Timing chain alignment - resolution

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southerly95
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Timing chain alignment - resolution

Post by southerly95 »

I have posted previously over our 65 MM not running properly after a new head and engine front end (symptoms were no tickover and very lumpy running).

I thought the prob was the timing chain being a tooth out because everything else was ok. So I made sure the sprockets were aligned as per the article in Technical Tips, see link:
http://www.mmoc.org.uk/members/chain.pdf

The article has a picture showing the sprockets with sprocket centres and dimples in alignment when on the engine. With this configuration our engine will NOT run properly. It is NOT possible to keep the crank at tdc and the camshaft precisely at 2 o'clock as well as keeping the centres and dimples in a line. If you do try keep them in aligment (as I did) the camshaft ends up at 1 o'clock as does the rotor arm on the dizzy with the result of poor running.

I fully expect to be challenged on this but will say in advance that for OUR 1098 A series this particular article is not entirely correct.

I hope I'm not being overly critical here, MMOC does a very good job and I am delighted with all the advice offered on Message Board, thanks, and best regards, John
kennatt
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Post by kennatt »

very strange that john. Just about every engine i've worked on is corretly timed if you make sure the dimples or marks are lined up with crank at tdc.Can't think why yours falls out of this.How did you solve the running problem by the way just in case I come across the same issue.I wonder if you have a different sprocket fitted,though would think they are all the same. Anyhow as long as you have the car sorted supose it dos'nt matter.. Cheers ken
Kevin
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Post by Kevin »

John I dont wish to state the obvious but it was definatly set up to TDC and not 180 degrees out.
Cheers

Kevin
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southerly95
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Post by southerly95 »

Just to confirm - with no timing chains on and cylinders 1 and 4 at tdc, the crank woodruff key was vertical, it is impossible to get that 180 degs out (unless cyls 2 and 3 are at tdc instead).

The reason I am posting this is that the priority is to get the camshaft at 2'o'clock regardless of the sprocket dimple positions. The new sprockets were identical to the old ones.

I'm very glad and relieved the car is running ok now - I've had the full front end off 3 times in the last 10 days! Still it's nice and clean under there now as a consolation :lol:

And finally, as they say, with the camshaft at 2 o'clock and crank at tdc do you think cyl one is on the firing stroke, or not??

Regards, John
kennatt
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Post by kennatt »

in this position simple turn the engine in the correct rotation and watch no one valves.If its on firing stroke the valves should not move as the piston goes down the bore,untill the piston comes back up the bore on exhaust stroke.If after you turn it after a short way if either valve moves then not on firing stroke
bmcecosse
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Post by bmcecosse »

Only explanation for all this is that the sprockets were not correctly 'dimpled'. I assume they were not original BMC items ? Where did you get them - as a warning to others!
Check for correct cam timing is that the valves on 'overlap' at TDC (any cylinder - it matters not a hoot) should both have exactly the same lift - of course you need a dial gauge to check this properly although a good estimation can be made with vernier caliper - near enough to say if the timing is one tooth out!! If it is - the difference in lift will be quite obvious.
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rayofleamington
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Post by rayofleamington »

I've never had to do anything more than line up the dimples - (I've never checked for TDC -I only checked that the rotor arm was pointing at #1 position in the dizzy to make sure the crank isn't 360° out). They've always worked first time, just by lining up the dimples.
Just regarding using the rotor arm position, it's worth to check that the dizzy drive is correctly positioned - if not, that can help to lead on to other mistakes.

I've done 3 or four of these over the years, although my last one was probably quite a few years ago!

If your old and new sprockets are the same, then maybe your engine has an aftermarket / non standard camshaft? Any difference in the position of the key slot would explain problems to get the cam timing right.
Ray. MMOC#47368. Forum moderator.

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bmcecosse
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Post by bmcecosse »

The position of the rotor arm is not at all important - that can be sorted out later if need be. Important thing is engine bang on tdc - and dimples lined up. Always worked for me ! Special/aftermarket cams can be set up more accurately using dial gauge and 'degree wheel' - to get the timing accurate to 1 or 2 degrees - but this is a waste of time for anything other than out and out competition use. For one thing - the chain soon stretches and spoils the setting - and in any case - who can say for sure what the very best setting is for any one engine. Only way is with hours of fun on an engine dynamometer!! BMC came up with a good compromise - and the dimples will work fine - unless they are wrongly marked on after-market sprockets. Hence the 'equal lift on overlap at tdc' check.
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