Best Cam belt for HC

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Esprit2
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Re: Best Cam belt for HC

Post by Esprit2 »

NickC and I have disagreed about the 90 degree twist test in the past, and we'll just have to agree that we disagree again today.

The twist test depends too much upon a practiced feel for how tight is tight to be recommending it as the correct method to someone who is installing a belt for the first time, or is at least inexperienced. The downside is bent valves and an expensive top-end rebuild. So in light of that, I'll continue to recommend that beginners use a gauge of some sort. I'm not a big fan of the Krikit, but it's a quantum leap more user friendly than the twist test done by unpracticed hands.

I have built a lot of 9XX engines, and replaced more timing belts than I can count. I usually finish off the belt install (after tensioning it with a Burroughs gauge) by giving the belt a twist. I've practiced it for 20+ years, and IMHO, I do know what a correctly tensioned belt feels like. And I'm confident that if a roadside emergency demanded it, I could indeed properly tension the belt by hand. I'm equally confident that most first timers, having been given instructions for the twist test, would get it wrong more times than not. Given the expensive downside, I don't believe recommending the twist test to them is doing them any favor.

My complaint with the Krikit is a low resolution scale that's not east to read accurately in the first place, plus a lack of consistency from one Krikit to another. Yeh, it will put you in the ball park, but it's hard to have confidence that the tension is really 'right'. The Krikit is much better than the twist test, and for it's low price it's okay... but it's still not great.

NickC wrote:
> Any reason blue trapezoidal belt needs to be tensioned different to the 90degree twist?

Yes. The blue belts are significantly stronger and stiffer, and they are harder to twist. For any degree of twist, the blue belt will have greater tension than the black belts (either one of them).

BTW, the black belt twist test isn't the '90 degree' twist test. The correct tension occurs slightly before straight-up-on-edge 90 degrees... it's more like the '80-85 degrees' twist test. 90 degrees is the lower tension limit, and by the time the tension gets to straight-up 90 degrees, it's loose and time to re-tension the belt.

richardw wrote:
> The Burroughs and Krikit gauges do seem to be slightly primitive and inconsistent tools for the job.

True-ish. But when you get right down to it, tensioning a belt isn't rocket science. I find it interesting that one gentleman professes with a passion that the twist test is more than adequate, and the next one thinks the Burroughs is primitive. Reality is probably somewhere in between.

I do like the frequency method best, but even it is less than perfect in terms of repeatability, and working correctly the first time every time. One of it's most notable imperfections is that it's belt-specific. Belts of the same general type but made by different manufacturers will each have a different frequency response. And different constructions, like the black HCR rubber belts verses the black HSN rubber belts, verses the blue HNBR rubber belts produce very different frequencies. The main problem with the frequency method is the lack of definitive frequency specs for all the different belts. It could be a great tool, but Lotus only released retroactive specs (ie, around 1998) for the Lotus OEM B-prefix HSN rubber HTD belt, and for the OEM V8 belts. That's it.

and richardw wrote:
> The engine has to be at tdc (I think) and what about temperature?

The conditions for tensioning the timing belt are clearly defined in the Service Notes.
⦁ Cold engine, normalized to 15°-25° C (59°-77° F) ambient temp over night, before starting it for the first time that day.
⦁ Rotate the crankshaft a minimum of one turn clockwise before setting either the #1 or #4 cylinder to TDC.
⦁ Use the Burroughs Gauge BT-33-86A 5-20 (ie, the Lotus OEM gauge).
⦁ Check the tension midway between auxiliary and inlet camshaft pulley
⦁ Set the tension to 95 units on the Burroughs, cold.

Just a note to illustrate the affect of temperature... If the trapezoidal belt is set to 95 Burroughs cold, as specified above, then the tension will rise to about 125 Burroughs when the engine is at full operating temperature. That's normal.

> I'm flummoxed as to why a decent automatic tensioner was never developed.

One was. It was originally used on the 907, then on the 910-912 prior to 1986 (see page 41.01A in the Parts Manual. The early 907 spring loaded, semi-automatic tensioner had some durability issues. It was articulated, the center hinge would wear, the metal would get thin, and it would break. The end result was the same as for the timing belt breaking... bent valves.

The tensioner went through a series of evolutionary changes, and the last version was pretty good, IMHO. I have it on three 907s, and really like it. Setting the belt tension is a piece of cake compared to the eccentric tensioner.

IMHO, the main remaining problem with the last iteration of the automatic tensioner was owner apathy. It needs maintenance like anything else does; however it worked so trasparently that owners just ignored them. Well, actually, they would have had to at least recognize it's existance in order to ignore it, and they never even did that... until it broke. Then it was the blasted tensioner's fault... right !? Not really. I think the final auto tensioner was quite good, and it was the victim of neglect. Either way, it got a bad reputation, so Lotus dumped it. I like mine, but then, I take care of it.

The eccentric tensioner is as simple as a nut and bolt, and hard for even the most inattentive owner to screw up. However, it's a pain in the butt to set the tension with one (barely nudge the eccentric and the tension jumps 15 lbs), but once set there's not much to go wrong. It has been dumbed down to the point of minimizinig failures. All you need to do now is replace the bearing on a regular schedule... every other belt, or every 48k miles, whichever comes first.

Regards,
Tim Engel
Last edited by Esprit2 on Tue Dec 05, 2017 20:51, edited 3 times in total.

richardw
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Re: Best Cam belt for HC

Post by richardw »

Tim, you have restored my faith in automatic tensioners - thanks for the tip about the later ones being best. Were these only ever fitted to the 907 and early 912 engine? I know the Excel SE never got one but I'm surprised that the Esprit didn't. I guess they can be fitted OK to the later 912 engine?

Thanks, Richard
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Esprit2
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Re: Best Cam belt for HC

Post by Esprit2 »

Richard,
The auto tensioners were used on 907-910 & 912LC engines with trapezoidal timing belts... pre-1986. There were two generations of eccentric tensioners on the very first 907s (Jensen Healey & early Lotus), but then came the auto tensioners.

When the round-tooth HTD belt was introduced in 1986, it was given a new eccentric tensioner and the auto tensioners went away. I don't know the exact introduction date for the HTD belt & eccentric tensioner in the Excel/ 912HC, but I presume it was at the very start of the '86 model year. So no, the 912HC and the Excel SE-SA never got an auto tensioner.

The early auto tensioners' center hinge was metal on metal. A steel hinge pin was pushed through the trunnions of the tensioner's die cast body. When the hinge wore, the steel pin was harder and didn't wear, so it was the more expensive die cast bits that took all the wear. There was no provision for replacing the worn-off material, so the joint just got sloppy, and thinner, and weaker.

In the last version, the trunnions were enlarged enough to make room for replacable Nylatron bushings. If you paid sufficient attention to maintaining the tensioner (and you should), then you could catch a worn hinge before it wore through the bushing and into the metal bits, place the cheap bushings, and restore the tensioner to like new condition... for pennies in parts plus a little labor.

On the other hand, if you're one to ignore maintenance until something breaks, then the bushings can still wear-out. Failure just takes longer.

Regardless of which tensioner you have, timing belt replacement should always include a thorough inspection of the tensioner bearing, and the tensioner itself. Replace the bearing with every other belt change, or at 48,000 miles. Replace the bushings whenever you have the auto tensioner apart to replace the bearing.

Properly maintained, and periodically given new bushings, there's no reason why the last generation auto tensioner can't be reliable, and outlast the engine. At the very least, out-last the tensioner bearing. Then while you're in there routinely replacing the bearing, also replace the cheap as dirt bushings, and you're ahead of the game.

Regards,
Tim Engel

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