TDC markings

Player#3 · Apr 21, 2014

Hi,

An strange thing on my ´75 Commando mk 3.

I took my cylinder head off and confirmed that when both pistions are at TDC the markings on the timing wheel are no where in sight. When I continue rotating to BTC then the mark is in perfect alignment.

Is something out by 180 deg ? How do I fix it.

Any idea what the markings on the piston crown mean. STD, B and I.D.1. I assume that STD means standard with no overbore but what about the other letters ?

Getting the head off was not so difficult once I had found all 10 bolts. The Workshop manual/parts list was not good for identifiying the location of the bolts, nut and studs used.

Regards,
Sean

Rohan · Apr 22, 2014

Remove the alternator rotor and refit 180 degrees. ?
Without recalling if there is more than one keyway to permit this...

You were doing well to find all 10 bolts.
Some folks try to force it after finding 9....

Nortoniggy · Apr 22, 2014

You must have a strange alternator rotor. It should have 2 scribed lines 180 degrees apart, one for TDC & one for BDC. The rotor normally only has one keyway slot. If it has 2 you could turn it round, otherwise scribe your own mark for TDC.

Ian

toppy · Apr 22, 2014

Check the rotor is fitted with a key don't run it without one as it may well slip and affect air gap or the crank nose diameter it fits on.
My mk2 has Alton Ekit which has no marks so you could do as I did. Use timing disc to find TDC then because the timing mark on my cover dont start at TDC I set the engine to 30 degrees before TDC removed timing disc refitted primary cover an scribed a line to match up with 30BTDC on timing marks of cover.
I have never had a mk3 but I believe yours shoul have a bolt that can be removed from crank cases then a special tool fits in to locate in hole In the flywheel to show TDC.

Hobot · Apr 22, 2014

That little detail seems to catch me out about half the time too. Start ignition timing process over again with proper rotor mark in range. Really helps to scar up either the right one or wrong one and lay bright finger nail polish in the correct groove. I knife edge mark TDC and also another at 30' then fudge judge slightly below for 28 or slightly ahead for 32' full adv. One thing that caught me out trying to find which jug was on compress to time, by putting finger over plug hole to feel the push off but at such slow turns it pushed off on exhaust too to fool me. You can see how much or not the factory dial is off to trust or fudge factor. A sure fire way is see which jug has both valves closed near TDC when the rotor mark in sight.

Player#3 · Apr 22, 2014

Hi,

Thanks for the replys guys.
It looks like I need to pull the cover to see what is under there. The timing mark from the inspection window looks like this. Mark happened to be a 36 deg when I took the picture.

I will let you know...

Regards,
Sean

L.A.B. · Apr 22, 2014

A Commando rotor usually has two marks, but earlier type Lucas rotors often have only one timing mark (and even earlier rotors have no timing marks at all).

The correct rotor timing mark doesn't coincide with any "0 degrees" mark at TDC.

If the crank is set to TDC, then the rotor mark will be off the timing scale.

Player#3 · Apr 24, 2014

Hi,

Thanks L.A.B for your comment.

I am slowly getting a grip on this now.

So basically the scale seen in the inspection window is only any good when viewed with the engine running and a timing light.

If I want to see some mark when crank at TDC then I need to make the extra marks myself.

Regards,
Sean

toppy · Apr 24, 2014

Better than marking TDC is to use a TDC finding tool. I got one off ebay that screws in to the spark plug hole but unlike others that do this this one has a dial gauge not just a plunger so you can set the engine quickly and accurately. It cost less than £20 delivered to my door to.

olChris · Apr 24, 2014

toppy said:
Better than marking TDC is to use a TDC finding tool. I got one off ebay that screws in to the spark plug hole but unlike others that do this this one has a dial gauge not just a plunger so you can set the engine quickly and accurately. It cost less than £20 delivered to my door to.

That sounds good ...... Dont suppose you have a link to the site where you purchased it, cos that will save this ol'frt exploring the WWW............

Matt Spencer · Apr 24, 2014

Knactuall , find a depth of 1/8 in. say , btdc . rotateing fwd & Backward . Find the DEGREES BETWEEN Ea. , Half That degrees IS T.D.C. .
this eleminates the lost motion over top with 1/2 in. slack in the bigends .

pete.v · Apr 24, 2014

olChris said:
toppy said:

Better than marking TDC is to use a TDC finding tool. I got one off ebay that screws in to the spark plug hole but unlike others that do this this one has a dial gauge not just a plunger so you can set the engine quickly and accurately. It cost less than £20 delivered to my door to.

Click to expand...

That sounds good ...... Dont suppose you have a link to the site where you purchased it, cos that will save this ol'frt exploring the http://WWW............

Google "piston stop".

And don't forget the degree wheel. This is also your opportunity to validate the timing marks on the scale attached to the outer. Mine is 2 degrees off having the 26 mark is really 28 degrees.

When using the piston stop pay very close attention the the valve position and make sure what ever unit you use does not interfere with them.

I scratched a TDC mark on the stator. You may never need it again but it comes in handy once in a while.

L.A.B. · Apr 24, 2014

pete.v said:
And don't forget the degree wheel. This is also your opportunity to validate the timing marks on the scale attached to the outer. Mine is 2 degrees off having the 26 mark is really 28 degrees.

And, of course the Mk.3 has an additional 28 deg. BTDC crank timing mark (slot) which lines up with the Mk3 crankcase aperture below the timing case (hexagonal plug). This mark is generally more accurate than the primary case timing scale but it can also be checked for accuracy with a timing disc.

pete.v · Apr 24, 2014

L.A.B. said:
pete.v said:

And don't forget the degree wheel. This is also your opportunity to validate the timing marks on the scale attached to the outer. Mine is 2 degrees off having the 26 mark is really 28 degrees.

Click to expand...

And, of course the Mk.3 has an additional 28 deg. BTDC crank timing mark (slot) which lines up with the Mk3 crankcase aperture below the timing case (hexagonal plug). This mark is generally more accurate than the primary case timing scale but it can also be checked for accuracy with a timing disc.

Yes, of course, MKIII. Lab, thank you so much for that. :wink:

Player#3 · Apr 25, 2014

Hi,

Thanks again L.A.B you know your Norton like the back of your hand ! Great clear pictures - help a lot.

Here is a an example of the locking tool http://www.ebay.co.uk/itm/251189111530? ... 1423.l2649

I noted that the scale on the inside of the chanin case is fixed with two screws and has sloted holes so if in noticing a few degree difference in markings you can just shift the scale to line up as it should do.
I put my own marks on the stator and rotor anyway so I will always know which to trust.

Regards,
Sean

TDC markings

Player#3

Rohan

Nortoniggy

toppy

Hobot

Player#3

L.A.B.

Moderator

Player#3

toppy

olChris

Matt Spencer

pete.v

L.A.B.

Moderator

pete.v

Player#3

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