replacing the piston and barrel

The old piston may well have been usable, but the trauma of its removal from the barrel had left marks on the crown. 

Similarly the barrel would be usable but really needs reboring and rechroming to be sure it would last.

 Luckily I was fortunate to get one of the last new old stock piston and barrel kits from Germany. 

Testing the old gudgeon pin revealed wear in the small end. The pin itself wasn't greatly worn but the bush was. There was no up-down  play but there was a tilting effect when the pin was tilted in the vertical plane. If you tried the same movement in the horizontal plane then tilt was barely perceptible. This told me that the reciprocating motion of the piston has taken its toll on the bush and even though it might be OK for a while, it's going to need changing.

I assembled a hotchpotch mixture of pressure pieces to use with some M6 studding to press the small end bush out. Rurning a small stepped adapter to locate inside the bush and press on it- but small enough to pass through the conrod eye. I used a narrow tube (old Quickly gudgeon pin) behind it to get the length of push. The assemblage is shown below after bush removal, and the bush itself is indicated by the pencil. Obviously these items were assembled around the con rod to press the bush out and into the short tube section opposite. It worked fine but I think would have been better with M8 studding and probably a centre piece to keep the studding and tubing in line as pressure is applied 

 Unfortunately I could find no NOS bushes and no remanufactured examples either. I decided I would need to make one although my chances of making anything better than the worn original are debatable. 

I have prepared by ordering some reamers at 11.9 and 12mm to test but in fact fate intervened and I found this odd looking bush advertised as an NSU Quickly small end on ebay.

This had a number I didn't recognise but even with no clear sense of scale this looked a bit too big for the Quickly. The label has also had the "ly" obliterated and a "50" added. All this made me wonder if its actually a Quick 50 bush mislabelled. It was supplied by Alphaautomotive who seem no longer to be with us- at least as a parts manufacturer (superceded apparently as Alpha Bearings") but I was able to track down an old NSU applications list for them.

Or zooming in-

You can see they list two bikes under NSU 49cc... the Quickly (1954-68) and the the Quickly 50 (1963-66). These bikes use different bearings so, as the Quickly used the same bearings throughout its life, I'm assuming that "Quickly 50" is simply a transcription error for "Quick 50". If so then this confirms that the odd number SE229 does seem to be for my bike. The manufacturing period is also in agreement. Finally I asked the seller to supply the diameter of the bush which he recorded as15mm. Again this suits the Quick 50 but is too large for the 13mm NSU Quickly equivalent. I therefore bought this bush and if its correct then I'll take its dimensions to use-as a pattern before fitting it.

One point that always worries me us that the bush has to be right but not too tight.... so how tight?I measured the bush removed from this bike at 15.039mm and the conrod eye 14.9. Obviously in fitting and removing the old bush it will have been crushed a little and probably hasn't expanded to its full size on removal. Nonetheless this indicates some 0.139mm of crush which seems enormous- I usually reckon about 1/1000th or 0.015mm in this case. However doing the same calculation for the Quickly small end and conrod eye shows the (extracted) bush at 13.016 and the conrod eye at 12.9- in other words 0.116mm of crush or a very similar value. There was however quite a wide variation. Maybe more squish is needed because the bush is hollow and so deforms easily.  Perhaps it therefore needs more squish than say a solid pin to make sure its tight. Of course I'm measuring the con rod eye with calipers and the bush OD with a micrometer so the accuracy may be a little suspect.

My plan is to first drill a  phosphor bronze rod to as close to the gudgeon pin od as possible without the sides becoming weak. I will then turn the exterior down to slightly larger than the diameter of the control rod little end eye. I can then insert the bush into the con rod eye, drill through the lubrication holes and then progressively ream out the bore until it accepts the new pin as a smooth sliding fit. I think if I try to ream it out too far before inserting it in the conrod, its walls will be too thin to take the⁷ machining.

The OD of the inserted bush measured in position  is very close to 15mm, so I ordered 90mm length of 15.8.mm phosphor bronze rod. I hope this will be manufactured accurately enough to turn down but just in case I also bought a length of 19mm. In either case 90mm length should allow me to make a few attempts.

Here is the new bush. In its virgin (non inserted) state it measures 15mm in length, 15.126mm in width (ie OD) and 11.95mm ID. Neither the old nor the new Gudgeon pin would fit this in its unreamed state although both fitted nicely into the old bush which was only 0.03mm larger in ID. Clearly this sliding fit is a close run thing. The bush started out at 15.125 mm OD and the extracted bush is 15.04 so some 0.08mm of squish?

I cleaned up around the crankcase mouth 

... and removed the barrel studs to make sure I could get a clean joint for the cylinder base gasket- although this also improves access for changing the small end and piston.

Inserted new bush using drawbolts- tapered side first as a lead in. Ensure both bush and eye are clean and lubricated. 

The new bush had no lube holes so these were drilled through using the conrod holes as a guide- great care to remove any swarf. Use a vacuum extraction.

Finally ream the bore. I used two adjustable reamers starting with an H3 7/16 t 15/32 

and finishing with type ST4 15/32 TO 17/32. It's essential to test the reamer in the old bush before using it to make sure you don't go too large and check gudgeon pin fit after each pass. When I'd got a smooth sliding fit I could check the bore with a fixed reamer and it looks like for future reference a fixed 12mm reamer should be just fine.

As an added complications I found that not all 12mm reamers are the same! I have 3 and only one (h8) reamer the bush correctly so o really its probably best to stick to the adjustable and use care.

Nice sliding fit. I had to clean out the cylinder studding threads before the studs would screw back fully. Note that the piston is directional. It was supplied in the new barrel and I've not removed it. As supped the arrow on the crown pointed forward: a check with the fb group showed that this is correct, the arrow would usually point towards the exhaust (ie forwards).

Piston in barrel as supplied

.. and refitted to the Con rod, barrel added. 

The piston was supplied with one gudgeon pin clip already in place. I warmed the piston with a hot air gun and chilled the pin in the freezer before fitting it through the piston and conrod from the cutout side. Don't forget to add the new Gudgeon pin clip in this side and to line the ring gaps up with the piston groove pegs before fitting the barrel. I mark the position of the pegs on the crown  as you can't see them during the fitting process. I can then check "by feel" to make sure that the ring gap is positioned directly under this mark as I feed the piston into the barrel. It's not necessary to use a ring compressor, you can squeeze with your fingers to pop the rings in as the barrel is lowered.