Can someone tell me if the de comp valve on the early (1970) RT1 is different to the later models.I have a barrel with a 12mm tapped hole.?? Thanks, Alan.

Last year of the reed valve RT (1973) had a different updated compression release valve I believe. 

70 and 71 would have been piston port.  I'm not sure the first year piston port (1970) RT model even had a compression release? 

One of the experts will come along with the answers. 

'70 RT1 did not have a decomp valve. '71 RT1B, '72 RT2 and '73 RT3 did. It was 12mm diameter thread. June of '73 Yamaha issued Bulletin # 381. This was a new cylinder for the '72 and '73 with a larger decomp valve with 22mm diameter. Not sure if any production '73s were made, if so it would have been late in the production run. The 22mm valve is also on the '74 360. The hole into the cylinder for the 12mm valve is 4.5mm. The hole for the 22mm is 6mm.
I believe some dealers may have installed '71 cylinders on '70 models for customers who complained of difficult starting.
I have a '73 NOS cylinder with the 22mm valve that I plan to install on one of my riders. I also have head gaskets that are 2.5 times the thickness of stock and Harley Davidson manually actuated decomp valves in the spare spark plug holes of my regular riders.
I need all the help I can get starting these as I get older.

Here's my RT1 cylinder with a decomp valve & was very small as i recal. I put the wrong link I'll try again later. Must be 3 different sizes from RT1 to the last RT3 having the Dt360A 22mm type.
Ok here's the pic i meant to put. RT1 & can just make out it has a decomp.
photos.app.goo.gl/hy9xsgw4PmDJYKvg8
But mine had the centrifugal advance flywheel mag so likely was a mix n match of 70-71 if "70 missed out on a decomp. Was a breeze to start like that. Think they went bigger on early reed 360's but i had the very last RT3 cylinder also which had a huge valve hole same size & thread as the swinging arm thread on a king quad or similar. I never had a valve so machined up one of those as a bung lightly pushing on a ball bearing against the hole in the cylinder. Anyway, you didn't want to know all that. I'll be teaching you bad habits.

Thanks for all the information.I did not realise that there was 2 different size’s.Thought I had bought a lemon.