SR643 motor on ebay?

montynormand

montynormand

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Joined
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Age
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central minnesota
OK, there seems to be some question wether or not this is a SR643, or the 643 yamaha that sno-jet used in the Thunder Jets. Reading on the New Hampshire Snowmobile Museum web site, the guy who posted it does not know. Seems he is getting different opinions. Im not in the market for it (would rather find a GPX433 for my SR chassis).
What do you think

http://cgi.ebay.com/1972-Vintage-Sn...190878241QQcategoryZ64679QQrdZ1QQcmdZViewItem

- Item #Item number: 7190878241
Monty
 
Monty,
This is a sr643 engine because it says R643-.... The r643 means factory race. Like your sr that you are building, r292 it should say on the side of the engine. For the 72 or 73 sr643?, i would say 72 because of the fins on head. Not for sure though.


PS Thunder jets, SR's ? They both used the same engine. (expect the t-jet 338)

So he could say it is a 650 thunder jet engine or a sr643 engine, samething.
 
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Sr643

Here is a copy of Gordon's post over on the NH site. Seems like yamaha used 3 different methods to id the motors. Leads for some confusion
Monty

Time Listed: Mon October 24, 2005 - 9:53 PM .. 11 hours, 25 minutes ago.
Name: Gordon
Email: nachbar@arvig. net
Phone: 218-963-2676
City: Nisswa Mn.
Category: General Info
Title: T-Jet / SR643 engine controversy
This is getting fun. This morning the tally was 3 that say it's an SR and 4 say T-Jet. By this afternoon it is 4 to 7 in favor of T-Jet. I had one guy say he knew for sure that his Yamaha SR643 had no numbers on it. Clay Stroup is the registery author and he says mines an SR engine. I still haven't had anybody come forward with an SR that starts SR643-xxx. Just R643-xxx stamped (like mine), ink printed R643-xxx or nothing at all. Thanks to everybody who responded. One thing is evident, there are SR's and T-Jets out there with all three varaitions. So if anybody with either a 1972 SR643 or a 1972 650 Thunderjet needs an engine, mines on e-bay with no reserve. Search for items for sale by realman625
 
Looks to be 72 SR643. T-jet would have no numbers just like if you bought the SR engine over the counter at a Yamaha dealer. You could buy SR motors back then. Many a yamaha SS ended up with a SR433 in it for power. 72 has angle spark plugs with 643ccR on both Cyl's and heads, also has only two total bolt holes to hold the intake. 73 SR has 643ccRB on cyl and heads, also has four total bolt hole to hold intakes.
 
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thanks for the input gentlemen. Wish I had the checkbook to go after it. Getting my little '73 SR292 race ready has proved to be an expensive task!
 
Yamaha had lots of crank issues with the 643. They sent racers a "STOP do not run!" Bulliten because of crank problem. So many motors didn't live long. Racers would swap or buy other motors to keep the 643 going and most do not have the original motors.

Yamaha did fix the crank issue in a 1973 Bulletin by adding a extra PTO bearing and new tapper crank.
 

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Is this the same reason that the little SR292 has the extra bearing on PTO side? seems like over kill on these little guys....
one of the reasons i bought my SR basket case was for the option to run in the 440 class in another year or two. I would most likely run the GPX 433 in there.
 
If it's a big bolt on bearing housing that bolts on the PTO side of the case PLUS under the engine mount, it's the update. The bearing plate that just bolts to the case is stock.
 
I thought the 72 SR engine on ebay should also have oil injection if it's a SR, but thats easy to unbolt.

another bulliten for ya Monty:
 

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I race small ovals here in central minnesota. I'm having alot of fun in the one cylinder classes.
Trying to build this SR, rebuild my GP's and trying like hell to finsh my new Yamaha Hauler for this season is wearing me out!!!!
I work out of town all week, so my weekends consist of one day for me, and one day to get all my parents chores done!
 
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