Bodywork, mudguards rear light and frame

Obviously the bike will need rust treatment and a respray so I thought I'd use this post to mention my approach to antirusting.

Generally, if it's a surface that shows, then I wire brush loose rust and rub with 400 wet and dry. I then degrease in meths and treat the rust with Jenolite. That's followed by an etch primer, filler primer (as needed). This is rubbed down with P1000 grit wet and dry. Where the filler primer wasn't enough to fill eg serious rust pitting, I use a skim of P1000 filler, rub down and reprime. Finally, I apply two coats of coloured paint and a clear lacquer. I have recently found an acid etch filler primer that cuts one stage out.

Where the surface doesn't show, as inside the forks or under the mudguards, I prepare as above but treat with metalmorphosis rust converter which dries to a flexible coat converting and sealing in the rust. I then spray with high zinc anti rust primer followed by stonechip or underseal. Inaccessible surfaces such as inside the frame are treated with waxoyl using a flexible spray extension tube.

I should add that Ford Dove grey and Ford Royal blue are very close matches to the original NSU colours and a fraction of the price of remixed classic colours. I don't think the names are coincidence, I suspect they may actually be the same paint. Both are indistinguishable from original colours if used as touch up over the original paint, but the grey in particular appears too yellow when used over grey primer. It may be that an intermediate coat of white would help prevent this effect.

Front mudguard

Whilst I was waiting for parts I started to strip the mudguards for repainting. The NSU crest at the front is held on by two pins that are fixed at the back with these plates. The equivalent of star lock washers today. 

Rear of bumper, NSU rest fixing pins and plates 

There were two small cracks just commencing that I sealed with weld and a slight dent that needed to be folded out. The rust pitting required filling as high build primer couldn't cope with it. I also found the external seam was tricky to access with a spray so I worked primer in there with a small brush. Id like to seal this seam but I'm not sure what to use as I want it over-painted and invisible. 

Working antirust primer into the seam.

Finished bumper.

Frame

As  I  had already removed the forks, I could prepare the headstock as I didn't want to damage it once it had been repainted. Using a long drift I was able to tap out the lower bearing cup.

... which dropped free.

The corrosion inside this cup was obvious.

I also noticed a small crack in the turn bump-stop bracket. As I have bought new rubber stops I'd like this repaired. I had intended to repair this before spraying but in the event I forgot and had gone through my anti-rust, prime and first coat procedures before I remembered...Duhh!

Crack in bump-stop bracket.

Masked off the upper bearing cup as I'm not changing this one and cover the ID plate.

Sadly this meant I had to wire brush off the new paint and weld up the crack and grind it back.

Crack repaired

Removing mudguard and carrier

The rear mudguard is held on by two vertical stays and a U shaped horizontal stay. These share their attachment to the forks with the two struts holding the carrier. These do not need to be removed to release the wheel, but I took them off anyway as I wanted to repaint the mudguard.

Bit crowded on the fork! Rear axle nut (lower) and upper bolt securing rear mudguard stays and carrier

Removing the mudguard stays and carrier struts

There is an additional nut and bolt securing the carrier to the top of  the rear mudguard. I suspect that this should be converted to a weld nut as its very awkward to unscrew unless you have also removed the rear wheel.

Carrier to mudguard nut and bolt.

The rear carrier can now be lifted away and this leaves the mudguard, with its stays attached, held only by the toolbox at the front. This is a simple clip fit onto the seat post and so can be clipped off and removed with the mudguard. The toolbox itself is removed by unscrewing the two M6 screws accessible from inside the rear mudguard. The upper one of these is an M6 cross head cheesehead screw with a  a nut inside the toolbox, whereas the lower is an M6 self-tapper. In my case the toobox was pretty much undamaged and even had its lid so I put it aside.
The mudguard stays are held on to the mudguard by M4 cheesehead slotted machine screws with nuts inside. In my case all bar one were rusted solid and had to be cut off. I will get new to replace them.

The RHS stay also carries the wire feed to the rear light held inside small welded clips. The wire then passes inside the rear mudguard where it is secured by more welded clips. These can be opened by levering with a small screwdriver to release the wire

Rear light wire feed inside mudguard stay and mudguard. caption

Rear Light mounting screws inside the rear mudguard.

