My 71 Ser 2 recently started losing power on a local drive. It sounded like a leaky manifold gasket, but that didn’t explain the power loss. I nursed it home and did a few tests, one being compression. With the exception of Cyls 1 and 2 (from Rad back to firewall), all was as normal 125 psi. However the other two had only 20 psi.
I suspected a valve or head gasket, it turned out to be the gasket (Payen) which was installed about 5 years ago and 15,000 miles.
The block is a late XJ6 donor L8 with all bores and bearings standard as my original was timexed at 40 thou over. As per the photos, the gasket failed between the cyls, which is a first for me, having owned the car since 1985.
Looking at the atrocious match up of the 4.2 head to the original designed block it is surprising that these gaskets don’t fail more often. My question is to all then is this… has anyone else experienced this and if so what is the solution.
I am shopping now for a bullet-proof gasket that can work with what Jaguar has mis-matched as factory workmanship. Any help appreciated, pix attached, Brian / Mytype
Get a Cometic multi layer steel gasket. XKs sell them. 125 psi is probably a bit low for compression on that engine but if it’s otherwise running ok I’d just fix the gasket and see how you go.
Lots of mix-ups there, Brian. Did you build the engine?
Jag cylinders are numbered in the other direction.
The block is 8L not L8.
The 4.2 head is essentially identical to the 3.8 head, it’s the block thst changed.
The four offset 4.2 squish areas perform no worse than numbers 2&5 which remain co-axial with their cylinder bores.
The above are not major issues, obviously, but tossing words like ‘atrocious’ around is perhaps a bit more credible when we get our own facts straight.
With regard to the gasket failure, this is uncommon, but head joint integrity does rely on meticulous assessment, rectification if needed, and technique.
Why was the engine stripped at the time you decided the original block needed replacing?
How many thou of bowing did the XJ block face have or did the 8L pass 100% flat before use?
Did you use your existing timing cover or use the later 8L cover?
How many thou out was the head and did it get skimmed? If skimmed, was it a milled finish or fly-cut?
Which way up was the gasket fitted?
Was the tightening sequence followed and at what point, if any, was the head retorqued?
I mostly ask the above to highlight some of the many factors involved in ensuring the correct function of the head joint, which is generally reliable in the vast majority of cases. Failures, when they do occur, are typically caused by overheating or other factors besides design. I would certsinly not fit another gasket without scrupulous checking of block and head for flatness and finish.
Ditto on the Cometic gasket. They are well designed and have great crush rings to seal around the cylinders. I got mine from Terry’s. Ideally you’d CC a head dome so you can pick the perfect thickness. Yes unfortunately the 4.2 cylinder spacing is far less than ideal. It scared me into a 3.8 personally.
If you aren’t just looking to get this back on the road ASAP, I’d seize this opportunity to do a mini rebuild, and I did just that 17 years ago. My car was in that 125 range when I got it and it ran like crap. You can actually do this in situ although I pulled the engine. Drop the pan, and unbolt the pistons one by one, get a proper sized ball hone to clean up the bore while being careful to mask under it with rags. Then clean the bore and clean up the piston and install again with new rings. Finally reassemble with new big and little end bearings. For just a few hundred in parts and a few days work, I got my 125 engine up to 165 again, a huge improvement with just a little cost and effort.
edit: missed the bit about your replacement block. Was it actually rebuilt already? Was your head ever rebuilt? 125 is really not what I’d call “normal”. It’s what I’d call worn. It’ll run but not great. I haven’t done a compression test on mine recently but I’m certain mine is still well above that even 17 years and 23K miles later.
Hi Erica yes the donor block was from an XJ6 and all the bores and bearing journals were standard. The only mod that I had to perform was for the cam oil line feed. On the original block there was a tapped hole in the rear of the block in the oil gallery. The XJ block did not.
I ended up putting in a brass T from the oil filter itself and 10 years later no problem. The block deck is nice and flat, the holes had the original cross hatching, it has proved to be a winner.
4 years ago I replaced all the crank and journal bearings in situ and honed the cyls at the same time and installed new rings. Based upon various responses from the members (thanks to all) I will be shopping for a Comectic head gasket and will have the head checked out at a locally recommended head shop.
The car sees about 5-6000 highway miles annually, it has the 8-1 pistons and perhaps the lower comp is in the head, stay tuned. B
Just one word of caution with composite gaskets, both the block and head finish must be smooth a “roughness” of less than 50Ra is required (and stated on the packaging). Having buttoned up my freshly skimmed block and head it leaked as soon as I filled the radiator and no refund from the supplier as “I hadn’t met the smoothness required”. I used an ordinary head gasket and 6000 miles later still no leaks.
