Jump to content
Click here if you are having website access problems ×

Sigma cam belt tensioner - Now a how to guide :)


DJ.

Recommended Posts

5 years ago I had to change the timing belt on my Sigma 125 after the original water pump failed at 13,000 miles. Fortunately, the one I fitted has survived 24,000, but I think it's time to change the belt/pump again.

In the meantime, I have heard that racers don't use the standard sprung tensioner as the belt can jump if the engine turns backwards during a spin (my Seven has been spun a few times by myself and my daughter *thumbup* )

Can anyone tell me how to do the suggested modification?

Thanks

Duncan

Link to comment
Share on other sites

Caterham used to do an item that would convert an auto tensioner to a manual one. Basically a new center boss/washer with a off center hole for a few quid. Not sure if they still do it as I couldn't find it in their stores - but that could well just be me! *biggrin*

Ivan

Link to comment
Share on other sites

I've looked on Caterham's site and three items seem to be needed:

Tensioner boss:

https://caterhamparts.co.uk/other/2763-tensioner-boss-sigma-150hp.html?search_query=sigma+150+tensioner&results=240

Tensioner washer:

https://caterhamparts.co.uk/other/2764-tensioner-washer-sigma-150hp.html?search_query=sigma+150+tensioner&results=240

And Tensioner collar:

https://caterhamparts.co.uk/other/2765-tensioner-collar-sigma-150hp.html?search_query=sigma+150+tensioner&results=240

I've also found a thread on Pistonheads that describes fitting, but the guy did it incorrectly. Apparently it is possible to modify the tensioner so the spring can be used to tension the belt correctly and then the Caterham parts are used to nip the eccentric part of the bearing as the mount bolt is tightened so the tensioner can't move any more.

I think I'll take some photos as I do it in case anyone else wants to try this.

Link to comment
Share on other sites

Great, I saw those bits but thought they looked different to what I remember but if you've found a thread about them then I must have miss remembered *thumbs_up_thumb*

That would be great if you could, I have a 150 and am heading towards cam belt change time again so want to check the tensioner while I'm doing it!! *smile*

Cheers

Ivan

Link to comment
Share on other sites

I have had a chance to try disassembling the Sigma tensioner. I practiced on an old one first to check that the dimensions were the same after modification. I am pretty sure I have it right, but there are people on here much more knowledgable than me. If you see something I have done wrong, please tell me, I won't have a chance to fit it to the car for a few weeks *smile*

The first task is to take the new tensioner apart. I released the spring tension using a vise to grip the arms before removing the pin, and I marked the spring and bearing unit to ensure the spring went back the same way (this may well not be imortant). The centre boss is peened over the top washer shown below, I protected the bearing from dust by putting tape over the top face and cling film under. Holding the unit in the drill press is a pain, I made a small block of wood to support it. In the photo, I have already started drilling into the top of the boss:

tensioner_1.thumb.jpg.430683d3ef37959e30f0c89513fc7e24.jpg

Here is a picture of the replacement parts supplied by Caterham.

 

tensioner_2.thumb.jpg.d409e04c5f3335285c7a3be2dfe65b64.jpg

Here are pictures of the tensioner bearing after the base plate is removed:

tensioner_4.thumb.jpg.d13a609c62862ff3948a02d1a3557537.jpg tensioner_5.thumb.jpg.d15774ec35d03916fe0a166dd6b3cf9a.jpg

And these are the parts to be discarded:

tensioner_3.thumb.jpg.aff961aef4c36fdad54596a8e2314433.jpg

The next stage is to carefully enlarge the hole in the base plate so the Caterham supplied aluminium boss is a snug fit. This was my first attempt on the old tensioner, the second one was much neater, but I forgot to take a photo and couldn't be bothered to dismantle the tensioner again *rolleyes*

tensioner_6.thumb.jpg.ee32c825e38e9a35cb5288a4de13aec8.jpg

Here is a photo of the boss fitted into the new base plate:

tensioner_7.thumb.jpg.8eae6a7bc0f2b4b2c5bddc2cedbf37ef.jpg

And here with the Caterham washer fitted:

tensioner_8.thumb.jpg.a3c7dab218db8b59609e072e66826ed5.jpg

The bearing assembly and spring are then put over this. Here is a view from underneath, you can see the spring pushes the back plate away from the bearing:

tensioner_9.thumb.jpg.6bc682ddcb606ea4433fb6e653339f5a.jpg

Here from the top you can see the Caterham tensioner collar in place on the top of the bearing. The raised portion appears to be designed to locate in the large hole in the bearing centre.

tensioner_10.thumb.jpg.1cb33c7cd2ee15c789df5059b9bcc9f7.jpg

The last stage is to re-tension the spring and replace the pin. As the caterham boss doesn't hold the unit together, I found a nut to fit the retaining bolt supplied with the tensioner and used this finger tight to hold the unit together. I then used the vise to pull the arms together carefully and replace the pin. Obviously when it is fitted, I will remove the nut and have to carefully hold the unit together as I attach it. The design of the modification is very clever, on assembly the correct belt tension is applied by the spring and then as the mounting bolt is torqued up, it clamps the bearing into a fixed position.

I hope this is useful to fellow Sigma owners.

Duncan

Link to comment
Share on other sites

  • 4 years later...

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
×
×
  • Create New...