CAM pulley markings

This forum is specifically for engine and gearbox posts, please try to keep on topic in any thread to make things easy to find

Moderator: Board Moderators

Post Reply
User avatar
Hawaiis0
Senior Poster
Posts: 4130
Joined: Sun Jun 30, 2013 18:39
Model: Excel SA (No 3); Elite 504
Colour: BRG; Dirty White
Year: 1986
Location: West Oxfordshire

CAM pulley markings

Post by Hawaiis0 »

Hi all, one for the Engine Guru's .

I have new CAM pulleys and noticed the alignment dots are not aligned. One is half a tooth out.

Are these purely a visual aid and it's the keyway that does all the important stuff?

They are all viewed from the EX side

Image

Image

Image
Nothing is fool proof. Fools are clever!

User avatar
rbgosling
Senior Poster
Posts: 1702
Joined: Sun Jul 04, 2010 22:55
Model: Lotus Excel SE
Colour: Midnight Blue
Year: 1990
Location: Rugby

Re: CAM pulley markings

Post by rbgosling »

The keyways are crucial for keeping the pulley aligned with the camshaft, but you need the dots to make sure the cams are orientated correctly when fitting the cambelt.

The fact that the teeth line up when the keyways are aligned is encouraging (assuming the keyways are perfectly aligned), but setting the cambelt up correctly won't be made any easier with the misaligned dots.

I can't recall how the dots were aligned (on a tooth or by a dip) on my cams when I rebuilt the engine, so I can't compare to a Lotus original, maybe I can get a photo later...

How come you have three pulleys there? Or is one of them the old one off the car? Why do the pulleys need replacing?
"Farmer" Richard

1990 Lotus Excel SE (Lilith)
2022 MG MG5 EV (not due to be a classic for quite a few years...)
2011 Nissan Leaf (Ragly - EV pioneer, must be due to be a classic one day)

User avatar
DavidOliver
Senior Poster
Posts: 688
Joined: Fri Nov 07, 2014 11:18
Model: Excel
Colour: Calypso Red
Year: 1985
Location: Malaga Spain

Re: CAM pulley markings

Post by DavidOliver »

If you still have the old belt installed set the crank to top dead centre (TDC) using the crank pulley tdc indicator.
Then check the existing pulleys for dot to dot coinciding on inlet and exhaust pulleys, these dots may not be centred on tooth or dip.
Note the position of the Auxiliary pulley dot, it should be in line with the auxiliary shaft and crankshaft centres, but if not this is irrevelant.
To change the cam pulleys it is convenient to loosen retaining bolts, remove and replace pulleys with the crank (and pistons) at 90º from TDC
to avoid valve to piston clash. If you loosen and remove the bolts before removing the old belt you will see that the keyways are nowhere
lining up, the keyway I believe is to set the cam for lobe grinding when manufactured..

If you have already removed the cambelt without setting the crank at TDC you need to carefully set the crank at 90º off TDC without rotating through the TDC position to avoid valve/piston clash. Then set each cam to the TDC position by lining up the dots (facing each other) with the new pulleys,
noting you have each pulley facing as inlet or exhaust accordingly. Again you will see that keyways do not coincide by a long way.
To set the crank ..exactly.... at TDC, and apologies to Richard for the next statement, with No. 1 plug removed and with a clean rigid rod
to contact the top of the piston turn the crank slowly to set absolute TDC and check the TDC indicator on the crank pulley. Dial indicators
with extensions are best for this.
Install the new cambelt etc.
The Auxiliary shaft and pulley setting is not critical, you will need to reset the distributor firing position with a timing light.


Hope this is not egg sucking.

Dave the cog.

User avatar
Hawaiis0
Senior Poster
Posts: 4130
Joined: Sun Jun 30, 2013 18:39
Model: Excel SA (No 3); Elite 504
Colour: BRG; Dirty White
Year: 1986
Location: West Oxfordshire

Re: CAM pulley markings

Post by Hawaiis0 »

The car is currently complete pending belt and sprocket replacement next week. There is visible wear on the inlet and exhaust pulleys. This is me just gathering parts ready.

