Basket Cases

[David Lahey]

I then got stuck into lightening the new piston and cutting the transfer port holes in the rear skirt. Have a look back at the earlier piston photos and you'll see how much metal has been removed.

[David Lahey]

Here are a couple of photos to show how the holes line up with the inlet/rear transfer port.
The idea is that fuel/air mix flows outwards through the holes and upwards to the combustion chamber as the piston is moving down. I think this idea was a patent and it was called Torque Induction. The first Yamahas with it came out in 1972. For it to work, the motor has to have reed valve induction and an (extra) transfer port that leads upwards from the top of the inlet port. Yamaha made a big thing of it at the time in their advertising.
By 1978 on other brands there was a reed valve arrangement that was different enough to avoid infringing the patent. The first of these I saw was on George McKenzie's PE175C in 1978 but it was probably on RMs before then.
Later again, the full case reed arrangement was adopted by everyone and continues today in KTMs and Yamahas.
The photos are taken with the piston at bottom dead centre and at the point where the transfer ports start to open on the downstroke.

[David Lahey]

Don't forget this piston is actually made for a DT200 or WR200 but the purpose of that little rectangular cutout in the rear skirt can be illustrated even in the TY cylinder.
This photo shows the piston at top dead centre and you can see the edges of the cutaway section at the top of the inlet port. It is to increase the duration that the inlet port is open compared to a full-length skirt. They could have just had a shorter skirt all the way across but doing it this way, the sections at the side of the cutaway reduce rocking of the piston in the bore.
This cutaway is not needed in a TY175 cylinder but there is no such thing as a purpose-made 72.5mm TY175 piston.

[David Lahey]

I hadn't lightened a piston before but wanted to lighten this one as much as possible to get it down as close as I could to the mass of a TY175 piston, to minimise vibration.
A while back I bought a rechargable Dremel tool and for this job I tried a few different contraptions in it. The best by far was the abrasive paper cylinder on a rubber drum. On this job I went through about 8 cylinders.
I think the mounted grinding stone would have been good too except it kept clogging up with aluminium.

[eagle]

I remember when i first started racing a Suzuki TM125, the local shop said to put a TS piston in , because the longer skirt & extra weight gave them a bit more low down power :) I don't remember any more vibration with the TS piston.

[David Lahey]

I remember when i first started racing a Suzuki TM125, the local shop said to put a TS piston in , because the longer skirt & extra weight gave them a bit more low down power :) I don't remember any more vibration with the TS piston.

No it wouldn't have expected it to have made much difference to vibration on the TM125. The 72.5 mm piston is about 25% heavier than a TY175 piston which is a lot bigger difference than a longer skirt on the TM125 piston would have made.

[David Lahey]

So this is the view with the piston at bottom dead centre showing the rear transfer port open fully open. This transfer port was an advantage over a conventional motor of the time with no rear transfer port.

[David Lahey]

Here are the latest photos of the TL125 tank that I have been bringing back from the dead.
I'm ready to weld the top back on the bottom but have run out of acetylene and am waiting for the welding shop to get going again after the holidays.

[David Lahey]

As part of this top end overhaul I wanted to set the combustion chamber squish to around 1.0 mm. The reason for this is that in 2005 I reduced the compression ratio to where I wanted it by fitting a thicker head gasket and it ended up with too much squish (2.6 mm squish).
So this is my big chance to remedy the situation.
After measuring the squish with the new piston, I've worked out that I can machine 1.4mm from the sealing surface of the head and use a thinner (0.5mm head gasket) to get my goal squish of 1.0mm.
After measuring the squish with the old piston, I was then able to work out how much metal I need to remove from the domed part of the combustion chamber to end up with the same trapped volume at TDC as before the top end overhaul.
I've heard that a modern concept with 2 strokes is to have a different shape to the domed part of the combustion chamber to what is in this head and I'll be changing this head to match that design concept. Don't ask me why it is better, but the modern idea is to have that chamber cylindrical with rounded corners rather than hemispherical.
The pieces of resin-cored solder sitting on the head are what I used to measure squish.
The reason that it is only "sort-of" progress is that my lathe is not big enough to machine the head so there will be a delay in progress on that front until the COVID situation allows accessing a bigger lathe and the tooling to hold the head.

[Kurt]

What inlet rubber do you run on your Big Bore TY175 David?

Our special TY175 has an RD350 inlet plus boost bottle to try and increase the air flow compared to the original dog legged TY175 manifold.

Lacks power though compared to Greg's 300 engined 240 on the hill climbs at Pacific Park in NSW.