Author Topic: Need some advice - Oil lubrication on MK 4 with a one piece main bush  (Read 3945 times)

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Offline twirl

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After a discussion with some friends I need some advice. As reported, I restored an MK4 Douglas (Douglas 7260/4 ready to go). But for some reasons I m not shure about the oil lubrication is reliable. As Eddie told in another topic, the MK4 normally has a two piece main bush. With only a one piece main bush my Douglas is different. On the T35 and  on the MK 1 and 3, too, I guess, the timing end of the crankshaft has a radial groove (see  pictures below). This groove allows a permanent oil lubrication to the crankshaft. The MK 4 crankshaft does not have a groove like that. Instead of this, the gap between the two bushes will ensure an continuous oil pressure to the crankshaft, too.
If this is true, the combination of a one piece bush with the MK4 crankshaft will be a problem. Only for the short moment, when the oil hole in the bush bearing and the oil hole in the crankshaft fits together, oil can flow into the crankshaft. I am afraid, that's not enough. Has anyone experience to give me an advice. I am thinking about to dismantle the engine again to spindle a similar groove in the built-in bush bearing to get a continuous oil pressure to the crankshaft. What do you thing about that? Thank you for your hints!


Offline eddie

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Hi,
    During the time that the Mark series was in production, Douglas tried several different configurations regarding the front main bearing. As you say, some cranks had an oil groove around the front journal, others had a plain journal, and a third type had the oil groove eccentric to the journal so that it only went half way round. I have also seen all 3 types with an extra groove going to the front of the journal - presumably to increase oil flow to the timing gears! The one piece and two piece bushes also added to the variety!
 If it gives you peace of mind, by all means machine an oil groove in the bore of the main bearing bush - this only needs to be about .030" deep x the width of the oil hole in the journal. Don't be tempted to go back to the 2 piece bush as they are more trouble than they are worth.
  The cross section drawing you have shown is of a late Mk1 engine with the coarse tooth timing wheels - it shows the main bearing bush being shorter than the bore it fits into - this is not right - the bush should protrude by about .010" and the oil retaining plate should be shimmed from the shoulder on the crank to give endfloat of about .005" - .008".
  If you are making a new main bearing bush, don't make the bore too tight - Mr Douglas recommended .003" clearance on the bore - anything less is likely to result in the bush nipping up and turning in the crankcase - causing the locating pin to damage the flange - the only remedy being another complete strip down!

  Regards,
                Eddie.

Offline twirl

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Thank you very much Eddie, for your Message. I will do so and machine an oil groove in the main bush!

Offline twirl

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Hi Eddi, some days ago I started to dismantle the engine. I found the timing end with a radial oil groove, but interrupted about 10 degrees, I guess.
My first conclusions were wrong. After reassembling the crankcase the first time, I wanted to check lubrication. First, I found the lubrication blocked. I was not able to press any oil from the oilpump to the main bearings. Having rotated the crankshaft a little bit, oil reached both main bearings without any problem. That's the reason I thought a radial oil groove is missing.
On the photo, you can see the extra groove going to the front, you mentioned. As John Mark in his manual recommended, I asked alpha bearings to block this groove when fixing the crankshaft.
After all, dismanteling was really unnecessary. With a little more care, I could have avoided that. But, after all, I enjoyed working on the Douglas again. Thank you for your hints.

Offline douglas1947

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Hi Twirl,

I work also on a MK4 engine (No. 7419/4).
It has also a one piece bush and the same oil hole and groove on the crankshaft, but NOT the extra groove.

When you have now the crankshaft out of the crank case, you should use the possiblity to clean the oil ways in the crankshaft.

Michael

Offline twirl

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Thank you Michael, for your hint. I asked alpha bearing to clean the oil ways when fixing the crankshaft. They reassembled the crankshaft with a pair of new pins, too. So I hope this will work.

Offline douglas1947

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Hi Twirl,

was it necessary to fit new crank pins (of the plain type)?
You should keep the old pins.
I have just dismantled my crankshaft and find out to repair it by renewal of oversize rollers.

Michael

Offline twirl

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Yes, a replacement was nessacary. The surfaces were to bad. I've got the old ones back, but can't imagine they are useful. Your method is very impressive, but I am really not able to fix a crankshaft myself. Good luck!