Another timing belt question

Belts, Plugs, Filters, Fuels, Oils, additives etc..... Told you this might morph into servicing and service items.

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Esprit2
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Re: Another timing belt question

Post by Esprit2 »

Pete,
The 907's design work started in the late 1960s. While it wasn't the first engine to use a timing belt, it was very early in the belt industry's learning curve. In hindsight, some things should have been done differently, especially the belt's path round the engine. Especially the placement and diameter of the auxiliary pulley, which resulted in less than 90 degrees of wrap around the intake cam pulley. The net result is that timing belts do not last as long on a Lotus 9XX as they do on later, more modern engines. For instance, the later Nissan 3.0 Litre V6 used the same 133 tooth trapezoidal belt.

If you compare Lotus change interval specs to other car manufacturer's specs for similar belts of the same era, you'll see that Lotus' specs were typically about 50% of mainstream manufacturer's specs for later engines. Like Nissan vs Lotus. That's a wise safety factor to follow.

For the modern/ current HNBR rubber belts, If Nissan (or Audi) specifies changing at 150,000 miles, you should consider half of that, or 75,000 miles to be reasonalble for a Lotus 9XX.

Regards,
Tim Engel
Last edited by Esprit2 on Tue Mar 23, 2021 20:43, edited 2 times in total.

Esprit2
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Re: Another timing belt question

Post by Esprit2 »

Hawaiis0 wrote:
Tue Mar 23, 2021 16:53
I feel that is sound advice. The 2 year rule was based (I feel) on old technology. Modern materials makes belts today far superior.

I have just changed my belt after 5 years in situ. And I do above average miles for an Excel owner.

Most will scoff at my insanity but the old belt was like new still.
I can't agree. The old technology that spawned the 2 year rule is old, yes; but it's also still current.

If you were talking about an clearance engine, your risk would be having the engine quit and strand you. That plus the cost of a tow plus a ride home. For the engine, replace the belt, and all is good again.

However, with an interference engine (which all Lotus 9XX are), if the belt fails while the engine is running, bending all the valves is the 'least' that is likely to happen, and it could be much worse. It's also possible to ruin the cylinder head and/or the pistons. A failed belt quickly becomes an engine rebuild. It's folly to put a cheap timing belt replacement above the risk of an expensive engine rebuild.

And time "IS" as important as mileage... YES, it is. And that is "IF" the car is in normal service. Sitting static under tension in one position for extended periods of time is harder on the belt than normal, regular running. For Garage Queens that don't get driven much, or cars in prolonged storage, the belt will time out in far less than the specified time. All too often owners will say, "The belt was replaced just before the car went into storage 5 years ago, so it's ready to go." Absolutely not!! That belt died in storage about 4 years ago. Don't even hook up the battery until the belt and tensioner bearing have been replaced.

The original black trapezoidal tooth belt was made with HCR rubber and a fiberglass cord body. That's 'old school', going back to the mid-late 1960s when the 907 was being designed. That HCR belt should be replaced every 24,000 miles or 24 months, and time "IS" as important as mileage.

The black, HCR trapezoidal is still made the same way today. Just because you bought a black HCR belt today doesn't mean it's now made with newer, better materials and can be run longer between changes. It's still an HCR/fiberglass belt with a 24K/ 24mo change interval.

The trapezoidal tooth belt's design is OLD school and out of date, and most of the cars it was used on went to the junk yard many years ago... Lotus cars are an exception. The market is to small to justify belt manufacturers investing in new technology for a dying segment, so until very recently, no rubber companies did. Today's black trapezoidal belts are still HCR/ fiberglass, and stil 24k/ 24mo.

Only recently did Gates racing introduce a modern version of the trapezoidal tooth belt using HNBR rubber and modern fibers in the cord body. That's the Gates Racing T104RB Blue belt ("Blue" is Gates RACING's trademark color). And only 'very' recently, the parent company, Gates Rubber, has started producing the same belt under the same T104RB part number, but it's black... not blue. Neither Gates nor Lotus has stepped up to spec a change interval for that HNBR trapezoidal belt on a Lotus 9XX, but it would probably be safe to say 50,000 miles or 7 years of normal use. With prolonged static storage, all bets are off.

*~*~*~*
In 1986, the first black "HTD" round-tooth belts were made of the same old HCR rubber with a fiberglass cord body. But it was stronger by virtue of being thicker ('more is better' theory), and and having a taller tooth to more securely engage the pulley teeth (less likely to skip timing). The change interval spec for that belt evolved/ changed over time, but 36,000 miles/ 36 months.

In 1995, a black HSN rubber HTD belt was introduced with a 72,000 mile/ 8 year change interval. If you have a black HTD belt, and don't know if it's early HCR or later HSN, then the wise approch is to presume it's the older HCR belt... it's old HCR until proven new HSN.

After all 9XX 4-cylinders were long out of production, a Gates Racing Blue HTD belt became available only through Lotus & Jensen-Healey specialists (Gates doesn't list it in their own catalog, or sell it through their own retailers). Like the blue trapezoidal belt, neither Gates Racing nor Lotus has stepped up to specify a change interval for that belt on a 9XX engine. There are Esprit owners who loudly claim they push that belt to 100,000 miles/ 10 years. I think they're pushing their luck with 100,000 miles!! But, since most Lotus cars don't get driven enough to pile on 100K miles very quickly, the 10 year limit would be reached first, and that's very realistic for an HNBR belt.

Just to reinforce a point... static storage under tension is very hard on the timing belt, harder than normal use, and it will perrish in much less than the standard time limit... less than half. The fact that a belt only has 1000 miles on it since new means nothing if the car gets little use, or has been in prolonged storage. The belt is dead, don't try to start the engine!

Regards,
Tim Engel

Esprit2
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Re: Another timing belt question

Post by Esprit2 »

Pete Boole wrote:
Sat Feb 13, 2021 20:19
In people's experience, once the timing belt tension is set how many miles does it stay within spec?
Pete,
A little redundant to my first long reply, but...
Getting back to your original question, if you use a Gates Racing Blue belt, either trapezoidal or HTD, it's very likely that you'll never have to re-tension the belt. One of the HNBR belts' advantages is it's stability. Once the tension is correctly set by turning the eccentric hub Counter-clockwise, it's very unlikely that you'll ever see enough change in the tension to require adjustment. Continue to check the tension regularly, but it's unlikely that you'll have to adjust it.

Also the Gates Racing Blue belts have the longest change interval. So if you wish to minimize timing belt maintennce, go with the blue belt. And of the two types of teeth, the round tooth HTD belt is the best for peace of mind.

Regards,
Tim Engel

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TrevorK
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Re: Another timing belt question

Post by TrevorK »

Interesting reply, Tim. Does it mean that my 1988 Excel belt change interval is 3 years/36,000 miles unless it's definitely a T104RB or an HSD belt?
What car is that?

Pete Boole
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Re: Another timing belt question

Post by Pete Boole »

Tim - thanks again for your invaluable input :D

Pete

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Hawaiis0
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Re: Another timing belt question

Post by Hawaiis0 »

TrevorK wrote:
Tue Mar 23, 2021 21:41
Interesting reply, Tim. Does it mean that my 1988 Excel belt change interval is 3 years/36,000 miles unless it's definitely a T104RB or an HSD belt?
I was going to ask to clarify the same as my newer evolved HSN HTD belt was supplied with a 50,000 mile reference. (Hence my 'scoff' of the 2 yr rule :roll: :oops: )
Umm! What to do next?

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