The objective here is to build a crankshaft that has a 68mm stroke and with mainshafts that are appropriate for the Mk 1 cammy engine to accept the bevel drive on the timing side and a taper on the drive side for the crankshaft shock absorber assembly – standard “K” practice.
Although I have a spare "K" crankshaft, this is of no use as it has a 74mm stroke and the starting point is an MOV crankshaft (an eBay find) that I have stripped and cleaned and which has the required 68mm stroke.
A few words about disassembling one of these crankshafts:
The first step is to remove the big-end nuts and press out the big-end pin – quite straightforward.
Removing the mainshafts, however, has to be done with care. When they were assembled a ¼” BSF threaded section was positioned midway between the shaft/flywheel interface into which a truncated screw was inserted and was put there to prevent the shaft rotating in the flywheel (in addition to the interference fit). It can be clearly seen in the picture below.
This needs to be removed and I do this by drilling to a depth of ½” with a 7/32” diameter drill
so that after pressing out the mainshaft (from the outside in) only the remnants of the screw remain and there is no damage to the flywheel.
After degreasing and with the smaller parts in the tumbler overnight (except the mainshafts, which will be scrapped), the collection of bits looks like this.
The separate tasks to rebuild the crankshaft are:
- Replace the big-end bearing, which is worn
- Remove material from the flywheels to give the correct width to fit the available space between the bearings in the crankcase
- Replace the mainshafts with those suitable for the “K” engine and with a central hole on the timing side to accept a quill
- Assemble and check
Before disassembling the crankshaft, I made quite a few measurements. One of the most important of these is the internal distance between the flywheels, “B” in the picture below.
The second measurement that is of importance is the available distance between the bearings when assembled. This was done by making 2 new solid “bearings” in mild steel that are exactly the dimensions of the actual bearings (22mm x 50mm x 17mm),
dropping these into place in the heated crankcases
and measuring the distance between them when assembled using a telescopic gauge that can be easily inserted through the main hole for the cylinder barrel.
The MOV flywheels are wider than the "K" flywheels and both were reduced in width to fit the "K" crankcases.
I have retained an overhang (0.040”) on the outer rim width of the flywheels.
I was very lucky to find new mainshafts. I was talking to Nick Payton on the phone a few weeks ago (I have known Nick for many years – since my early days of Velo-ing) and he mentioned that he had a couple of KTT mainshafts. He sent me these in the next post
which are exactly what I needed. The section that fits into the flywheels is the same diameter and they have the correct dimensions to accept the bevel drive on the timing side and a taper for the shock absorber assembly on the drive side.
A bit of work is required to set them up. On the timing side, there is a drilling along the axis of the shaft that supplies oil to the big end and for which a radial 1/8” hole needs to be machined to line up with the corresponding hole in the flywheel. However, the axial drilling in the mainshaft would have needed to be extended and, in my opinion, would have come too close (or, worst scenario, broken through) into the hole that was originally used to grind the shaft between centres.
I therefore drilled the radial connecting hole at the end of the existing drilling and made a short slot (0.1” deep) to connect it to the hole in the flywheel.
The other end of the timing-side mainshaft was bored 5mm diameter to a depth of 0.75” in readiness for accepting a quill.
Both shafts have a moon-shaped segment removed to allow the big-end cage to fit (see the 1st and 3rd pictures down from the top of this page). The diameter of the recess in the flywheel is 2” and to remove material of the correct size in the end of the mainshaft I used a 50mm diameter x 4mm thick slitting saw mounted on a mandrel.
This slitting saw is made of HSS and needs to be run slowly using small cuts and plenty of lubricant as the material is tough and the shaft is case hardened (Grinding would be better – or machined before heat treatment but I don’t have these as options. There are also alternative ways to machine this using either the milling machine with a smaller carbide cutter or the lathe but the way I used has a far simpler setup although HSS is not the best material for cutting through case hardening!). The machining operations on both shafts turned out OK.
The positioning of the cut is important to ensure that the oil feed hole (slot in this case) in the shaft aligns with the hole in the flywheel feeding the big end bearing on the timing side and, on the drive side, the cut is positioned in the correct place with respect to the exit hole for the timed breather.
