13 replies to this topic

[floormanager]

Hello,

 

Can anyone give me a clue how to time the cam with a muti-keyed crank sprocket like this? 

 Screenshot 2026-04-15 154235.jpg   31.5K   1 downloads

 

Align the dots on the two gears initially, then check the cam timing.  Lets say it's at 102 degrees and you need 106.  Then what?  Do you remove the gears, turn the crank until the next key and refit?  I can understand the offset keyway method, as it turns the camshaft a few degrees but I can't get my head around the multi-keyway crank sprocket.

 

Thank you.

[PoolGuy]

It's a combination of moving the crankshaft pulley and chain position on the pulleys to achieve the correct timing. It's a bit more of a faff than using a vernier, but you only do it once.

[GraemeC]

http://www.romacusa....Fs/JP5629. .pdf

[Spider]

From memory, they have 5 slots. The 3rd one is 'nominal', where the dots 'line up' and the other 2 either side of that advance of retard the timing by 2 degrees for each slot.

 

I ended up using a felt tip marker to mark out a dot for when using the other slots. I did that but fitting up without the chain, then fitting the crank sprocket in the other 4 alternate slots and putting dots with the felt tip marker, one for sot one, 2 for slot 2 etc until I had all 4 marked. You could do the same with a centre pop.

Then fit it up on the nominal slot, measure the cam error and move the the slot required to get the timing I was after.

 

[floormanager]

Most useful, thank you.  Glad I asked but I can't get in my head how it works. you have to rotate the crank, so why the extra slots? I wish there was a video of it.  The instructions still confused me.  I'm thinking of just getting an offset key and be done with it!

 

 

Attached Files

•  Screenshot 2026-04-15 173322.jpg   51.01K   3 downloads

Edited by floormanager, 15 April 2026 - 04:34 PM.

[Ethel]

You won't rotate the crank (by more than the few degrees to correct the timing).  You'll remove & refit the sprocket  so it's a 2  or 4  links farther left or right on the chain. When you refit the sprocket to the crank it will line up with a different key slot on the pulley that will give the corresponding change in cam timing because the sprocket key slots are offset instead of the key.

 

I can't see the extra marks in your photo, but you could do the dial gauge stuff to determine what the cam timing is on the middle slot. Mark the chain & sprocket, for reference, then slip them off the crank so you refit the sprocket to the chain by the required 2 link steps to align the appropriate slot with the crank key. When you've got it back together you'll find the crank sprocket dot is out of alignment with the cam sprocket dot by the same number of teeth/links. As Spider suggested, you could count over by the appropriate number of teeth (from the original dot) and make your own mark to align with the cam sprocket dot.

[floormanager]

Thanks.  There's no marks on the crank socket for each key, just the standard alignment dot.  I get that the purpose of the additional keyways is to move the cam, to achieve the correct timing, like an offset woodruff.  If so, why then not just move cam wheel a couple of chain links and forget about the alignment dots? 

 

In my head, that if you align the dots on the cam wheel to the centre of the crank, to achieve the correct timing, you need to advance the cam (eg from 102 to 106), you are going have to move the crank away from TDC, hence the need for the additional keyways.  Moving back to TDC will move the cam around.

 

Selecting a different keyway at TDC and aligning the cam sprocket dots to the crank centre, will not alter the timing.  I might just get some offsets!

[Ethel]

It will. If you forget the about the cam sprocket dot, it's clear you can fit it in any of 7 different  positions without moving the cam or crank: just putting the sprocket teeth in to different links in the chain. Or at least you could if each slot was in exactly the same position between its nearest teeth - it because that position moves by a couple of degrees from slot to slot that you get a full range of cam timing adjustment.

[timmy850]

The relationship between the crank woodruff key and the dot on the crank sprocket will be very slightly different for each of the different adjustment slots.

If the adjustment is 2 degrees at the cam this should be 1 degree at the crank which is pretty hard to tell by eye.

You can’t just move the chain on the camshaft sprocket by a few chain links as this is much too coarse of an adjustment

I’d go though and measure the first spot at least twice to make sure you’ve got the measurements correct. Then once you know your value and if you want to change it , it’ll be a bit of trial and error to find the right spot

[Ethel]

I know what  you're saying, but moving the chain on the sprocket (or, more clearly, the sprocket on the chain) is the only way you'll move the sprocket to align a different slot with the crank key.

 

If you didn't move  the sprocket, relative to the chain, you'd get a tenth of 360 (crank) degrees for the 2 out of 20 teeth between each slot - give or take the 2 degrees.

[Spider]

 I'm thinking of just getting an offset key and be done with it!

 

I've gotta say, I don't like them and easy to end up with mistakes. I find it easier, cheaper and faster to use Off-set Keys.

[GraemeC]

The instructions are poor - the pictures at the bottom are the same, the one on the right (for retarding) should be different.

 

Essentially each slot moves the crank sprocket by 2 teeth and a fraction.  Therefore you reset/rotate the crank back by 2 teeth and a fraction to get it roughly back where it was relative to the cam. The two teeth make no odds as you wouldn't need to move the cam to get the chain & sprockets back on, they just throw the previous marks out.  The fraction is what gives you the +/- degrees as you will *have* to move the cam slightly to get both sprockets back on.

 

I would say, rather than trying to understand them, measure what you have at standard dot-to-dot, work out what you therefore need and use the appropriate slot and follow the instructions. Re-measure to check it has achieved what you want.

 

I'm the opposite to Spider - I like them as it saves having a bunch of offset keys to hand (which are expensive). Yes you can measure and then order the one you *think* you want, but sods law says when you come to fit it the result isn't what you expect!

Edited by GraemeC, 16 April 2026 - 08:58 AM.

[floormanager]

Thanks everyone.  I know its me not getting my head around it, I think it must be my age.  I did time it up a few years ago but want to replace the backplate, hence trying to understand it again. 

[Ethel]

I'm sure you will. Lay the sprockets & chain on  a chunk of card so you can mock up the fitting & easily play around with different crank slot positions & draw some lines. Even better if you have some scrap board to nail some bottle tops, or wateva, to as simulated shaft ends.