Classic Dirt Bike mag - Project TY

[JC]

Does anybody know if the Project Ty article in the first few issues of Classic Dirt Bike mag is TY250 or TY175?

[vandy010]

175, and then i believe it was offered for sale.
i'd need to read the article again but i think the barrel was resleeved then bored to take a TY250 piston and then a spacer was fitted under the barrel for correct piston clearance. something like that anyway. i belive it's a common mod to the 175 to do that.

[JC]

Thanks Vandy,
Thats probably the 200 kit for TY175s that tytrials.co.uk offers
Altho the 250 piston has an 18mm gudgeon while the 175 has 16mm

[David Lahey]

When a TY250 piston is used to big-bore a TY175 motor, shouldered sleeves are usually fitted in the gudgeon pin holes of the TY250 piston and yes a barrel spacer is needed.
There are other pistons that are easier to use ie KDX220, Yamaha (something like Blaster) quad and my 205cc TY175 has a piston originally intended to big-bore WR200 and DT200 motors, and doesn't need a barrel spacer or gudgeon pin bushes - and is 72mm diameter.

[JC]

D,

Did you get a liner/sleeve made up locally to fit the 72mm piston? If so, who made it?

[David Lahey]

Yes the TY175 big-bore job was done locally. Labour mainly by brother-in-law George McKenzie and the donkey work cutting the ports in the sleeve and assembly by myself. The sleeve is second-hand Suzuki GS1000.
To use a 72mm piston there are a few extra things needed compared with using a 70mm TY250 piston in the TY175.
Machine out the crankcase hole for the new liner.
Weld up the head nut holes and redrill to allow longer studs to go right though (to provide a wide enough head gasket sealing surface)
Machine the head to get the compression ratio and combustion chamber shape right.
Cut holes in the back of the piston to match the TY inlet port.
Make a 72mm head gasket and bigger base gasket.
Lighten the piston as much as possible.

It was George's idea to hot up my TY175 to see how well it went.
George is a legend with motors and race tuning, and runs a business in Brisbane producing amongst other things, killer performance motors for VMX bikes, and is no slouch as a rider either - being an Aussie title holder in both VMX (pre-75 class) in the 1990s and lately in Observed Trials (pre-65 class). If you are thinking of getting a TY320 motor or a big-bore TY175 motor built up, he would be just the man.

By the way, the TY205 does perform strongly and is great in big sections but the quicker motor (standard flywheel mass) combined with the short wheelbase of the TY175 frame does make it less relaxing to ride than with the standard TY175 motor.

[JC]

Yes, I've heard of George's reputation.

Sounds like heaps extra work compared to usung 70mm piston.

Does the 320 conversion use an 80mm 360 piston? Does that require macining out the cases too?

[David Lahey]

No I haven't worked that out but expect that it would.
I haven't pursued it because I reckon the motor is perfect for trials as a 250. Steve Holzhauser has a 320 Majesty and reckons the motor is not that good for modern trials because the flywheel has so much flywheel mass added to tame the motor response down that it takes ages to lose RPM after jumping up a big obstacle (while the rider balances stationary), before it can be ridden gently again. When these bikes were developed, bikes were not allowed to stop in a section and riders never used the clutch so the heavy flywheel mass wasn't a problem. A 320 would be a blast for blasting up hills though.

[subanator]

All this big boring must screw up the porting, is this compensated in anyway? Could end up with a pig if not looked into properly.