NSU Quickly S23 Restoration and Repair.: Crankshaft truing 1: Making a truing stand

Rebuilding the crank will be necessary whenever the big end develops play. This could be due to wear in the crank pin, rollers or conrod, and as you can't predict which has failed its best to replace the lot. Fortunately there is a rebuild kit available for the Quickly, although at a bit of a price (around £60 at the time of writing). Unfortunately the cost of a professional rebuild then puts the cost of rebuilding your crank close to that of a new replacement. To my mind this isn't green as well as being bad for originality! Disassembling the crankshaft requires a press, but this is useful for a load of stuff and if you're going to do a couple of these then it will soon pay for itself. Luckily I already have one so I'm going to try my hand at a crank rebuild. In theory it's straight forward, but of course it has to be accurate if it's not to shake the motor apart! The new crank has to rotate true to its centre and the webs need to counterbalance the movement of the piston. This means the crank must be both "balanced" and "trued" . Provided you are not swapping for con rods or pistons of different weight, the crank webs should still balance the oscillating parts and balance can be ignored (I hope, obviously this wouldn't be true of high performance motors but I'm hoping to get away with it here).

Truing however is a different matter and unavoidable in any rebuild. I would also strongly recommend that this be done on any second hand crank you may acquire- even if the threads look great. Obviuously any wobble in the crank will be constrained by the main bearings. This can only lead to rapid wear but further, these bearings wont remove the wobble entirely. Since the magneto pole clearance is of the order of 0.15mm it will not take much movement before the flywheel contacts the magnets as it rotates.

If the crank is out of true then you need a way of rotating the crank and determining whether there is any wobbling movement off-centre (run out). You need to know how much movement there is and also in what crank position. This means making a stand, although I'm told rotating it by hand in a lathe chuck should also work. I do have a lathe but it seems to me that the whole process would be easier to understand if I had a stand, so this is how I made a primitive version of one.

The base is made from a 300 mm length of Box section, 75x25mm with a 1.5mm wall. To this I'll fit two uprights to hold the crank and either cut slots or holes in the base to allow the uprights to be moved so as to accommodate different sized cranks.

The uprights are made in 3 pieces: A base and a top plate, each made from a 60mm length cut from 50mm wide, 4mm thick steel bar and an upright of 150mm length 25mm Square box section. These will be welded together with holes cut in base plate to allow it to be fixed to the base, and in the top plate to allow the bearings to be fitted. I drilled 8mm holes at 10 and 40mm centres from one side of each plate, 10mm (top plate) or 15mm (base plate) from the top edge as shown.

Top abd base plates as shown, 4mm steel, 50mm wide bar

I drilled corresponding 8mm holes in the box section to match those in the base plate, and continued these through the box section, opening them to 22mm holes on the underside. This will allow me to bolt the bases to the box section and use a small socket or box spanner to tighten the bolts from underneath. The towers are designed to be high enough that the piston can rotate between them so that I can check balance as well.

The top plate carries bearings on which the crank will rotate, but also needs a 10mm square slot cut from the top edge centre (20-30mm from one side) where the bearings meet so that the crank can slip down and rest on the bearings. The assembled tower pieces are illustrated below.

Assembled tower sections for fixing to the box section base.

For rotation I'm using Im using 4 6382RS 8x25x9 rubber sealed roller bearings. The bearings themselves must be positioned quite close to the webs so that the bearings contact the straight and non splined sections of the crankshaft and this is quite short on the clutch side.

This means the bearing fitments must be as flush as possible so I used M8 flanged socket head screws, 20mm in length as these were the least bulky I could find. The bearings were secured using an M8 nylon washer as a distance piece and a nut n the back so that they could rotate (see below).

Bearing fitment using 20mm socket flange capped screws M8 and nut. I used a nylon spacer between the bearing and the top plate to allow rotation.

The length of shaft on the clutch side is short- you could use the narrower section further along but this will mean that the bearing would need to be raised. I found I could rotate the shaft without scraping in this position.

There is more room on the magneto side

Provided that everything is cut true and welded caccurately then everything should fit immediately. In my hands this wasn't the case and I was forced to construct 1 holding tower complete with bearings and bolt it into place. I could then assemble the second tower but without welding the top piece to the upright. I could then experiment with the position of the tower and the height at which the top plate was fixed so that the shaft could lie both horizontally and straight across both bearing pairs. Eventually I got a good orientation but it was untidy on the base. I welded the second tower into position rather than using the bolt holes because its position turned out to need more adjustment than I'd anticipated. It was simply easier to find the right position and tack it into place there. After all, any adjustment for crankshaft length only requires one moving tower.