Can anyone confirm that the compression test port (hole next to the spark plug) on a DT175B is m14x1.25? Mine is missing and its not called out in the parts breakdown I have.

Yes. And plug for hole is still available from Yamaha.

Awesome, thanks Mark. Do you happen to have the Yamaha part#? Not sure if it’s cheaper to get a plug from McMaster Carr or try to get a Yamaha replacement. Any idea what the factory plugs are made of?

Found the part number 90340-14022-00

Rustie,

Why not use a second spark plug? That's what is in my RT-3 right now. That's what everyone that didn't like compression releases used, back in the day. Comp releases didn't filter the air that was taken in by the release (when the piston goes down, it sucks air in, along with the dirt, sand, whatever). I needed one in the 1972 Grand Prix held at McGinn's Motorcycle Park in December, in the snow, in Sacramento. I was running first in the 500 Novice Class when I fouled the first plug ever in my RT-2. Swapped the plug cap, and picked up in 3rd place, which is where I finished.

Best of luck, and STAY HEALTHY!

Hammer

That's the part number for the earlier version... should work still work though. Partzilla, RMATV MC, or dozens of Yamaha online parts catalogs will show the part number for the 75. Look in the "cylinder" section.

IIRC, the equivalent Suzuki part is much cheaper... try looking up one for a 73 TS400.... they're like $3.

Ok, I can only find one Yamaha part number for anything from 69-79. I did find the Suzuki plug you recommended
www.partzilla.com/product/suzuki/09247-14025

You're right on the Yamaha... bought one a couple years ago and it had a specific part number with the 3 digit model code prefix but looks like Yamaha now superseded to the generic "bolt" number.