On both mainshafts I found that the diameter of the portion that enters the flywheel was 0.002” larger than the mainshafts that I had removed from the MOV crankshaft. This would have given an interference fit of 0.004 and would end up in either broaching or distorting the flywheels when pressed in. The surface of the mainshafts is too hard to machine by any means other than grinding. As these mainshafts are new (with good centres) and would originally have been finished by grinding between centres it was possible to remove this small amount on a cylindrical grinder. I don’t have cylindrical grinding in my workshop (only a tool post grinder – which is not sufficient here) and Peter at Jayess Tools did a great job in finishing these accurately to size to give a 0.002” fit.
The hydraulic press registered around 5 tons for pressing in the mainshafts and using Loctite 638 as a lubricant.
As I mentioned earlier, the mainshafts in the MOV crankshaft had the additional “belt and braces” fixing of a ¼” BSF threaded section positioned between the flywheel and the shaft. I had taken care not to encroach on the threaded portion when I drilled them out before removal as the plan was to replace them. As there is only “half a hole” after the new mainshaft had been pressed into place I couldn’t use a regular twist drill to reform the hole – it would wander off-centre, so I used a slot drill to reform the clearance hole
to be able to re-tap the thread back to the original size
before inserting a Loctited high-tensile grub screw.
The big-end had quite a bit of up-down movement before I had dismantled the crankshaft and would need attention. There was some visible wear on the ends of the pin on both sides where the bearing cage had been rubbing.
This phenomenon has long been recognized as an issue in caged roller big-end bearings (see, for example, Phil Irvings book Tuning for Speed, 3rd edition, page 84). There are a number of mechanisms of how a soft material can erode a harder one (aluminium vs hard steel in this case) and I have no idea which of those mechanisms applies here – anyway, it happens. The damage it not all the way around the pin – it occurs either side of TDC and the loading on the assembly at this time of the engine cycle is quite complex – inertia loads resulting from the deceleration/acceleration of the piston, change in the direction of rotation of the connecting rod relative to the pin and cylinder pressure acting on the piston crown.
When I measured the central portion of the pin where the rollers are in contact, I found it was completely unworn and measured exactly 1.250”. Similarly, the bearing outer which is pressed into the connecting rod, is also unworn and measured 1.625”. Incidentally, when I measure these using a micrometer, I use a set of slip gauges
to check the calibration of the micrometer as absolute accuracy is needed here.
The really good news is that the rollers, which have nominal dimensions of 3/16” x 9/16”, were found to be 0.0015” undersize – which would have resulted in an additional 0.003” clearance in the bearing.
A new set up rollers
and the big-end is “as new” in terms of bearing fit. I can live with the wear that has occurred to the outer part of the pin and which is not too important for the functioning of the bearing; future wear on the pin extremities will, in any case, probably be reduced with new rollers fitted. Total cost of the rollers (including delivery) was 17 GBP compared to the cost of a new big-end assembly at 400 GBP so I’m more than happy with this outcome.
The final stage was standard crankshaft assembly: the crankpin was inserted into the timing side, ensuring that the oil holes were aligned, and the nut on the outside of the flywheel tightened. I put a small black mark with a felt tip pen on the pin to indicate its position - just in case it rotates when tightening the nut and putting the oil hole out of alignment. The bearing and connecting rod were then put on and the pin inserted into the other flywheel, pushed “home” and the nut screwed on. Adjustments were made (with a soft-headed mallet) before final tightening with plenty of checking for alignment on the crankshaft alignment jig.
I ended up with runout of 0.001” on the timing side and 0.0015” on the drive side which is not too bad for a crankshaft that is probably around 90 years old.
Although the crankshaft has been rebuilt and basic checks such as central positioning of the connecting rod etc. have been made there is still work to be done to complete the fitting of the crankshaft. The balance factor needs to be checked and the distance between the flywheels is around 1/8” less with the MOV big-end/connecting rod assembly compared to the “K” crankshaft which means the mainshafts are 1/16” further inboard which creates complications in fitting the shock-absorber on the drive-side. The location of the crankpin (closer to the centre resulting from the shorter stroke) also caused a problem by interfering with the main bearings which needed sorting out ….more next time.
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