I got one for my '74 850 from: RMA Engineering, California,phone# 408 - 578 - 4032. Cost $110.00, it took a little work to fit it but well worth the effort, the instruction sheet is very comprehensive.You will find,if you fit that one, that the cam chain is tighter than with the stock tensioner, so far that does not seemto be a problem -- 600 miles since fitting it.
Good luck. James.
I have had one on my 75 MK3 for several years and around 4500 miles or so---no problems--yet. Good instructions--you do have to use a file to remove some aluminum material from the boss at the bottom inside of the case--no problem--plenty of excess material--just work carefully.
Best to install a new cam chain at this time also. And all new seals and gaskets. Rework the oil pump as well since your in here--tighten up the clearances.
I have just purchased another tensioner I plan to install on a 74 MK2A 'm working on. Perhaps the best feature of this is when you install it, you don't have to fool around with the tensioning procedure. Kind of tricky if you don't have a cut-away cover on hand. Not to mention that there are several different schools of thought on how to go about getting the proper tension. Perhaps this may make for another forum topic someday.
I notice a lot of guys install these auto cam chain tensioners and reckon what a great job it does. But at the end of the day what advantage is it?
The chain runs in oil so there is minimal wear. I would think this mod does not affect performance or handling. Maintenance on this item should be minimal.
Am I missing something that makes this a must have piece of gear?
It is not intended to produce an increase in HP or lower your ET in the quarter mile---it does help to eliminate the chain whip and perhaps make the engine run a bit smoother. By chain whip I mean when you decelerate and accecelerate in rapid succession the chain being under constant tension dosen't create a timing problem--ever so slight.
I like it for just being another small improvement or perhaps another update on an old design. Every little bit helps.
Maintenance is minimal but it is one less job that you have to fiddle with if you buy the tensioner. For me the pleasant surprise was that it gave that final little tweek to the tune up. It went from a 2 kick starter to a one kick starter and it gave me a more stable idle.
In reply to dgwilson, the workshop manual recommends checking/adjustment every 5,000 miles. To do the job you need to remove the ignition and autoadvance unit and the timing cover. You are then supposed to support the intermediate gear spindle with either a cut-down timing-cover or some other special tool. You will probably then have to replace the oil pump cone-seal and possibly the timing cover gasket and then reassemble and retime. There is a theory that the spate of engine blowups associated with the early Combat engines was at least partially due to fluctuating ignition timing, when cam chains became a bit loose after a couple of thousand miles of being thrashed.
In short, the automatic chain tensioner will not give any major performance gains, but will remove a time consuming and potentially problematic chore from your list and hopefully also maintain consistency of timing set-up for longer.