I finally got down to Brisbane a few weeks ago and so the story about the top end rebuild can continue.
There's a fair bit of technical stuff later in this story so I'll do a basic version first then give the detail:
Basic version:
The rings were stuck in their grooves in the piston in my 210cc TY175 motor. This reduced the performance. It was put together in about 2005 and had done countless trials since then. I could have cleaned the rings and the grooves and put it back together but George McKenzie who is the brains behind this motor had told me that Wossner were now making a piston that is better than what went into the motor in 2005 so that's what is happening now.
OK more technical stuff now:
The original piston that was used to fit the 72.5mm (big) bore was actually free to me because it was one of a batch made for an unnamed motorbike engine workshop in Brisbane and used to convert DT200s to 230cc and to put it mildly, they didn't work out well. More bluntly, every one they fitted seized. They worked out after a bit that the pistons had been made with the wrong amount of ovality. All pistons are oval at room temperature with the intention that when they are at operating temperature they become round. They need to be made this way because pistons grown more in one axis than the other due to the extra metal supporting the gudgeon pin.
Anyway in 2005 my old faithful 1975 TY175 cylinder had run out of oversizes and George convinced me to give the 210cc big-bore idea a go. George knew of those unwanted DT200/230 pistons and got one for me. The
TY175 sleeve was bored away completely and a new, bigger sleeve was fitted. It was made from a second hand
GS1000 sleeve. The ports were hand cut about 1 mm undersize and then taken out with porting tools to match the port dimensions after it was in place. The head was welded and machined to suit the bigger bore diameter (was 67 mm now 72.5mm) Standard bore is 66mm and nowadays you can buy 68 mm pistons and bore the standard sleeve to suit. You can't go much bigger than 68mm though with the standard sleeve.
The cylinder studs were lengthened so they ended above the head so flat nuts could be used instead of the long steel nuts that are standard. The stud holes in the head were reduced to the size of the studs. If not for this, there would be no sealing surface for the head gasket (the insides of the cylinder studs are very close to the OD of the sleeve). A weakness in having such long studs is that the motor has to be removed from the frame to get the cylinder off. Otherwise, all good.
The bottom end was disassembled so that the hole where the sleeve fits in could be machined bigger.
To get around the problem of the incorrect ovality, George hand-shaped the 2005 piston to get the ovality to where he reckoned it should be. In one of the photos you can still see the marks on the old piston where George did this bit of magic.
George also gave it a bit more clearance than normal to make sure it didn't seize and consequently it made a bit more more piston noise than a standard engine. I got used to the sound and greatly enjoyed the performance for all those years.
When the motor was first put together in 2005, it was very strong everywhere and a bit too responsive for the way I ride trials. First off I retarded the timing to dull it down a bit. It was also a fair bit easier to stall than a standard engine so I reduced the compression ratio, experimenting with different thickness (home-made) head gaskets. I ended up happy with the way it went at low revs but being a bit retarded it wasn't ideal at higher revs. I then fitted an electronic ignition with an advance curve and set it so that at low RPM it was where I had it before and the advance curve moved the timing to standard timing at higher RPM.
This was pretty good except that with the head gasket about 0.75 mm thicker than the standard thickness, I ended up with too much squish clearance which made for an annoying little bit of detonation just off closed throttle sometimes when it was fully hot.
A couple of years ago George told me that Wossner were making a piston that would work in that motor and that's what is in the photos. Being made by Wossner, we are very confident that this piston will have the ideal ovality.
Because the first 72.5mm piston was hand shaped, the skirt was actually a bit below normal diameter for a nominally 72.5mm piston, so George was able to hone the cylinder out a bit to provide the perfect cold clearance for the new 72.5 mm piston.
Things about the photos:
The piston is made for a
DT200 or
WR200 which have case reed induction. They only need that little cutaway at the base of the skirt. The TY175 is pre-case-reed
design and
used holes in the piston as part of the rear transfer porting back then. That's why I will need to cut holes in the new piston to make it like the one I cut the holes in back in 2005.
The new Wossner piston has a flat top near the top edge. Normally piston crowns have a gentle curve all the way to the edge. However remember that this piston is made to make a DT200 or WR200 into a 72.5mm bore and they have done this to the piston so that the DT200 cylinder head doesn't have to be machined to a bigger diameter combustion chamber.
It will be interesting to see if I can get away with this piston crown feature in my motor and still have a good squish design.
Alpina in 1976 and they were definitely a cool thing at the time. Almost indestructible and a wonderful shape and lighter than the equally strong but very heavy steel Bultaco bars. Anyway those bars went out of my life when I sold that Alpina.
Cota 348.