Image

The Third ignition auxiliary sprocket (same part number ) is just incase as is the crankshaft sprocket

This is all new to me so my friendly mechanic is getting the task.

The manual quotes 4 different DOT MOP angle colours. How crucial are these and could I have a mix? I just asked for standard 104 's.

8 years/ 30000 miles in ownership pulleys not changed on my watch.
Nothing is fool proof. Fools are clever!

Pete Boole
Senior Poster
Posts: 3711
Joined: Tue Apr 04, 2006 21:28
Model: Elite
Colour: Monaco White
Year: 1974
Location: Nottingham

Re: CAM pulley markings

Post by Pete Boole »

Either the dots are in the correct location wrt the keyways or they are poorly made. Maybe the vendor can explain why there is so much difference. I'd always assumed that the dot represented the correct position, not the adjacent sprocket tooth.

The dot colours are referenced to the cams themselves - you need the correct pulleys for the installed cams.

Pete

User avatar
Tanz
Senior Poster
Posts: 2071
Joined: Thu May 03, 2007 22:05
Model: SE
Colour: Calypso Red
Year: 1988
Location: Wolverhampton

Re: CAM pulley markings

Post by Tanz »

Not sure if this is helpful but these are my original cams before I replaced them.

Image

And these are the replacement ones

Image
Cheers, Phil
Never take life seriously, nobody gets out alive anyway!

User avatar
bash
Senior Poster
Posts: 2210
Joined: Thu Nov 03, 2005 15:34
Model: Se, V8
Colour: white
Year: 1986
Location: Doncaster
Contact:

Re: CAM pulley markings

Post by bash »

Eyup Stu, I was wondering if you had resolved this. Rover v8 pulley keys are notorious for not being aligned and I used a piston stop and dti, not seen this on the Lotus engine before.
Bash
Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.

User avatar
Hawaiis0
Senior Poster
Posts: 4130
Joined: Sun Jun 30, 2013 18:39
Model: Excel SA (No 3); Elite 504
Colour: BRG; Dirty White
Year: 1986
Location: West Oxfordshire

Re: CAM pulley markings

Post by Hawaiis0 »

The solution was to set the engine to TDC and mark the old and new pulleys against the woodruff keys for safety.

Using the 2 matched pulleys as inlet and exhaust to maintain the similarity and the 'odd' one as the aux, we removed everything and replaced with new in the marked positions.

All worked well, except for the CAM shaft pulley being a pig going back on, and it fired up first time beautifully.

:D
Nothing is fool proof. Fools are clever!

User avatar
bash
Senior Poster
Posts: 2210
Joined: Thu Nov 03, 2005 15:34
Model: Se, V8
Colour: white
Year: 1986
Location: Doncaster
Contact:

Re: CAM pulley markings

Post by bash »

Every day is a school day, well done.
Bash
Two things are infinite: the universe and human stupidity; and I'm not sure about the universe.

Esprit2
Senior Poster
Posts: 252
Joined: Fri Oct 26, 2007 17:15
Year: 0
Location: Minneapolis, Minnesota, USA

Re: CAM pulley markings

Post by Esprit2 »

Hawaiis0 wrote:
Thu Mar 11, 2021 08:57
I have new CAM pulleys and noticed the alignment dots are not aligned. One is half a tooth out.

Are these purely a visual aid and it's the keyway that does all the important stuff?
Where did you buy the pulleys? A friend recently purchased similar looking pulleys from (?) either SJ Sportscars, or Lotusbit... I'm not sure which. Either way, I think they're originally made by Kent Cams. They were sold as 104MOP, and the actual MOP is something else... not correct. That says nothing about your pulleys, but to me it opens the door for some doubt about the accuracy of the pulleys. I suppose that's just messing with your head.

