PCV on an XK120?

With lock down still in full effect and based on a conversation with someone who’s done this a number of times before… I thought I’d experiment with fitting a Positive Crankcase Valve to my 3.4 XK engine (utilising the existing crankcase breather outlet and a vacuum feed from the inlet manifold).

My aim here is to reduce the amount of oil passing through the rear (and possibly front) crankshaft seals. I think the theory goes, that by submitting the crankcase to a slightly negative pressure, when the engine is shut down oil is not ‘forced’ out of the seals or is greatly reduced.

The engine is in great overall condition - doesn’t use oil (but does leak it) has both good compression and oil pressure so I’m not trying to make up for poor rings etc…

Has anyone tried such a modification?

No need for a valve, but for something to catch flames (steel wool, or the mesh screen). To the inside of the air filter is how I know it.
I’m afraid you can’t help leaks after shutdown much though, only while running. This under the assumption that the 120 vents to atmosphere, so no oil is forced out; it might remedy leakage while running which I know can happen and badly so.

Crankcase vapors are flammable. A PCV hose must be routed to inside an air filter or an air filter intake plenum where it is contained, not the intake manifold because it would affect running, and not just attached to outside an air filter where it can ignite if you have a backfire.
You may be interested to know that the Mark V had PCV in 1949, long before it was required in CA.

I was wondering how one would run the mid-sixties and up style of PCV since the systems I am familiar with used manifold vacuum, controlled by a PCV valve, to draw fumes from the engine. Of course, looks like you fellas are talking about a more “passive” system, routing the fumes into a very slight depression within the air cleaner canister, instead of letting them vent out the supplied breather. Ford used both a strong vacuum w/valve and the passive approach on the same engine.

Even the last XJ with fuel injection and all had it connected just before the throttle body and it worked fine. The PCV valve is a strange invention and I don’t think it’s needed.

Prior to 1963 many, if not most, U. S. cars had what was called, IIRC, a road tube that vented crankcase gases. It was an open tube that vented the crankcase. When the car was moving the slipstream passing over the end of the tube sucked gases out of the crankcase for the rest of us to enjoy.

So Just a quick update - I’m in dialogue with these folks who appear confident that their adjustable valve can be installed to work:

They certainly seem to have supported a variety of different engines. Once the design is settled upon I’ll post here for comment :wink:

Just to be clear, the Mark V does not have a valve, it’s Positive Crankcase Ventilation is just a port with a rubber grommet connecting the rocker cover with the air filter, so crankcase vapor is sucked into the air stream to the carbs. There is also a road draft tube on the side by the valve lifters, which all the pushrod engines had. In this case I think it pulls air in rather than pushing it out here.

I don’t quite see the need for a valve. My '74 XJ12 does not have valves, just a vent tube by the left cam cover running over to the air filter housings.

I was talking about the valve. Of course I want the gases to be burned, like Rob says, we don’t see the need for a valve.

Nor do I. Did any E-type engine have a pcv valve? Not that I’m aware of. The crankcase vent system that starts at the front of the head and ends at the point between the carbs and air filter seems to work without any kind of valve.

The valve is only needed if manifold vacuum is used in conjunction with a hose with a rather large ID…Ford used about a 3/8" hose…a small conduit could be used, thereby serving as it’s own restriction, but a hose that small would be subject to plugging up.

Also, the carburetor must be retuned to compensate for the vacuum " leak"…worth the effort?..not to me.

In theory the engine should be air-tight and any blowby should have been measured before to a degree, if it is still combustible at all. Leakage should be negligible.
And I wouldn’t use manifold vacuum if avoidable. A pipe to the air filter won’t do any harm, won’t alter the tuning I‘d say, and it will be easier(?) to do.

Certainly easier to avoid using manifold vacuum…too many issues there, and I don’t know what would be gained, actually.


In theory the engine should be air-tight

Surely this can’t be the case as the engine has that sodding great filtered breath in the middle of the cylinder head??


Certainly easier to avoid using manifold vacuum…too many issues there.

Could you elaborate? I’d assume that if I could position a vacuum take off in the middle of the manifold this would be OK? I know that servo brake kits are sold for XK’s and the vacuum must come from the inlet manifold…

So in consultation with ME Wagner, we’re thinking of connecting the PCV valve to the existing breather assembly, locating the vacuum from the centre of the manifold - I suspect that the air filter assembly will not create anywhere near enough draw to operate the PCV valve. so don’t see I have any other option for vacuum). The last issue is where we draw new, fresh filtered air into the crankcase (running the assumption that the front crankcase breather is the ONLY breather on the XK engine?).

I could use the dipstick hole with a filter OR install a separate filter in the cam covers… neither are ideal, so continuing to seek inspiration.

For reference, take a look at the following 6 cylinder install. In this install I’m assuming that the oil filler cap is also a breather

if you suck on there, besides some blowby, nothing should come out.
maybe you need to seal the dipstick! just a thought.

brakes need manifold vacuum, but the crankcase is not important to safety and does not need a past - carburettor high (low…) vacuum source.

as we all seem to agree, you need neither valve nor high draw. you also don’t need to draw air through the crankcase. the front one at the timing chain adjuster is the only one. i thought all you wanted was to install something to pull a slight vacuum on the crankcase while running.


I can tell ya what Ford did. They used a carb spacer plate under the carb with a vacuum port and length of hose attached The other end of that hose accepted the PCV valve, which was then inserted into the valve cover via grommet. The air cleaner provided the other valve cover…v8…with a filtered fresh air supply. So, you had a cycle of fresh air in, “fumed” air out, thereby flushing the crankcase fumes.

Of course, the system saw varying vacuum levels so it’s operational characteristics were variable as well. But this is were the valve comes in…it does react to these changes, your guy can explain more on this.

Now race engines often omit the fresh air supply in order to maintain a vacuum in the crankcase.

which apparently isn’t neccessary on the XK, or at least it does work well enough without…? if i was lucky enough to have a 120 i would get new seals or live with it. if anything, i’d put a hose to the air filter and use later XK parts for that. done.

what makes a pcv so desirable?

what makes a pcv so desirable?

I’m basing this project off a ‘hunch’ (and having spoken to an instrument engineer who’s installed plenty of these sorts of valves curing engine leaks) - that by introducing a slight vacuum I should be able to reduce the rear crankshaft oil seal leak I have. The engine is no where near requiring a rebuild and to fit modern crankshaft seals the crank has to be removed and machined - not a small job :)))) where as installing something like this variable PCV looks relatively straightforward and not too invasive.