I think the restricted dowel hole is at least twice the diameter of the lube port, so definitely some pressure buildup in there.
And, there are different sized supply holes in the several dowels I have stockpiled…so some of the thinking here changed over time. We did have a large discussion about this…with photos…a year is ago?
Likely then when mine was going together. I was able to buy the J/LR one from CJ and transplant my OEM dowel over. So far so good 
OK, so now I have a problem. I don’t have an old tensioner, as I am converting from the original spring blade type. So I have no locating dowel to swap over - I may have to run it as is and see what happens. As it was supplied like this from Jaguar, I would assume it should work like this…
Hi Roger,
That begs the question I tried to ask yesterday about the size of the bleed (lubrication) hole on the face of the rub block. That hole bleeds off oil pressure and controls the back pressure on the that plunger, and therefore the force on the chain. If it was properly engineered, I believe you will not need the orifice in the “dowel”, or the dowel itself for that matter.
Tom Brady
Sorry Tom,
I failed to answer your question yesterday.
I’ve now checked the holes, and am now more confused. I understand your reasoning above, but I now wonder if the Jaguar Classic part is faulty. I’ve attached photos:
Yes, there is a hole in the rubber pad.
But this hole doesn’t seem to go through the metal backing. There is a depression (I think) down inside the tube, on the reverse side to the rubber, but I don’t think there is a patent hole there.
There is a small bleed hole at the point where the tube part joins the head.
So, is the part faulty and that hole should be through to feed the hole in the rubber pad?
Do I take a chance, and drill it through? It seems odd to have a large diameter feed hole, but just a pinhole allowing a bleed out from the tensioner.
At this rate of confusion, it’s probably simpler to go back to the blade spring!
Thinking about it more, it may be intended to be like this. There is a large hole to let oil into the plunger. The only way oil can get out of the plunger is through a small spray hole in the plunger shaft. This spray hole is positioned in line with the path of the chain (and with the 3/16" blind hole in the rubber) and will therefore spray oil onto the chain before the chain reaches the tensioner. This should lubricate the chain, and maybe allow oil to collect in the blind hole, acting as a reservoir. I’ve never seen a tensioner with a hole here before, and suspect it’s an improved design.
I’m tending to think now that this doesn’t need the ‘dowel’ peg with smaller hole.
Welp that would help lubricate the plunger 
It’s hard to see what’s going on inside. It isn’t just a smooth machined surface. Maybe it can flow around that bit in the end. Try covering the tiny hole with your hand and blowing in the end and feel whether any airflow comes out the shoe.
Erica, no, nothing with the pinhole blocked.
Sorry, I might have been editing my post when you responded - see my reasoning for the hole being where it is, above.
I think you’re correct, that the tiny hole is so close to the shoe that it would spray down onto the chain. Is that hole exposed even with the piston inserted all the way? When everything is brand new it barely all fits together and that piston is bottomed out in the body.
Not sure about the reservoir theory. It’s kind of a pathetic reservoir. With nothing spraying it would be full of sludge shortly. It may just be the case that it’s part of the rubber mold and when they changed to this method they didn’t change the mold.
I won’t know until I assemble it all, and the block and head are still at the machine shop. The hole uncovers very early, so hopefully it’ll be OK. I think you’re right about the hole in the rubber - it seems most likely to be a leftover from the original design.
The dowel serves to locate the tensioner body AND to keep the filter gauze seated against the shoulder in it’s bore. You could replicate that with a hardware store spacer. Of course, careful assembly of the tensioner to the block could also obviate the need for the locating dowel.
The oil spray hole does appear to be an improvement, as you have reasoned, since it would shoot oil down toward the bottom sprocket. And since the supply hole is far larger than the bleed, plenty of hydraulic pressure would be available to tension the chain at higher revs.
If it was me, I would certainly try to use the improved (hydraulic) tensioner system.