The Rear light is at this stage still attached to the mudguard but is removed by undoing the two mounting screws inside the mudguard. Again these were slotted cheesehead screws which seems an odd choice for an area prone to corrosion. Luckily I was able to unscrew both and remove the light. I will deal with the light and its problems in a separate post.

Removing the rear light.

The rear mudguard was now free. Checking it I could see that it had a couple of problems: it was cracked up towards both vertical stay mounting holes. The LHS was a minor issue but the RHS was worse. Both needed welding.

Crack to mudguard stay mounting holes on RHS

Damage was easier to see once the area had been abraded. I welded this up and welded a reinforcement washer on the inside.

Damage more visible after grinding back.

Towards the front LHS of the mudguard a large piece of padding had been inserted. This appeared to be because the welded seam in the mudguard had separated and I assume that this stopped the vibration. I removed the foam packing and cleaned up the area before welding the seam back together.

Foam inserted in mudguard split seam

Removing the bodger's packing

I was then able to sand the mudguard, fill it where necessary and repaint. A word of warning though- do not ever use Hammerite primer- it appears to be incompatible with any paint other than more Hammerite! I used it because I had nothing else at the time but eventually I had to remove it all from the koutside with paint stripper and start again! I left it on the inside of the mudguard and oversprayed that with a coat of white Hammerite. This I then covered in grey stonechip. After three coats I got an acceptable finish

The rear number plate

Nothing much to say here, the plate was bent and dented which I attempted fix by careful hammering. It was also badly pitted with rust so I wire brushed it treated it with jenolite and then skimmed the rusty areas with P1000 filler which could be sanded back.

Finally it was primed again and over-coated in satin black. I treated the rear of the plate with grey stonechip.

Number plate preparation- filling the rust damage with fine surface filler.

Preparing the Rear frame.

I removed the small plastic trim piece at the base of the seat tube. This is held on with really meaty M6 cheese-head screws (cross-head). I don't know why the attachment needs to be so strong. The trim has metal eyelets around each screw hole presumably to stop it from being crushed. Predictably in my case one was missing, leaving only a rusty tell tale to show it had ever existed! I will need to find a replacement.

Missing M6 Eyelet

It was also clear that the trim piece has cracked at the front and at some stage had been poorly repainted- two more jobs to deal with.

Crack at front of seat tube trim.

The rear light wire feed is also secured to the inside of the rear fork (RHS). Its held there by spring clips. Press down on the top and lever backwards to pop these out.

Rear light wire feed secured to inside of RHS fork arm

Spring clips on wire.

The wiring loom (such as it is) was now actually free since I had disconnected the headlamp nacelle and magneto earlier. The whole loom looks to be cracked and old so I will slip new wires into the sleeves and replace it as much as possible.

I also removed the motor from the frame, I hadn't meant to do that but in the end I thought now was as good a time as any so went ahead. That is described in the section "Considering the motor".

The rear frame was now clear so it was degreased and de-rusted before being treated with Jenolite, Primed and sprayed in Royal blue.

Rear Carrier

The carrier is incredibly complicated for what is a simple job. Mine has two hinged clamps, one of which is sprung and is more reminiscent of a bicycle carrier than a motorcycle rack. It's not really clear how these clamps work together and although the springs could be disconnected, it looks like a fiddly job.

I decided to short cut this so I degreased the rack thoroughly and then soaked it in 2% citric acid for 3 days. This lifted the rust and loose paint. The rack was then rinsed and wire brushed before being treated with Jenolite and Kurust before cleaning in meths, priming with zinc primer, and repainting in Dove Grey.

Not perfect, the rack really needed complete disassembly and sandblasting but this process should at least stabilise it and look OK.

Considering the motor: piston, rings, pin and sprockets

When I bought this bike it was actually the second one that I'd looked at. The first was described as freely turning when I bought it, but later the vendor contacted me to tell me that in fact the piston was seized in the bore. I pulled out of that sale, so when I found this one at Kempton Park I was quite pleased that the vendor immediately told me that it turned over freely and proceeded to peddle it like crazy to prove it. It seemed to me in fact, that perhaps it turned over too easily and there seemed to be almost no compression. Still, at least it was turning freely. The time has come now to investigate the motor and discover why there seems to be such low compression so time to remove the head.

