Problems Getting engine timing on S3 N/A LC

[G_Reaper]

Hi Everyone I have a few questions. I have replaced many timing belts on various Lotus cars using the 900 series engine including S4s, S3 HC Turbo, S2 etc however this S3 G ar is giving me a real head ache. The car has been off road for many years I cut the original timing belt off as it was in a bad way. I ordered a new Gates belt 5146 however I received a Blue Gates RPM T104RB belt which was supposed to be an upgrade as they had no standard belts in stock.

1) Has anyone fitted one of these blue belts? If so was there any problem getting the timing to be correct?

2) Is one tooth on the crank pulley equal to 10 degrees?

3) Can anyone provide a photo of their cam pulleys with timing marks lined up in relation to the cam boxes.

I will try to post photos of the cam positions with the engine at TDC. If the engine is at 10 degrees before TDC the cam timing marks align.

Any other observations advice would be greatly appreciated.

Cheers Mike

[snowrx]

1) I am running the T104RB on my Turbo Esprit, it's the same tooth count and a mm or two narrower, and went on fine.

2)My crank pulley has 20 teeth, so I would assume 360*/20 would give you 18 crank degrees per tooth.

3) Is one of these shots what you're looking for? 

[G_Reaper]

Thanks that's good info. Have you any photos of the cam sprockets looking from the rear of the engine? Did you fit your belt when the engine was out of the car?

[G_Reaper]

Here is a photo of my timing marks. When the engine is approx 10 degrees after TDC the cam sprockets line up? The photos show the engine at TDC.

Edited August 21, 2023 by G_Reaper
text missing

[Sparky]

Has the head been skimmed?

[Escape]

The cam sprockets have 40 teeth, so 9° per tooth. If those are one tooth off, that could explain what you're seeing.

[G_Reaper]

Those are the sprockets having been perfectly aligned prior to the cam belt fitting and after one engine revolution of the engine. When the cam timing marks are aligned the crank is approx 8 to 10 degrees after TDC no matter what I try I either end up before TDC or after. I cannot get the timing marks aligned. The engine has been rebuilt many years ago so it could have been skimmed but  I think that would have to be a severe skim to create a marked difference to the timing surely? I've never come across that before and I have rebuilt quite a few engines? But is this a known result of head skimming? If so perhaps I need some vernier sprockets??

Cheers Mike

Edited August 21, 2023 by G_Reaper

[910Esprit]

It would be pretty straightforward to put a dial gauge against both No1 in/ex valves to confirm what point they are each opening relative to TDC (Which should be the same for both).   That will prove definitively if you have a sprocket misaligned, or something else is going on.

[jonwat]

12 hours ago, G_Reaper said:

Here is a photo of my timing marks. When the engine is approx 10 degrees after TDC the cam sprockets line up? The photos show the engine at TDC.

 

Those belt teeth don't look very well defined, is it a round tooth belt or trapezoidal?

[910Esprit]

And the prize goes to John - T104RB is a round tooth profile belt. 

Edited August 22, 2023 by 910Esprit

[910Esprit]

Or maybe not...  There seems to be a lot of misleading info in the online catalogues - some say Trapezoid, some say Round.  Most illustrations are definitely round profile.  But Mike's pictures above look suspiciously round.  I'd be tempted to go back to a standard trapezoid belt, as the semi-automatic tensioner needs regular examination regardless of the belt

[drdoom]

Absolute fail if indeed the belt is of round tooth form, those are trapezoidal wheels.

[Sparky]

5 hours ago, 910Esprit said:

I'd be tempted to go back to a standard trapezoid belt, as the semi-automatic tensioner needs regular examination regardless of the belt

Absolutely this.  Why have an 'immortal' belt when you need to regularly inspect other stuff?

[G_Reaper]

4 hours ago, drdoom said:

Absolute fail if indeed the belt is of round tooth form, those are trapezoidal wheels.

This belt was supplied as an upgrade to the 5146 standard belt I ordered from a Gates distributer as they had no stock left of the 5146. I assumed this was a correct direct replacement for the 5146.

[snowrx]

The black/ blue layer looks undefined in photos, but it is a trapezoidal belt as used on some Nissan Z cars.

I'd switch to a round tooth if someone wanted to post me some pullies, but this does for now-

[G_Reaper]

On 21/08/2023 at 18:30, snowrx said:

1) I am running the T104RB on my Turbo Esprit, it's the same tooth count and a mm or two narrower, and went on fine.

2)My crank pulley has 20 teeth, so I would assume 360*/20 would give you 18 crank degrees per tooth.

3) Is one of these shots what you're looking for? 

Hi Steve are you running the later spec HTD cam pulleys? Further note I have tried timing the engine with the belt on both ways I know in the photos the belt is on the wrong way around but the results in trying to time the engine remained the same, I now have a Standard Gates 5146 to try out. 

Edited August 22, 2023 by G_Reaper

[G_Reaper]

3 hours ago, snowrx said:

The black/ blue layer looks undefined in photos, but it is a trapezoidal belt as used on some Nissan Z cars.

I'd switch to a round tooth if someone wanted to post me some pullies, but this does for now-

So the T104RB is the correct belt for the Trapezoidal pulleys it appears?

[G_Reaper]

OK I have found the answer the belt T104RB is classed as a Trapezoidal Belt not round tooth.

[jonwat]

Looks pretty bloody round to me

[Escape]

Other websites list it as round tooth. The difference may not be clear in a picture (never trust a picture on a website anyway, half of those are generic and not for the specific part no!), but should be evident looking at it.

[G_Reaper]

I can stop the speculation regarding the T104RB profile.  The official specification states "Trapezoidal". I have placed it against the original Gates 5146 Trapezoidal belt and it matches exactly as stated earlier on this thread it is the colouring which seems to fool the eye. The blue in each tooth is semi circular but the outer part around the blue on each tooth is square. I have tried it against my set of later HTD cam sprockets and it is no way round in comparison to the profile on the HTD pulleys.  The profile can be seen on the photo next to the HTD Cam Sprocket.

[lotus-62]

20 hours ago, snowrx said:

The black/ blue layer looks undefined in photos, but it is a trapezoidal belt as used on some Nissan Z cars.

I'd switch to a round tooth if someone wanted to post me some pullies, but this does for now-

what is the big advantage of a round tooth belt / pulley if I may ask?

[Sparky]

Chance of slippage is massively reduced.

[drdoom]

1 hour ago, Sparky said:

Chance of slippage is massively reduced.

 . . . why the car manufacturers long ago went there.

[G_Reaper]

Does anyone have the Gates part number for the HTD HC belt? Now Ive found my set of HTD pulleys I am thinking of fitting them instead.

Cheers Mike