good even in all,

it’s been a little while since I posted anything as it’s been a busy time for me with various personal issues but I am back in the garage albeit in a limited fashion.

my ‘70 rt1 project is a labour of love and I try to work stuff out where I can but I could do with some input from you guys please. 

my original barrel and piston are not the worlds best although serviceable in terms of compression. I have procured cheaply another barrel piston and head in better condition but am unclear of what they actually are. The head appears identical externally to mine stamped 275.  

the piston seems to be stamped 76 and inside the skirt it says Yamaha Izumi 4.  This I have checked on line and it seems to indicate 4th oversize. Not worried about that as I won’t be doing thousands of miles on it. So this seems ok

the barrel/jug has no cutaway machining for a decompressor and has the correct inlet type (non reed). The fins are great on head and barrel. But.  There are no cc numbers in the casting and everything I can see online has the cc stamped. 

is this a common thing or is it an indication of a different barrel type? MX perhaps?

could someone shed any light?  Ideally I need to know I am ok to use all three items on my enduro. I can take more photos if it helps. Sorry if I am being thick and missed something obvious already on here.

hopefully the link works to the pictures. 

thanks as ever for your time

ian. 

share.icloud.com/photos/008U2_5u6lVPEg0KdHh2vPRNw
 

The 275 stamped head is a 70-71 RT1 360 enduro head. If you look up the part number on partzilla or babbitts you'll see the enduro part number starts with 275 where the M part number starts with 284.
As for the cylinder i don't think its an M cylinder because the fins just don't look to be cut back to clear the GYT pipe. 68-71 Yamaha used GYT kits, it wasn't until 72 when they came out with a true MX bike.
Are you sure its not a 250 cylinder? measuring the bore will determine that.

Im probably wrong on this but if that is a 360 cylinder, i believe the 1970 model had the retard mechanism on the flywheel so Yamaha didnt put a decompression release above the exhaust. Ive heard they did that only in 70 but ive also heard that was only done in 71? and that they both had the decompression releases still. Hopefully someone here will know more about that but right now im thinking you have a 250 cylinder.

Thanks for the swift reply enduronut. I agree the 70 didn’t have the decompressor. Hence why I thought it was an indication it was the right one for my year.

I don’t have a super accurate micro meter but a cheap slide rule one which isn’t really big enough. But the piston measures over 80mm but under 81mm on the skirt as best I can see. I was assured the piston barrel and head all came off the same engine. I think the piston measurements indicate 360

If the barrel had the cc size on it I wouldn’t have queried the set up.

When I take the existing ones off the bike I will be able to compare everything including ports etc.

Does that help?

Many thanks

Are you sure piston isn't stamped "75"?  That would be 3rd over and oversize is stamped on top.

You could measure the port heights and compare them to yours. 1970 RT1 did not have decompressor, that feature started in 1971.  1970 would have had the advance mechanism in the flywheel to retard timing for starting instead of compression release.  

P.S.The above is general.  Check your VIN to identify model year.  But even then if you have a model that was originally sold outside the USA, sometimes features and paint colors varied.  For example, Japanese market models sometimes got features and paint scheme a year early.

I type too slow.  LOL

I wanted to add something...  Yamaha did sell "MX" (aka "M") models before 1972 and the "off road only" prepped version you could buy at the dealer did not always come with a "GYT" cylinder.  In the early years, they actually had 3 cylinders for many models.  Enduro, MX, and GYT.  

Hi mark.

I do have a 1970 RT1. Vin and flywheel set up as you say.

My issue is with the pictured barrel piston etc that I have bought separately. I was told they were off a 70 rt360. The piston could well be a stamped 75 now I look again. I have added a new photo to my link. It’s just the fact there is no CC number stamped on the barrel

I will check and compare the recently acquired one against the barrel on it when I remove it. I just wondered if forum wisdom would say “aaah…. No CC stamp means xxxx”

Don’t want to waste your time.

share.icloud.com/photos/061-S_Yrg-aBvUUVVbESAEe2Q

Cheers for the input.

3rd over piston.

There could be something about the displacement missing that means something.  I don't know what it is though.  My guess is "early production"?  Lots of experts on here, hopefully one of them has an answer.