Firstly disconnect the decompression cable- you can disconnect this at either end but as the head was gummed up it was easier to release the solderless nipple at the decompression lever.

Solderless nipple at lever end of decompression cable.

My first clue that all was not well came when I discovered that the cylinder head nuts were simply finger tight, and unscrewed easily using a box spanner and finger pressure alone. In fact, removing the cylinder head was so simple that I had done so before I even thought of taking any photographs! The top of the piston was lightly carboned and the cylinder head itself clearly needed a decoke. 

Inside cylinder head

I then removed the barrel which just pulled off. At this point it became clear that in order to make the motor turnover freely (and presumably to ensure a good sale) the PO had removed all of the piston rings and only loosely reassembled the unit. I feel it would have been nice if he'd told me these things instead of emphasizing the fact that the bike turned over - when it was quite clear that it only did so because he had fudged it- Caveat Emptor eh? Sometimes my faith in human nature is a bit shaken. We're all enthusiasts- we could at least be upfront with each other. There was also a circular mark on the piston crown. The motor had clearly been seized at some point and the vendor had then freed it by hammering the piston down with some form of drift.

The good news was that the barrel itself appeared completely undamaged. There was some marking on the inside of the Chrome but absolutely nothing that could be detected with a fingernail it's more of a bloom in the finish than a scratch or damage. If this was a body part I'd have no hesitation in polishing it out- its that slight. Overall I think the bore has survived remarkably well. 

Looking inside barrel- hard to picture the marks I mean

Certainly I don't feel the need to buy a new barrel. I measured this one as best I could using Sealey bore gauges and discovered the measurement to be 39.395 with minimal ovality; this indicates a standard and unmodified bore. In fact it's impossible to bore these cylinders out since that would remove the chromium plating on the inside but nice to know that the wear isn't significant.

Sadly, things did not end there. Checking the piston and big end for play I discovered rapidly that the little end was clearly worn. Probably as a response to the hammering process that was used to free the piston. Of course the damage may have been there before hand, I can't really be sure. The big end was a little bit more difficult to interpret. There was no up-and-down movement which clearly indicates the bush is not completely useless. The bottom end also slid easily between the two crankshaft webs as indeed it is supposed to. However there did seem to be quite a lot of transverse tilt movement; i.e. without moving the big end, the top of the con rod could be tilted across the motor by some 5 to 10 degrees. I've no idea whether this indicates excessive wear or not. It could for instance imply that the big end bush has worn like a bell-mouth at each side, the centre section being unworn preventing up down movement but the opening towards each end allowing some tilting motion. Of course, there has to be some clearance between the crank pin and the big end bush and so some degree of tilting is to be expected. I am unsure exactly how much is excessive and consequently I don't know if the  crankshaft needs any attention. Certainly changing the big end is a large job. It's not excessively difficult but it will be very expensive. A new crankshaft seems to be of the order of £180. However a new piston is relatively cheap at £40 for a piston set including new rings circlips and gudgeon pin and changing the piston itself is a relatively simple job that doesn't even require the motor to be taken out of the frame. I decided to press ahead and fit a new piston into the existing barrel, refit the head and test the motor to see whether it will run. I should then be in a better position to assess the state of the gearbox, the clutch and of course the big end.

The piston is attached to the conrod by means of the gudgeon pin, this is turn is locked into the piston with circlips at each end. The pin should be free sliding but in two-strokes tends to gum up in the piston and often needs a tool to remove it. NSU speak of a pin punch to knock the pin towards one side, but that sounds like a perfect way to strain the big end bearing... Hmmmmm. Larger motors usually use some form of puller but these were way too large for this motor. 

Anyway the first step was to remove the circlips, there should be one each side of the pin.... LHS- no sign of a circlip

RHS circlip present and grooves for its removal.

