Reason why the early XK engines changed the front engine mounts? A flawed, weak design?

Hello all, I just talked to a friend that cleaned the front of his early 1951 XK120 block in preparation for Magnafluxing. After removing some grease he found two of the threaded bolt mount holes for the early style, front-mounted half round type, engine mount had cracks on the block around the holes. Is this why they changed the style of mounting? At first I said I bet it was in an accident but he suggested that perhaps they fatigued the block as a result of a less than ideal design. It is something I will have to check on my early engines I have as spares. Thanks, Gary

Do you mean the C.2331 block or the C.4820 block?
The first and second front engine cross plates did not have angle struts on the rear going to the block.
The third version did, which added stability, but there are no holes in the C.2331 block to attach these struts.
My cross plate had a stress fatigue crack running upwards from the bottom hole where it attaches to the front timing cover. The holes are too close to the edge of the piece.
There was also a change to the timing cover; it got thicker around the bolt holes, probably for the same reason.

Cracks in the plate, or cracks in the block? The early plates flexed a bit every time you pushed in the clutch. I could see cracks in the earlier plates. Moving the mounts back to the sides also took some of load off of the transmission mount, which was also different, early to late. When did the trans mount go from the slab to the angled bobbin? Maybe about the same time?

FWIW, my early-- pro date, 62 Oct-- E Type still had the bobbins. Not sure why they went to that goofy spring plate. The bobbins seems to work fine.

Bobbins or round mounts began to be used on 120s finished in July 1952 according to Porter.
However, they were on Mark Vs beginning in mid 1950, as a little over half of them had them. The change involved drilling a couple of extra holes in the chassis.
Mark VII never used the slab mounts.

672027, April '52, has the slab mount. Unknown whether it had the braced front plate, as the original engine was gone when I got it.

I found a welded repair to my front engine mounting plate. Mine has the C2331 block, so can’t use the angle stiffeners. I wonder if there’s any reason the plate couldn’t be remade in a slightly (or much) heavier gauge steel…? Hardly rocket science…!

Are your horse collar plates a single thickness, or are the bent-over ears overlaying and welded on? I modified this one, which I am sure was a reproduction, to work with the different timing cover bolt layout of the later blocks.

Mike, mine is single thickness (haven’t measured it) and the ends where it goes on the engine mounts are just bent over at 90 degs.

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Hard to tell from the photo, but the plate above is a welded assembly made up of 3 pieces. I think -somewhere- I have another just like it.

Mike, yours would have had the angle struts.
The cross plate is a single piece bent at the ends.
XK120 engine mounts 007
I don’t know that I would make a whole new thicker plate, as we won’t be racing/abusing these cars. I would be mistrustful of a simple surface weld repair. I used a cutting wheel to open up the crack first, so I would be sure to get full penetration of the weld on both sides. 11,000 miles later its still ok. If you’re worried about the crack developing again, you could build up some weld below the bolt hole to add some strength there where the stress is greatest, or weld a bushing over the bolt hole and use a longer bolt.

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Some of the earlier timing covers had a passenger-side boss that doesn’t quite meet flush with the back of the horse shoe engine plate. If yours is one of those, then you’ll be wanting to fit a spacer so that it does. The spacer on mine was missing and as a result, a previous owner simply torqued the bolt there contorting the plate until it met the boss face.

Hello all, my friend and I are astonished at the outpouring of well intended and helpful advice. Many thanks for that. I spoke further with him and he wanted to point out that the front alloy engine cover shows no damage in the area through which the bolts that attach the front engine support pass through. He is now thinking that perhaps since the cover is not cracked in those two places where the block mounting holes are cracked, perhaps someone in the past has inserted too long or perhaps oversize bolts in the block thereby cracking the block. I have never heard of the front mounts causing a fracture in the block where they mount although I, like others who have commented, have seen the mounts themselves cracked. In light of the fact that the original alloy front engine cover is not cracked like the block, perhaps the cause was indeed made by human error and the issue is not a design flaw.

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