[v12-engine] Tensioner topic

From Kirbert:>Interestingly, that’s not how mine broke. Mine broke the arch itself

into about three pieces, as somewhat visible in the pix on


But that may be how I broke it, which was not by retracting it but
rather by releasing it too abruptly – I basically just tripped the
latch. That was before anybody told me the things were brittle.

The one from the HE being rebuilt was broken only on the pin bore.
While figuring out a retrofit kit that one was flexed from normal bowed
position to fully flat many times without harm.

The other was broken at the pin bore and had two large cracks on the arch.
That shattered when trying to flex it.

Sweet! Are you able to lever the screwdriver against something, a
bolt on the damper plate or some such? Or are you directly pushing
with the screwdriver?

As it happens there is a spacer for the damper hardware just in the
right place for a fulcrum.
I checked afterwards and using it as a fulcrum point did not disturb the
damper settings, but it is something to watch.

compressive force needed is about 20kg. My estimate is the danger of
breaking the pin bore is about 50/50 on the average engine using the
recommended technique. It will be more like 5% using the

Agreed. I wonder why nobody came up with this idea years ago.

Another idea was tried but it did not work out too well. This was second
best but it looks after all it will be ok for most DIY V12 fixers.
As mentioned you can get by up to a point by not retracting the tensioner to
get the cam sprockets out.
Putting the sprockets back on is a struggle if the tensioner is not retracted.

I jury rigged the HE and tried it with the new retrofit tensioner retracted.
Even then it is tight to get the sprocket the last couple of mm onto the end
of the camshaft. I only just made it with the normal two hands we all have.
A second person to help makes it a breeze.

Tensioners are a very expensive spare part. Starting with the metal
parts from the old tensioner I made up a steel strip spring with
riveted on acetal wear track to replace the broken plastic part.

Exactly the fix I suggested years ago. I’m glad somebody finally
tried it. Your part is visible in the pix of the screwdriver
retracting method.

That tensioner in the photo was the first prototype made with a sample of
spring steel with bits out of the junk box welded on for the 3 pin bores,
the spring retaining pin and the latch pin. The relative geometry of the 5
holes is what took most time to figure out, and a cheap and simple way of
fabricating a steel item for small scale batch production to get all 5 holes
just right.
On this one the holes were not optimum and I had to turn down the neck of
the latching rod to get the thing to latch.

One thing not possible to duplicate is the way in which the original arch
has tapered webs. That makes the OEM tensioner slightly flatter when
retracted. Still, the retrofit one is as near as possible for steel.

After that prototype I had 30 springs punched out on a CNC press. The pin
bore is now two pieces of laser cut 5mm steel with 3 holes as required. I
welded the next prototype more carefully to get the latching rod working
without modification. The welding jig is half done to make the 30 final
units. The welding will be done by an expert with a MIG welder, not by me.

The only hitch will be if the tooling for the OEM tensioner was changed
sometime and the latching rod modified, My guess is the tooling was
probably not changed because it actually works quite well in service. It is
a clever design when you look closely at the OEM unit, the only
weak point being the method of retraction. If the cross sectional area of
material around the lower pin bore had been increased by 50% very few would
have broken, bar a major overheating of course.

It won’t break at the arch, either. I expect that, if this design
breaks at all, it will be that the “acetal wear track” somehow gets
ripped loose from the metal arch – and if you’ve attached it
securely, there’s not much chance of that happening.

The acetal is 5mm thick. This is the recommended engineering plastic for
the job.
It is held on with 12 X 5/32" pop rivets.
The two broken tensioners showed very little wear on the track surface.
It probably takes a long time to wear it out.

Richard Dowling, Melbourne, Australia, 1979 XJ-S & 1985 XJ6.

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