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Repairing a Douglas 350 comp crank.

Started by AndyH, 01 Sep 2015 at 22:26

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I hope this is of interest to someone, having spent nearly two years rebuilding a very old comp that has been in the family for 40 years you can imagine my disappointment at the big ends tapping slightly on only my 2nd short road trip.

So I stripped the crank out and sure enough there was 10 thou play in one big end and 20 thou in the other.

So having asked about a little and tried a few local companies who didn't fancy the rebuild I decided to have a look at it myself. Certainly a journey into the unknown as I have never done a crank before and was a little bit unsure if myself or the equipment I had was upto the job.

I had also read all the articles on here regarding crank rebuilds and decided this was going to have to be handled very carefully to have any chance of success. Many thanks to Eddie for all his informative responses.

So this is the story of my first crank rebuild and some of the pitfalls. I am certainly no expert and this is not designed to be a tutorial, merely my observations, good and bad.

I have a 1950's Harrison lathe, so I started up by mounting the crank between two centres and checking the run out, this was pretty good just 2 thou either end on the shafts. I checked the webs were parallel by measuring them outside to outside at the top and bottom and they were nearly spot on just 0.04mm out. There was 9 thou clearance side play on the big ends.

Now luckily I have a spare crank for a Plus model that's been in a parts box for 30 years and the big ends seemed like they might be okay, however it has a longer bearing mount at the front so won't fit in the comp and it was absolutely filthy as well. I checked this between the centres and it had 8 thou of run out on the shafts, 10 thou clearance on the big ends and again the crank webs were just 0.04mm out.

First problem was getting the bearing off the end of the crank, to achieve this I cut two pieces of 30mm solid alloy bar about 25cm long and slotted them into the recesses under the bearing edges and pressing on the end of the crank with the nut on pressed the crank through the bearing.

Next problem was getting the cranks apart I only have a 10 ton bench press and I wasn't sure it was upto the job. Firstly I needed to make a support for the crank which only has about 15mm of clearance between the webs and requires a specific shape to fit under the web and around the big end. I found some 12.5mm plate and after a bit of Plasma cutting and angle grinding managed to make a support. A piece of brass was turned to 29mm as a press drift to push out the pins. However the 12.5mm plate was just not thick enough and was bending in the press, however putting the original 20mm support supplied with the press underneath it cured this and sure enough I pressed the first pin out. Amazing a 10 ton bench press did the job and soon enough I had both cranks apart, labelling everything as I went.

I pressed the sludge end caps out of the Comp crank which were steel and the alloy pins out of the 90 crank, couldn't believe how much sludge there was in both. Sure enough the crank pins on the comp were worn these were the original parallel type. However the 90 pins looked like they had been replaced at some time, they were stamped with Alpha and were slightly stepped higher in the middle, aside from being filthy they looked okay and the 90 had bigger bearings running directly in the rods unlike the comp rods that were sleeved.

So after spending a good while cleaning everything and deciding the 90 rods and pins looked okay I decided the best course of action was to leave the 90 pins in the 90 centre web and simply use the comp crank ends.

At this point it is worth mentioning that the oil only flows through the centre of the first big end pin before being directed up the centre of the middle web and then splitting to oil both big ends from the sides of the centre web so it is essential that the centre web and pins is assembled the correct way around to achieve this.

Okay reassembly, well like everything Douglas nothing is ever simple, firstly I needed to make some half moon 1.3mm shims so that I could press flatly on the crank webs as they are cut away right across the pins.  Having located some 5/8 bar as per Eddies suggestion I was relying on this to align the crank up by slotting it through the crank web and pin centre.

I started with the front web and my press was straining as it got closer and closer. I am sure the purists  here won't agree with my next step but I used two 6 thou feeler gauges against the sides of the rods between the webs, one on the inside and one on the outside and pressed the crank until they tightened.

Having removed it from the press, the 5/8ths bar just slid out so I knew it was aligned however I then checked the outside width of the two crank webs at the top by the big end and the bottom, they were quite a bit out, .2mm to .3mm wider at bottom. I just popped it in the vice and gave it a gentle squeeze at the bottom and it creaked a couple of times and hey presto it was within 2 thou. It was almost as if it wanted to go back into position, side clearance on the big end was now 9 thou.

So far so good. I repeated the process with the rear web and again found that the webs were not parallel and needed pressing together slightly at the open end, again they just seemed to click into position. Okay the moment of truth, back in the lathe for checking and fully expecting to have to be hitting something with a big hammer. Not the case just 2 thou of run out on both shafts, couldn't believe it. 9 thou side clearance on the big ends and just 0.07mm out of parallel on webs overall.

I think the 5/8ths bars and ensuring the webs are parallel at all stages are really important.

Wow nearly there. Not one of the pin inserts was re-usable, be aware that if you take them out you will have to make new ones. I re-made them in aluminium, first ones with 4 thou clearance but were too tight to fit. I had to redo them with 2 thou clearance, you also need to be really aware that the side webs of these plugs are only 6mm wide max to avoid covering any of the oil feed holes.

Well I am feeling pretty pleased with myself just need to re-fit the crank bearing and plate behind it, my first effort at this was terrible, the washer is tight on its mount and it turns out is not held in by the bearing and the bearing wouldn't turn as the washer was not seated properly. So I cleaned up the inside of the washer a little still leaving it a tightish fit and refitted the bearing. Then I got thinking if this washer comes loose it would create a real mess hence the previous crimp/stamp marks around its mounting edges. I did it again and this time, fitted it tightly and crimped/stamped it to its mount in about 4 places.

Just as a last check using an oil can I pressurized the oil feed system from the front feed and sure enough oil appeared at both big ends.

Job done. I'll let you know how it goes but it just shows, some patience a 1950's Harrison Lathe, a 10 ton bench press and a couple of dial gauges.

Hope this was helpful to anyone considering having ago at doing a crank themselves. The most time consuming part of the whole operation is making up the plates and drifts for the press.

I have included a few pictures of the finished crank and my left over parts.



Hello Andrew,

you have send a very interesting article of the crank overhaul!



Hello Andy. Well done with the Competition model crankshaft renovation and the resulting article and photos,but I was surprised by the nine thou' big-end sideplay. Was that standard for the Comp? All the best, Ken.


Hi Ken

I had access to three cranks, one comp and two plus 90 and when I measured them that's what they all were within a thou so I just went with it. Hope I am not wrong.