I picked the pin out with  needle nosed pliers. I couldn't find one on the other side so either this piston never had one or its been removed once already!- This could explain my tilting conrod ??? I do possess a pin extractor but this pin is too small for even the smallest fittings. I'm certainly not going to hammer this already dodgy fitting so I improvised a small valve spring compressor, receiving socket and small drift to force it out. This worked very well but it was fiddly holding everything in alignment. Given the tightness of the pin I tend to think it hasn't been removed recently.

Valve compressor, punch and receiving socket to remove gudgeon pin.

Once removed I could examine the piston- rings obviously missing and their grooves a bit gummy. There were some longitudinal striations on the skirt but nothing matching them in the bore.

Given these marks on the piston- and the circular hammer marks on the crown I don't want to reuse this item. It could be cracked so rather than risk that I ordered an new piston kit complete with rings, pin and clips and 2 little end bushes (I only need one but I'm expecting that I might not get it reamed right the first time!). I also ordered 2 pairs of circlips in case I need to re-install the piston on a new crankshaft. 

Piston crown showing circular hammer/drift mark

Cylinder head and Decompression valve.

The decompression valve was jammed in this bike. It had to come out for cleaning and checking. I depressed the spring washer with a screwdriver and could then use the flat of the blade to push the "C" clip towards the valve stem and unclip it.

The C clip washer and spring just lift off. The valve itself was still jammed so I tapped it downwards with a wooden drift.

And pulled it through

It was simply gummed up with sticky deposits and carbon. I cleaned the stem on a brass wire wheel and cleaned the head thoroughly. I ground the valve in using lapping paste to generate a nice even grey seat to both head and valve



Before rebuilding the spring system and retaining it with a new "C" clip. Checking operation showed that it now moved freely.

Little end (Gudgeon pin) bush

I had already detected wear in the gudgeon pin/little end bush area, although the jury is still out on the big end I went ahead to renew the little end bush. I found that one of the old leading link suspn bushes was just about the right size  to act as a driver for the bush. I made up a small extractor using some M6 studding, two nuts, a socket large enough to receive the bush and a stack of washers. I turned the end of the link bush a little in the lathe to create a step that could locate inside the little end bush as it pressed it in.

Components of home installer/remover for little end bush. Note the leading link bush  was turned down for a mm or so to create a locating lip 

Home assembled extractor

This pushed the bush out easily but you have to take care that it pushes out straight. You can then fit the new bush using the same system but without the driver as you can push the new bush in with the washer alone. Insert the little end bush bevelled end first using some oil to ease it in. The bush has three lube holes for oil splash, so make sure one lines up with the groove in the top of the conrod.

New bush entering conrod

... and home.

New bush installed.

This process causes the bush to close up internally so it will need to be reamed out to fit the gudgeon pin before use.

 Once reamed ... I used adjustable reamer H1 3/8-7/16" to ensure a nice sliding fit. Check for pin fit as you go adjusting the reamer by half a turn between attempts. The pin for this piston was noticeably larger than the old one.

Note added: I have now fitted several small end bushes and there is a variability. The bush used here was from Krippl and was too small to accept the gudgeon pin (even the old one) before reaming. It was a tight fit in the con rod eye and reaming was essential. I have found that some bushes appear larger internally and will accept the gudgeon pin before they are fitted. These presumably rely more heavily on being crushed to generate a tight sliding fit to the pin when they have been pressed into place. This makes them more sensitive to any wear in the eye of the con rod which would reduce the "crush" and hence the tightness on the gudgeon pin. If you do find your bush is of the larger type then check the con rod eye for wear before fitting; measure both side openings and if there is any difference insert the bush through the larger opening so that its only crushed as much as needed to fit the eye. Further if the Bush is relatively loose in the eye it can be secured with a little Loctite 620 high temp retaining compound or it might rotate in the eye and close the lubrication channel.

Before fitting the piston I inserted one of the pin circlips. Note that the cut out port in the piston skirt has unequal sides. The longer side should face rearwards and in this piston that was also indicated by the letter "A" on the crown

 This piston has two identical rings. They are also symmetrical so there is no special orientation. Just align the gaps with the stops.

 You have to heat the piston before the  gudgeon pin will slide in easily. I used a hot air gun. You'll need a cloth to hold the hot piston.

