TY250A with two spark plug holes?

[mcman56]

I have a TY250A with two spark plug holes.  Did they come that way or could it be an aftermarket head?  Online parts lists don't seem to show two but a drawing in the service manual looks to have two.  I remember having a DT1 with a compression release but don't know if it was OEM or after market.

[feetupfun]

Standard TY250 twinshock head has a single central plug hole. None of the TY250 twinshock heads came standard with a decompressor or a hole for one. The first series of 434 TY250 cylinder has a boss on the front of the cylinder that would allow the use of the type of decompressor that Yamaha fitted to some bikes, that opens a small hole between the cylinder wall above the exhaust post into the exhaust duct.

There are plenty of other 250 Yamahas that have a head that will fit on the TY250 cylinder and have two plug holes. The bike your head came from could probably be identified using photos of the head.

Aftermarket Yamaha heads in the 1970s were made to look spectacularly different to OEM Yamaha heads in a bid to make it obvious visually that they are aftermarket.

[mcman56]

Clearance holes in the fins look a bit crude and not OEM so maybe a modification.  However, it looks like there is a cast in boss where the plug threads in.  

 

 

Edited August 29 by mcman56

[guy53]

It sure look like an original 434 head  with a modification made in an unconventional way. if it work, use it. The original plug wire must be short.

Guy

[feetupfun]

Here are photos of a TY250 head to show the cast-in boss that mcman56 is referring to.