Author Topic: Mark crankcase  (Read 4922 times)

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Offline eden1941

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Mark crankcase
« on: 02 Jan 2012 at 16:02 »
On stripping my Mark 5 engine I find there is a small hole just under the rear main bearing through to the clutch housing.Can anyone tell me what it is for?Also,on the lower fork yoke l h side is a 8 mm approx hole.What is this for?Cheers Mick.

Offline eddie

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Re: Mark crankcase
« Reply #1 on: 02 Jan 2012 at 18:10 »
Hi Mick,
             I fear the hole in the crankcase is a DIY mod that someone has attempted. I have heard of crankcases with minute blow holes that persist in leaking oil into the bellhousing. The best cure is to lightly countersink both sides of the hole, then insert an alloy rivet and peen it over. The 8mm hole in the lower fork yoke is there to take the locating peg for the optional extra steering damper that was listed for the Mark series machines.
             Regards,
                          Eddie.

Offline eden1941

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Re: Mark crankcase
« Reply #2 on: 03 Jan 2012 at 12:16 »
Thanks for the prompt reply Eddie,the hole does have a screw in it at the clutch side.Fig 12 in the workshop manual appears to show this hole with no screw.Mystery.Cheers,Mick.

Offline eddie

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Re: Mark crankcase
« Reply #3 on: 03 Jan 2012 at 13:17 »
Mick,
        The hole shown in Fig 12 of the owner's handbook is one of three that take short studs and nuts to retain the clutch release assembly.
          Regards,
                        Eddie.

Offline David Lawrence

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Re: Mark crankcase
« Reply #4 on: 04 Jan 2012 at 12:54 »
Hi Mick,
It seems that Eddie has covered all your queries, but a thought regarding the mysterious hole, the plus series had oil mist for the cork clutch, this was introduced via a small hole bored through the tail of the crankshaft, it could be that a previous owner had experimented with cork clutch linings and needed a bit of lubrication.
Eddies suggested cure does seem a simple and effective solution!
Have a good rebuild
Dave

Offline eden1941

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Re: Mark crankcase
« Reply #5 on: 05 Jan 2012 at 11:48 »
Hello Dave,you might have hit on something.The clutch does have cork linings but the small hole in the crankcase is blocked off.This hole is right next to a brass bush containing the main bearing.Is this standard? It seems very neatly done.The problem I have is oil pouring out of the clutch housing when the engine is running.I did find a camshaft core plug had moved a little but not enough to leak this much oil.Grateful for any ideas,Cheers,Mick.

Offline eddie

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Re: Mark crankcase
« Reply #6 on: 05 Jan 2012 at 14:53 »
Mick,
        The Mk4 & 5 Douglases had bronze liners in the crankcase to carry the rear main bearing. These liners were in the form of a headed bronze bush. Now, is the hole you have found part hidden by the flange of this bush? If so, then there should be a shallow headed pin in the hole and a corresponding scallop out of the back of the flange to accommodate the head - this was done to prevent the bush from turning. Maybe the bush has been replaced at some time and doesn't have the scallop (which should have a depth about half the thickness of the flange). Normally, the bush stands proud of the crankcase by a few thou, so should get clamped into place by the clutch release assembly, so may not be reliant on the pin to stop it moving. If you don't want to disturb the liner, you could tap a good fitting pin into the hole from the inside and then peen the hole to prevent it from working it's way back out.
    If there is any sign of looseness in the camshaft plugs, remove them and clean the area, then Loctite them in. Then, to be double sure, centre punch around the plugs to make sure they cant move. Plugs have been known to get dislodged and then get caught up in the clutch release. Be careful when rebuilding the motor - do not try to tap the timing gears onto the camshafts as this can force the cams back until they push the plugs out. It is easier to fit the gears prior to assembling the crankcase halves as there is only just enough room in the timing chest to get the gears on the camshafts.
     Good Luck with the rebuild.
               Regards,
                         Eddie.

Offline Mike of Woking

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Re: Mark crankcase
« Reply #7 on: 06 Jan 2012 at 14:13 »
Mick,

Are you sure the hole that Eddie refers to goes all the way through from the bell housing to the crank case chamber ? I currently have my engine split  and can confirm that my hole is blocked off. Are you sure that this is the source of the leakage or could it be that the oil seal is leaking or the flange on the back of the clutch release mechanism is loose and not sealing.
The oil seal should be 49mm OD, 30mm ID and 11mm thickness x R4 (metal housing). However this is not a standard size nowadays. I managed to get (today) from BRT Bearings Hanworth Middlesex  (0044208 898 8561 also on the web via google)the following: 50mm OD, 30mm ID x 10mm thick R4. This is perfect fit on the crankshaft, but means you need to open up the clutch release ID from 49 to 50mm bore. No problems here - any machine shop can do this - more importantly it means you can use a standard and available oil seal in future off the shelf.
If the source of the oil leak is the hole then plug it as Eddie says. If it is leaking round the back of the bush then it is more sinister. If as Eddie says the flange is not slightly proud when inserted and hence is not being clamped by the clutch release mechanism then you can always put a circular shim or a gasket between the clutch release mechanism and the crankshaft bearing support bush - this will then provide the required clamping force. Basically the bush should not be allowed to rotate as it will wear the crankcases.

I hope this helps,

Mike

Offline eden1941

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Re: Mark crankcase
« Reply #8 on: 06 Jan 2012 at 15:20 »
Thanks guys,the setup is just as Eddie describes it so at least I know nobody has been experimenting.I fitted a new seal[from the club]previously and on stripdown this appears to be working ok.I also fitted a gasket behind the clutch actuator and was most dissappointed when the oil continued to flow.Hence the current stripdown.Oh joy.

Offline David Lawrence

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Re: Mark crankcase
« Reply #9 on: 07 Jan 2012 at 00:25 »
Mick,
Just a passing thought about your oil, as Eddie seems to have covered all the eventualities regarding the crankcase have you considered that the oil may be coming from the oil seal in the gearbox, or even a crack around the rather thin seal housing in the"8" plate of the gearbox?
As you have cork clutch linings are they a replacement for the standard circular lining or as someone fitted a Plus clutch plate, in which case it would be alloy with a number of small diameter corks.
Heres to an oil tight clutch housing!!
Dave

Offline eden1941

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Re: Mark crankcase
« Reply #10 on: 07 Jan 2012 at 13:59 »
Hello Dave,I did at first suspect the gearbox but Eddie explained any oil passing the felt seal is channeled back into the gearbox,I checked the 8 plate and there were no signs of a leak.Also the gearbox oil is new and green whereas the leaked oil is black,indicating the engine.The clutch plate has numerous circular corks so seems to be a 90+.Cheers,Mick.

 

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