundreds of other bikes, deemed not worthy of full inspection!

Nigel