MK IV engine set up clarification

I am about to check my 1948 MK IV engine which is the 1 1/2 litre. There are discrepancies in the Instruction Book and Service Manual I have so I need clarification what are correct for this engine size and year:
Timing: 5 or 10• TDC
SPARK PLUG GAP 0.020 or 0.025”
Distributor points gap 0.010 or 0.012”
Tappet gap 0.012” on inlet and exhaust or 0.015” on inlet and 0.018” on exhaust.

Can anyone identify the inlet and exhaust tappet positions for this engine as the Manuals don’t say. Picture just says item 3 is Exhaust and 4 Inlet, and I know cylinder 1 is towards the bulkhead - the Block drawing shows what I assume are 4 round inlet ports and 3 square exhaust ports.

Ex, Int, Int, Ex, Ex, Int, Int, Ex

The Standard Triumph manual for the same engine may be helpful. It is 102 pages and can be downloaded from here:

On page 42 it says Inlet .012" and Exhaust .015" cold.

Thanks Rob. I will look through and compare the data.

I started to check the Timing set up on the engine, but must be having a senior moment on the MK IV !! Cylinder 1 piston at top of travel, distributor cap off, and then ooohh… I can’t find the TDC marker on the flywheel and despite having a camera looking into the spy hole whilst slowly cranking around the engine by hand nothing appears (and I’ve been rotating the engine fully around several times). Am I mistaken as to the timing set up on the MK IV or am I looking in the wrong place? What is the “marker” supposed to be?

One nice thing about symmetrical valve timing is that TDC is easy to find because the inlet and exhaust valves change over. So when they are equally open you have reached it. If you want the other stroke you just need one complete turn of the starting handle.


Here is the TDC arrow mark on my 2-1/2 Litre.

There is a viewing hole in the top of the bell housing.

Are you looking through a hole at the bottom?

Oops!! I’m looking in the bottom hole and not the top one. Better get my directions right before rechecking again. Thanks

Hello David, some redundancy in finding the answer to your questions is required.

For valve clearances, the service manual lists 0.012" inlet and 0.018" exhaust, but with engine hot. Many of us do these checks with engine cold, and therefore use different numbers cold compared to hot. I do not have experience on the 1 1/2 and do not know what the factory listed for cold measurements.

For your timing mark work, use double caution. There may be a convenient arrow on the flywheel (again, I have no experience on the 1 1/2) BUT the service manual recommends putting the flywheel 1/4 turn remounted if worked on. You should not rely on the arrow to indicate top dead center without redundant confirmation by other means. Instead, search for the arrow and use if useful, but be certain to determine TDC by direct measure, either as Peter describes or by measuring piston height via dial indicator or both. Then, you might find it convenient to make an arrow and indicator mark somewhere on front of block and crankshaft where a timing light may safely shine. I use a small arrow bolted to timing chain case cover and a painted line on the damper for a 3 1/2 engine.

I’m going with the cold engine setting of 0.012” and 0.015” to set up as the engine is now partly stripped down. I will double check on flywheel and piston heights before making any timing adjustments. Thanks everyone for the feedback and information.


I made this little plate when I was rebuilding my engine. I had a degree wheel that I printed from the net to determine the 10 degree mark and a chisel cut onto the pulley as the reference mark.



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That’s a good and neat idea that simplifies checking and setting. Thanks

There is NO viewing hole in the top of my bell housing !!! Is there a difference between my late 1948 1 1/2 litre version and the larger engine cars? I’ve measured the piston heights to get TDC but wanted to double check and see if I could find a marker on the flywheel (and what position that was at).

Also specifically for the 1 1/2 litre is the timing setting 5’ or 10’ before TDC ? - my Service Manual and Instruction Book for this year/engine size give different settings.