Timing belt tension

[eltax91]

Hi all. I have a 1.8 MK2 MX5 which is undergoing a bit of a winter overhaul. I'm reasonably mechanically minded, but over the last few days have slowly been making my first attempt at a timing belt change. I've stripped everything down slowly, marking it all up and only managed to have 1 stuck bolt, which resulted in me ordering a new timing cover

Anyway, once thing I'm not sure of, despite watching many video's and reading many guides online is if I have the right tension on the belt. I've changed the water pump, idler and tensioner pulley and also fitted a new tensioner spring. I know these are self tensioning, but the spring didn't seem as "reactive" as the old one did when its pulling the pulley back in. I've rotated the pulleys several times and the timing is still bang on, but would hate to put it all back together and find it's slipped a tooth!!

So, I made a YouTube video of the tension in the belt, comments welcome.

http://www.youtube.com/watch?v=xKCsMonhGJI

[Prizam]

That does look a little slack.

[Jaguar steve]

Checking belt tension between the cams can be misleading as in some positions valve spring tension can move the two cams relative to each other slightly when the engine is stationary - hence the slack in your video.

As a very general rule of thumb you ought to be able to twist the belt with maximum finger and thumb force through no more than 90 deg on the longest run of the belt. As you have a spring tensioner belt tension ought to be correct. Provided you have fitted it correctly and timed the cams up right I don't think you have anything to worry about.

For peace of mind turn the engine over several times and check the belt tension remains consistent. If you're still not convinced refit the old tensioner and see if there is any difference between the two.

[eltax91]

Thanks for the thoughts guys. I might spin it over a few times by hand and then re-tension it.

[CrutyRammers]

Steve is right I think - it does look rather slack but I've noticed the same thing between pulleys as the springs relax and the cams turn slightly. If you turn it over by hand a touch and keep tension on, I'd expect that slackness to dissapear.

I find the whole thing nerve-wracking to be honest! But I think there's probably a tendency to over-worry when it comes to cambelts.

[eltax91]

CrutyRammers said:

Steve is right I think - it does look rather slack but I've noticed the same thing between pulleys as the springs relax and the cams turn slightly. If you turn it over by hand a touch and keep tension on, I'd expect that slackness to dissapear.

I find the whole thing nerve-wracking to be honest! But I think there's probably a tendency to over-worry when it comes to cambelts.

Good idea. Next time I get chance I will see if the slack vanishes with the cam's being turned.

In this case, with the mx5 being non interference then I'm not so worried about it being bang on.

[ch427]

Autodata suggests a load of 10kg is applied to the belt between the cam pulleys and the deflection should be between 9 and 11 mm.
Do you have the tension setting mark on your crankshaft pulley/sprocket?
Just to clarify what year is the car?

[eltax91]

ch427 said:

Autodata suggests a load of 10kg is applied to the belt between the cam pulleys and the deflection should be between 9 and 11 mm.
Do you have the tension setting mark on your crankshaft pulley/sprocket?
Just to clarify what year is the car?

It's a very early mk2, 1998 on an r registration.

There are four lugs on the crank pulley and the plastic lower timing cover has a gauge on it with 'T' and '10' marked on it. I can't refit the old one as I had to damage it to remove it, a replacement is on the way already.

What are these lower timing cover marks for?

[ch427]

Not sure on those specific marks but approx 60 degrees btdc there should be a tension setting mark on the crank sprocket by the looks of the diagrams.

[eltax91]

ch427 said:

Not sure on those specific marks but approx 60 degrees btdc there should be a tension setting mark on the crank sprocket by the looks of the diagrams.

Thanks very much for helping. But sadly I don't follow you. With TDC No.1 the woodruff key is at 12 and the marker is at about 12:30 if that makes sense

What I don't understand is how those have anything to do with setting tension.....

[ch427]

I can try and send you the diagrams if you pm me your email address it would be easier!

[eltax91]

ch427 said:

I can try and send you the diagrams if you pm me your email address it would be easier!

Done

[Krikkit]

Looks too slack imo. Doesn't matter on an MX5 anyway, does it? They're non-interference engines!