Timing chain

[Moises]

Has anyone installed a tensioner on the Jowett timing chain?. Thank you

[PJGD]

Adding a conventional chain tensioner to the "slack" side of the chain would be virtually impossible because the oil pump and distributor drive shaft are in the way.

The easiest solution is to put an o-ring (Dowty 200-246-4470) in the trench between the two chain sheaves, and this serves the purpose. I have used this solution, and it has also been used by BMC on some of their engines.

Jowett Timing Chain Tensioning.png

[Moises]

Thank you very much, Philip. That's a great idea, and I'll follow your suggestion. Based on the reference, I think it's a ring with an inner diameter of 113.89 mm (4.484") and a thickness of 3.53 mm (0.030"). What material is it made of: NBR, Nitrile, or Viton?

[PJGD]

Either NBR or Nitrile should work well. Viton is unnecessarily expensive and has a higher temperature rating than is needed here.

[Moises]

Thank you Philip. Best wishes

[David Morris]

Hi Moises, This sounds like an excellent idea! However, for the 'O' ring Rear Timing Gasket modification, we found that Vitron rings are essential, as the Javelin/Jupiter oil temperatures are high and the other materials just didn't last well. Personally, I would recommend using Vitron, as the slightly higher costs could be off-set by the work involved in having to fit replacements if the lower-spec materials fail? In dis-assembling many Javelin/Jupiter engines over the years, I have once seen a front timing cover where a previous owner had fitted a small sprung-loaded mechanical tensioner, but unfortunately I don't have any photos.

All the best for 2026! David

[Moises]

Thank you David.
Happy New Year 2026

[PJGD]

David's reply is valid, but the answer may depend on whether your car has an oil cooler or not.
Nitrile (Buna N) is rated up to around 120°C which is probably OK if you have an oil cooler, but Viton is better if you don't have a cooler since it is rated to around 210°C or even higher, which your oil should never approach.

[Srenner]

120* C is getting towards the upper limit of safe for mineral based oils, most of which are good to 120* but can get to 150*. Synthetics can stand more before they begin to break down, but cost more and are more prone to creep past seals. My motors tend to run the oil about 20* C hotter than the water once things get up to temp. Really hot oil is an indicator of motor problems.

[Forumadmin]

Oil temp varies as it travels around the engine. Friction and shear stress at the oil pump and bearings and cylinder walls raise the temp and contact with the engine block which is mainly at the water temp will cool it. Blow by of combustion gas and worn piston rings contribute most to oil temp issues..Poor water cooling or valve, timing or mixture issues will raise water temp and hence the block temp. A finned sump or oil cooler will assist overall engine temp as well as oil temp. But note cold oil of less then 80c is detrimental to bearings so fit an oil temp regulator.

[PJGD]

I fully agree that an oil temperature thermostat is a near essential add-on device. I had a readily available MOCAL oil thermostat on my Javelin for many years. Recently though, I took a critical look at that device and I have some reservations. If you look at the drawings that I made of it after taking it apart, you notice that it is assembled from two identical aluminum die castings and there is a fairly significant flow restriction at the inlet and outlet ports. This is due to the draft angle taper in the bore which goes from just under 3/8" down to 5/16" which gives a resulting flow area of 0.075 square inch. Consider that the oil gallery in the filter housing is 7/16" which gives a flow area of 0.150 square inch, in other word it is twice that of the Mocal thermostat ports. As a result, I have drilled out the ports in the Mocal casting to 3/8", giving a flow area of 0.110 square inch which is an improvement but not ideal.

The bottom line is that we could do with a better thermostat to protect our engines. I have designed such a unit but not sure when I would get around to making an example.

Mocal Oil Thermostat.png