Do you have access to an accurate means for measuring the angular location of the keyway? Like +/= 1/4 degree? For instance, if you work for a manufacturing company, does 'Tooling' or 'Quality Assurance' have an optical comparitor? Or a modern equivalent?

The keyway's centerline is slightly offset from a vertical centerline that passes through the centers of one tooth at the top of the pulley (ie, at 0 degrees on a compass), and another tooth at the bottom (ie, 180 degrees on a compass). The keyway is at the top of the bore, some small angular distance to the left or right (centerline to centerline), differing depending upon the MOP. Tell me the angle of your pulley's keyway, and the pulley's advertised MOP, and I might be able to work out whether the keyway is accurately placed. The drawing at the following URL is clipped from an original Lotus part drawing for a 100 MOP, blue dot pulley (don't complain, it's the only drawing I have). The layout would be similar for all the other MOPs, but keyway will have different angular dimensions on the other side of the vertical centerline (100 is the only MOP wiith the keyway to the right of the centerline).

https://tinyurl.com/f2bzytfz

*~*~*~*
Yes, the keyway's location is the critical dimension/ feature, and the timing dot is just a visual aid that's not always located as accurately. With the non-adjustable OEM pullleys, the teeth can only be indexed on the timing belt in whole tooth pitch increments. One tooth pitch = one tooth width + one gap width = from any one point on one tooth to exactly the same point on the next tooth. One tooth pitch is 9 pulley degrees, or 18 timing degrees... a lot. Typically, the dots don't mean much, and you could just as effectiively paint the entire tooth the dot is on that one color... green for 104 MOP. Then just align the the two teeth as close to 'on the centerline' as possible.

In a perfect world, the two timing dots are supposed to align with one another 'ON' the imaginary centerline between the two cams. As in your photos, the two dots are usually offset to one side of a tooth or the other... not centered. So when the dots are on the imaginary centerline, most of the teeth will be offset slightly above or below the imaginary centerline. That's normal. And since you can only index the pulley teeth on the belt in whole tooth pitch increments, the next positiion in either direction is a long ways away. So as long as the imaginary centerline passes through each of the teeth that the timing dots are on, the non-adjustable OEM pulleys are aligned as closely as they can be. For perfection, you would need adjustable pulleys... and they're a whole 'nother pain in the butte to set-up.

For a typical street engine, the non-adjustable pulleys are close enough. For a perfectionist hotrodder looking for every last fraction of a horsepower, adjustable pulleys might be worth the effort.

All that presumes the keyways are located within tolerance (+/- 1/4 degree).
"IF" the keyways are inaccurately placed, as on my friend's now black pulleys, then it would be nice to know how far off they are. Is it enough to be concerned about? Since the pulleys in your photos appear to be very similar to my friend's 'new' pulleys, I have some doubts.

With stock valve diameters, and a stock or lower compression ratio, the cam timing would have to be off by more than three teeth to bend valves. I doubt that is a concern. But if the keyway isn't perfectly right, performance may not be as expected.

Regards,
Tim Engel

User avatar
Hawaiis0
Senior Poster
Posts: 4130
Joined: Sun Jun 30, 2013 18:39
Model: Excel SA (No 3); Elite 504
Colour: BRG; Dirty White
Year: 1986
Location: West Oxfordshire

Re: CAM pulley markings

Post by Hawaiis0 »

Hi Tim great write up but a fraction too late. Now fitted so measurement is not possible. I was hoping your input would have appeared last week before the effect but no worries. Suppliers were LB and Kemp for reference. Installation was reasonably uneventful apart from a mischievious key on the smaller cam pulley wheel. Rotation and tension were great and timing perfect. Drive was lovely as i elected to leave off the aircon belt pending a future need to recommision the whole system. Afew extra horses were released back into the pot.
Nothing is fool proof. Fools are clever!

Post Reply