Ignition timing mark and degrees before TDC

I have been getting ready to put my distributer back in place and check the ignition timing statically, the Manual says there is a mark for TDC that can be seen through the clutch housing, cant see that yet, however my question is, they say to set the ignition timing you get that mark in view which is TDC

The ignition timing should be 10 degrees BTDC how do you get to that ? I normally use a motorcycle degree marker which you put in the spark plug hole and back off from TDC the amount of degrees you need, but the Jag spark plug holes are at a Sharpe angle, so that may not work
tips / advice

You count teeth on the flywheel. It’s somewhere in the Service Manual. I think it was around 3 teeth.

Seems a bit open to variation, and not very accurate, I like to have my motors timed as perfect as I can get, this all stems from a experience with my 1939 Rover, where if I pulled up outside a cafe or a bar where people were sitting outside I warned them that the car may backfire after a few seconds of stopping it, which it did, to everybody’s amusement and several people ducking
looks like I will need to get the ignition timing where it should be and place some reference points on the crank shaft or other point

A flywheel for a 3-1/2 Liter Mark V has 104 teeth.
That’s 3.46 degrees per tooth, so 3 teeth is 10.4 degrees, close enough.
After you get it set, you can tweak it in a bit with the micrometer adjustment on the distributor.
Yes, the flywheel and the hole in the bell housing aren’t the most convenient places to put the timing marks, so I put a dab of white paint on the damper and front mount.

Can you mount a degree wheel on the damper, either temporarily or permanently?

Yes, the time to do it would be when the engine is out.
Mark V crankshaft 005

Here are my marks on the Mark V.

The 2.5L SS does not have a damper, but the idea would be the same.

It is possible to find TDC through a spark plug hole with a wood dowel stick and external gauge. The flywheel mark does have difficult access and also may have been rotated by a previous remount to crank. Once TDC is found, it is not hard to add some paint marks on the damper and a pointer on the front of the block. For my Mark V, I have put the marks on with markings both for the real TDC and for TDC where I have placed the pointer. The pointer is positioned to make the use of a timing light easy to see.

I did the same as Roger. I had a 6mm/1/4" rod in No 1 - the rear - plug hole and extending over a half metre. I used elastic to hold the outer end up under tension by attaching it to the propped bonnet above and with the inner end resting on the centre of the piston. This length of lever means that the slightest change in the piston’s position is magnified greatly and a check is easy by measuring the distance from the tip of the rod to the edge of the bonnet. You have to go backwards and forwards a few times until you are satisfied you’ve set TDC.

Just remember that you should always do the last adjustment in the clockwise direction. Why? Because when you turn the engine back, you’ve introduced the lag in the timing mechanism so that the backlash in the chain and gears means the distributor won’t be exactly right.

I use circumferential measurements to mark the advance points on the damper. E.g. 10° is 1/36 of the circumference and easily converted to mm. Also, I don’t hesitate to push the advance further as modern fuels have a different burn rate. You can take it up to just before ‘pinking’. I manually advance the distributor gradually and listen to the engine. It usually improves until you go too far, which tells you that the happy spot is just back a bit. A few final adjustments on the road might improve performance further.

If you are interested in the scientific smoke and mirrors associated with modern fuels you could acquire a copy of ‘Classic Engines, Modern Fuel’, by Paul Ireland. The extensive tests were done on an XPAG engine as used in MGs from '39 to '55. The bottom line is that these engines only need the lower octane fuels, 91/93.

1 Like

I just drew three lines, 0, 5 and 10 deg BTDC onto the damper at 6 o’clock with a white paint marker. Makes it easy to check timing with a light. Of course when these cars were made there were no timing lights! :laughing:


1 Like

Thank you guys for all the input, I shall report back how it all works out for me.
On another note I am looking for a suppler in the US of BSF nuts and bolts, does anybody have recommendations
Thanks you all

One choice:


1 Like

I just noticed that the timing spec for Mark IV is 10 degrees, where with Mark V it is 5 degrees. I wonder why they changed. Maybe something to do with the manual vs vacuum advance.

Access to the “timing hole” is troublesome:

I did as Roger and Peter suggested with the dowel in No 1 spark plug hole to see TDC, then I marked up inside the distributor cap:

I then made up a simple bracket, painted it up, and fitted it under a nut on the crank case cover:

I used a small file to notch a mark in the pulley wheel and painted this white so it could easily be seen.

I have then been able to use a strobe light without needing access under the car. My 1.5 litre MK IV was set to 10 degrees TDC, but sometimes with different fuels in the UK I did find soother runner at 8 degrees or so.

I did also mark with white paint the gear teeth inside the bell housing (through the access hole), but I never bothered going under the car when I tweeked the timing once I’d made my bracket and marker.

Oh, I thought the access hole for the timing mark on the flywheel was at the top of the flywheel cover !

Mine is on the top.

Not easy to see from the engine bay.

You have to get the plug wires out of the way.

1 Like

Probably another difference between the 1.5 litre MK IV (with the Standard and pre-war engine) to the bigger brother 2.5 and 3.5 litre cars.

There is no top viewing hole on the 1.5 MK IV:

And no plug wires go around the back on this 4 cylinder engine:

I was told by the Jaguar Heritage Museum that the 1.5 Litre engine was built, set up, and bench tested (at the Standard Factory I assume) before it was fitted into the car on the Jaguar production line. This seems sensible to me, with the Timing correctly set up, otherwise you would need an apprentice laying under the car as someone top side adjusted the timing set-up !!!

Having a top viewing hole is an advantage, but never the less not a user friendly postion for an easy to set-up and adjust maintenance job.

Whatever the case, adding your own markers and using a stobe light makes the jobs much easier.

Easiest of all to set the timing when the engine is on the test bed.

Here are some 4 and 6 cylinders on test at Foleshill.

That fixed pointer is a really good idea. I will make a couple and add them to my Mark V and SS.

1 Like

Greetings and Happy Holidays,
One method I used on my MK IV, 3.5, for top dead center. I drilled-out the ceramics on an old spark plug and inserted a wooden dowel so that No.1 piston hit the dowel close to top dead center. Marked the damper. Reversed the rotation (by hand) until the piston touched the dowel. Marked the damper. I then split the difference between the two dowel marks, which displayed a fairly accurate TDC.


Thanks all for the reply’s and advice, I have ordered a motorcycle timing tool which goes into the spark plug hole and will see if I can use as is, if not, I will add an extension to the unit to make it more visible and accessible, I have used these before in the past with good success


I tried the motorcycle timing tool but alas the probe piece was too short
so I bought an extension for it which did the trick, but doing it by yourself is a bit tricky as the probe gets jammed between the piston top and the plug hole because of the angle the plug hole, carefully removing the probe and inserting it again to get the feel did the trick whilst kneeling on the apron between the wings and leaning over the engine
thanks for the help