Britbike forum Forums British Motorcycle specific Brands Triumph bulletin board Primary chain tensioner blade width

On my 1964 TR6R, the original primary chain tensioner was 1 inch wide. The part number was E4149 (now 70-4149). In my parts book, that number has E6061 (now 70-6061) pencilled in next to it. Looking on-line, several dealers say that the later number, 70-6061, fits unit bikes from 1963 through 1972.

The difference is that the 70-6061 is 1 3/8 inches wide*, but only the spring steel backing is wider. The rubber sliding surface is the same width on both blades. It looks like the wider blade will fit on my bike, but I am wondering the reason for the extra width. Is it made that way to fit later bikes, like 750s or triples, or is it supposed to be stronger? It looks to me like a "will fit" afterthought method for eliminating one part number from inventory. The wider blade would overhang the mating parts and look like a bodge. I suppose I could use the later blade and grind the edge off to make it fit like the original.

Any insights?

Ray

*Edit: I later found that the 70-6061 is 1 1/4 inches wide.

Hi Ray,

Perhaps the thicker steep blade is for the triplex chains? Just a thought...

Cheers,

Steve

Hi Steve, that could be, but the surface that the chain rubs on is not any wider, only the spring steel part is wider. I guess it's not a big deal. I can make the wide one work. I was just curious as to why they changed it. No one but Klempf's still lists the narrow blade on their website, but I will do some calling after the weekend. The one that broke was from Klempf's, so I want to get one from somewhere else.

Hi Ray, I have T140 blade in hand. I have detailed photo of '69 blade from motor job I did. Both are very close to 1-3/8" wide.

The 650 rubber is narrower for 2 row chain.

The slot for adjuster spoke is in center of blade.

Looking at photos of inside bottom of primary cover the early & later covers are the same here. I find depending on gasket sent I must trim gasket to clear at bottom on 650.

Looking like the wider steel with 2 row rubber will work good. Actually it may stay flatter than the narrow one?? I didn't know there was narrow until now.

Don

Thanks, Don. Here are some pictures to show what I am talking about:



The left end of the slipper blade (Item #11) fits over the post (Item #10), which projects 1.00 inch from the inner wall of the primary. The parts book diagram shows the slot in the slipper blade off-centered in the wrong direction, but the rest of the picture is good. The 70-4149 slipper blade being 1.00 inch wide will fit onto the post with no overhang. It can be installed without fully removing the stator from its mounting bolts, though the stator does have to be slid outward to install the blade.

Here is the top view of the broken 70-4149 slipper blade that I just removed from my primary chaincase. The rubber pad and the spring steel backer are both 1.00 inch wide:



Bottom view:



Broken end:



Here is a top view picture of the 70-6061 that dealers are selling to replace the 70-4149 slipper blade:



The rubber sliding surface is about the same width as on the original narrow blade, but the spring steel backing is 1.38 inches wide*. This means it will extend .38 inch past the mounting post (Item #10 in the diagram above). It also means I will have to slide the alternator stator further off the mounting studs, which is problematic for the stator harness.

* Edit: I later found that the 70-6061 is 1.25 inches wide, and will extend .25 inch past the end of the mounting post.

I could use the wider blade and grind the edge back to make it like the original, or I could wrestle with the stator and install the wider blade.

I realize this is an inexpensive part, and I don't mean to make a big deal over it. My question is why did they make the blade wider for no apparent reason? Did it make the spring stronger so that it won't break like mine did?

This is a rather critical part. I was lucky that my chain only had 3/4 inch of up-down free play with the tensioner lying flat in the bottom of the chaincase and doing nothing. That is still double the spec, but the chain stayed in place. If my chain was a little more stretched, it could have jumped the sprocket and stuffed the whole engine.

they make a a replacement that is both wider at the back
and is ground down at the front , to fit behind the stator

the wider back might be an and Improvement against metal fatigue ?

How bad was your original Ray, do you still have it? Two reasons I ask. I have also had an after market one break as yours did. The one I replaced it had a slipper pad made from non oil resistant rubber, it consequently disintegrated very quickly coating everything in the primary case with a sticky black residue that was no fun to clean. I ended up fitting a used one I found in a box of crap under the bench.

quinten, thanks, I had not seen that version.

A friend who used to have a side business selling Brit parts still happens to have one of the wide blades in his remaining inventory that he got from Coventry Spares 20 years ago. I think I will buy it from him and grind it as per the feked.com picture you posted. On the other hand, if the wider backing gives an improved strength against metal fatigue, maybe I should leave the full width on the whole blade. Like the old saying, a chain is only as strong as its weakest link.

Rod, thanks for chiming in. It's great to hear from you again. Sadly, I do not still have the original blade*. Seven years ago, I was having an issue with losing primary oil. It was going out through the harness sleeve on the stator wiring. (see that story HERE) Because the oil was going away quickly, the chain often ran with little or no oil and therefore it stretched. My old tensioner was arched up quite a bit to compensate. I was scared it might break, so I replaced it with one that actually did break!

So, seven years ago, I installed a new alternator, a new chain, and a new slipper blade. This bike gets used rather sparingly, so it has covered only 6,000 miles since then. I would have thought the blade would last longer than that. It had only a slight arch to adjust the chain. I was doing routine maintenance on the bike this week, and found the chain with too much slack. No amount of turning the adjuster made any difference at all. I pulled the outer case and found the broken blade.

*Edit: I later found my old blade in my storage bin where I store every shred of historical junk that was once a part of this old bike. Since I have convinced myself that the newer style, wider blade is better, and it was already installed, I plan to keep it for now. If the new one breaks and the engine survives, I will put the original tension adjuster blade back in.

Just to wrap this up, I got the newer style tensioner that my friend still had left in his inventory. I found that it is not 1.38 inches wide as I had read in an on-line description elsewhere. It is 1.25 inches, which makes it a bit easier to slip in behind the alternator stator without fulling removing the stator. I decided not to grind anything off the edge of the new tensioner, but to leave it full width in hopes that it will be stronger. Here are some pictures for comparison's sake:



Surprisingly, although the new tensioner blade is wider than the old one, the actual rubber strip on the old one is wider by about .055 inch.



The old blade (70-4149):



The new blade (70-6061):



I'm no expert at Failure Analysis, but I will theorize that the narrow piece of the old blade broke off first, which put the full load on the slightly wider section that remained. That section was not strong enough to hold the load by itself and eventually fractured off as well. This spring is not working in a static situation. When it is tightened to adjust the chain, one surface of the blade is in compression while the other side is in tension. Now add the vibration of the chain links rubbing over the surface and consider the jaw dropping power of this 650 Triumph engine, and it is no wonder the old blade broke.