Thanks Lee,
As my car never had the hydraulic tensioner, I’m using the Coventry Autocomponents conversion kit, which uses a brass mounting block. This comes with a new Rolon tensioner with no dowel. The instructions explain that the hole in the brass block that would have taken the dowel should be used instead for the XK150-type brass filter instead, so presumably the filter should stay in place - but that said, the Rolon tensioner they supply has the pinhole oil inlet hole, not a large one as the Jaguar Classic has. Once I have all the parts together I need to do some dummy build-ups to see what’s what. However I just don’t believe that Jaguar Classic, i.e. as close to the factory as you can get, would sell a tensioner with all the shims etc. but not an essential part like the dowel. It’s not available anywhere sold separately that I can see, so I would expect it not to be necessary. I’ll report back once I get to that stage!
They may be counting on the fact that every car that used a hydraulic tensioner came with the dowel so it could just be moved over. I feel better with it on because it pins the device to the block so the bolts are only trying to hold it flush to the block rather than hold it in position.
I might not necessarily expect a replacement tensioner kit to have the dowel, but a CONVERSION kit, I WOULD expect IT to have one.
Mr. Brady asked about the need for 2 shims and after refreshing my memory on this, I believe the extra shim allows the use of BOTH the conical filter AND the locating dowel. I have a vague memory of shortening the length of my original dowel so that it did not protrude too far out of it’s bore because of the filter’s presence…an extra shim woulda saved me the trouble. Also, I had assumed my engine came with the filter as original, but now it looks like it wasn’t introduced until the 150. (Maybe that’s why I couldn’t find the thing when it came time to reassemble my engine and ended up buying one, lol.)
Found the thread about tensioners: April, 2020 “Chain tensioner-Rolon”…good read. “E” forum.
Hi Lee,
Interesting comment. Viart (on P. 192 of the 140 book) shows the hole in the block for conical filter C.13457 , and notes it was used from engine G.4431. I suspect that’s the break point from the blade type tensioner used on all 120’s and some early 140’s.
Moss has a decent diagram of the retrofit hydraulic tensioner they offer for 120 and early 140, as well as the original blade style. They also show a filter behind the tensioner, not conclusive however.
Here is a picture of their offering.
This looks like what Roger King has, and no filter is shown.
More food for thought.
Tom Brady
The Manual shows the filter became standard starting with engine #V.1191, XK150. G4431 was the start of the hydro tensioner ONLY(no filter) according to the Manual. The Manual does note that engines prior to #V.1191 can accept the filter without modification during rebuilds, however, but does not talk about any extra shimming of said tensioner body to make room for the filter’s shoulder.
Just to be clear - my engine is number 3736, so has none of the hydraulic tensioner parts as standard. I bought my conversion kit from Coventry Autocomponents, whom I believe make the kit and sell it to other suppliers. Their instructions state that the dowel is not needed, but the hole is retained in the brass block to use for the filter if the owner so desires. The filter is bought separately. That said, the tensioner that comes in their kit is the Rolon modern one, with the fine pinhole where the dowel used to be. I don’t know if using the Jaguar tensioner with the large oil inlet (dowel) hole will be OK in this application - but surely it must be. If a dowel pin was required here, I’m sure Jaguar Classic would have made it clear that one must be obtained from somewhere and fitted to their tensioner. They cannot seriously have expected that the buyer would somehow know the dowel should be fitted without that being made clear.
I mean, would you buy a piston, and still expect to have to source the gudgeon pin yourself?
Brady’s photo of the Moss offering DOES show the dowel with feed hole…interestink.
Given that that kit was made by CA, I think that just illustrates that it is pure chance what tensioner you get when you buy one. I’m really not bothered about the dowel, as all the experts I have spoken to say you don’t need it, including C&G who know early Jag engines inside and out and prepare a lot of very expensive cars for very wealthy clients who would get very upset if their timing gear failed in short order. I’m more concerned over the size of the inlet hole for the oil. Mine is ‘dowel sized’, with only a small hole for outlet (and around the sides of the piston). The Rolon’s inlet is ‘pinhole sized’, with a slightly larger outlet on the rubber face, as far as I can tell. Would mine produce the required oil pressure? I don’t see why not, but I’m a dentist, not a hydraulics engineer…