 Slip it in until it contacts the clip on the other side and then insert the second clip using needle nosed pliers.

You can  the then ease  the cylinder down on top of the base gasket. Its hard to find a small enough ring compressor but its easy enough to squeeze them in with fingertips as you lower the barrel. The cut-outs in the spigot are a big help.

Assembled the motor, but it wont start! Turns out I have no spark so time to dive into the magneto I think.

In the meantime I will check the final drive sprocket and remove the pedals.

The pedals are easily removed, unscrew the cotter pin nuts and tap the pins out to allow the pedals to rotate. and break the tapers. The retaining nuts are then undone (all normal threads to anticlock to remove).

The first thing I found was that there was a considerable end float on the pedal shaft, seems to be about 2 mm! This seems excessive but it could be acceptable considering that this is just a pedal shaft so this needs to be checked.

The pedal shafts can then be removed. The shafts are marked for left and right. The pedals can be unscrewed from the shafts but the Right hand shaft has a right hand thread (anticlockwise to unscrew pedal) and the left hand shaft has left hand thread (clockwise to unscrew pedal)

Before gaining access to the sprockets I needed to remove the drive chain, described under rear wheel removal.



I found the sprocket teeth were worn sharp and a few were chipped.The final drive sprocket was also in poor condition and so both will need to be replaced.

.

To remove the rear sprocket I will need to remove the rear wheel (see appropriate section). To remove the front sprocket I wedged it with some wood (handle of a wire brush was great) so that I could unscrew the retaining nut and remove it and its washer.

Removing the sprocket itself will need  the correct puller. I'm told a three legged puller will work, but not in my hands. Maybe a beefier one would succeed. Anyway I don't want to damage anything so I got the dedicated tool. However at this point I was overtaken by events.

Removing the motor

I would have liked to test the motor and change this sprocket in position as its easier to do both whilst it's in the bike and supported. However parts were late coming and I needed to get on with painting the frame and for this the motor needs to come out. Ah well its only 3 bolts and I can refit it later anyway. The three bolts are shown below, spanner is on the first of them

Engine mounting bolts, spanner shows rearmost bolt.

Forward mounting bolt

These bolts simply unscrew and the motor drops down. Note that there should be an anti shake washer under the head of each bolt (lhs) . One of mine was missing and one broken. All 3 bolts are installed from the left, the rear two have washer, spring washer and nut fitted on the right. The front-most bolt screws into a weld nut on the frame.

Engine mounting bolts remove- note missing anti-rattle washer on centre bolt and no nut needed on the front bolt.

The motor is quite light and easily manhandled. However it's an awkward shape and doesn't stand up on its own. Any further work on the bench would need an engine stand. I was looking forward to making one but they are available at a very reasonable cost.

Replacing the crankshaft sprocket

The crank sprocket is on tight! I therefore invested in the special tool. I have to say I wasn't impressed. It is a solid lump of metal but it's been meanly thought out.
Firstly the bolt is so wide it will push on the nut if you leave it on to pretext the threads. It would have been easy to chamfer the end (as I did) to make sure it pressed safely on the crank. Also the press bolt was a coarse thread which means a lot of force is needed to shift the sprocket: a finer thread would have reduced this.

 The main problem tho is that it measures 47mm across the flats. I don't know about you, but that's larger than most would have a spanner for. In fact it was larger even than my adjustable wrenches could cope with and obviously it won't take a socket. It would have taken a little longer but milling down to 28mm af would have been appreciated!

However, after much heaving I did remove the sprocket. The oil seal underneath looked in a poor state and was caked in crud. I will change it but I suspect I'll be stripping the motor later so I'll delay it and do it then if that's necessary or I'll do it on its own later if its not. I compared the old and new sprockets and received a nasty shock! The sprocket was so worn I was worried that my replacement was correct. This bike has clearly been greatly abused and this doesn't bode well for the condition of the still untested motor! 

Old (lower) vs new (upper) sprockets. Wear is extreme! 

Greasy gritty deposit around the oil seal. This is presumably what has worn down the sprocket. 

New sprocket installed.