Cam bearing cap stud OD

I have run into a problem with a part I ordered from one of the usuals. They are the long studs that secure the tappet block and cam bearing caps to the cylinder head. The OD of the un-threaded portion on the stud is too large to fit through the cam bearing cap. I have attached pictures comparing an old stud and a cam cap to a new stud with measurements. I am in the process of thinking through a solution: ream cam caps or grind studs. What would you do?

Cam bearing cap ID: 0.308 in
Old stud OD: 0.294 in
New stud OD: 0.309 in

I would choose option 3: Send them back.

Mike Eck

New Jersey, USA

www.jaguarclock.com

'51 XK120 OTS, '62 3.8 MK2 MOD, '72 SIII E-Type 2+2

1 Like

I’m with Mike… No way in HE** would I ream the bearing caps or grind the studs (weakening them). Send them back and with you data. Have you tried any other sources??? You might try Jaguar Heritage (no affiliation) http://www.jaguarheritage.com/ as an OEM source.

Happy Motoring,

Dick

'74 OTS
'99 XJR
1947 Stinson 108-1 “Voyager”

Joe,

Did a fast check and C29283 is available (OEM) (https://jaguarmerriamparts.com/parts/index.cfm?make=Jaguar&year=&searchText=c29283&action=oePartSearch&siteid=215718). David Manners has them (http://www.jagspares.co.uk/Manners/PartSearch.asp), same price as from Jaguar Heritage… Hope this helps,

Dick

Before doing ANYTHING else, I’d check to make sure this isn’t an SAE to metric thing. Lots of this engine changed to metric around 1984. I dunno if that included the cam bearing cap studs.

Just re-read your post noting CAM Caps - NOT Main bearing Caps. So, disregard my earlier post. You may check with sources cited for availability of OEM studs.

Happy Motoring,

DIck

Merriam Jaguar (https://jaguarmerriamparts.com/parts/index.cfm?make=Jaguar&year=&searchText=c30335&action=oePartSearch&siteid=215718) has OEM Anchor Studs for $11.80 each.

Dick

Frankly, this is one part of the Jaguar V12 design I’ve long had issues with. Bearing caps need to be very accurately placed. Typically such an assembly would have dowels or other features to ensure precise location, not just during initial assembly but also whenever the engine is disassembled and reassembled.

For some such assemblies the accuracy of the location is ensured using shoulder bolts with very precise diameters on the shoulders. With these cam studs, though, the location is a threaded hole, and you simply cannot tap a hole and expect a close-tolerance location to result. If I had been the designer, each of these studs would have been surrounded with a little metal sleeve that served as a locating dowel.

For cams, with the relatively weak stock springs on the valves, it’s not as critical as you may imagine.

Just hit the same problem.
On the engine beibg rebuilt the heads were corroded beyond sensible repair.
Bought 2 brand new heads at reasonable price.
Had to change over all the studs and the tappet blocks.
The engine was unbelievably filthy and I did not mark the caps for each bank, meaning not sure which cap was for the A or B bank.

Did not take long by trial and error to see which cap fitted which bank simply by gauging how tight the camshaft felt when tightening the cap.
However, when all caps were fitted and tightened down the camshafts were rock solid.
My guess was the studs from the old heads were not quite the right size or in the right position.
That makes no sense because the studs came with the tappet blocks.

Maybe the stud holes in the new head were slightly different pitch.
Should not happen, however that engine was in production a long time and tooling is subject to wear and tear.
The studs were metric.

The actual line boring of the caps in the tappet blocks is spot on, not a problem.
I drilled out all the stud holes on the caps a bit at a time. After overdrilling by up to 0.5mm per cap now have them all tight and camshaft rotates.
Sounds strange, but think about it.
The studs are not usually a very tight fit, and as Kirby says they are not located in dowels to give a precise position for the caps relative to the tappet block.
The studs are there to keep pressure on the cap so it holds the camshaft.
The position of the cap is set by the line boring so it is concentric with the camshaft.

There was no choice but to drill out the caps. It all looks good, the camshaft rotates and it is held exactly where it should be.

FYI, I have notified the supplier of the studs. Waiting on a reply. They initially reported that they had never received any customer concerns before, having sold many sets.

While I wait for a reply, Wiggles, are you in agreement with Richards that reaming the stud holes in the caps is a reasonable action to take?

Wellllll…maybe.

If indeed the cam rotates easily with all caps torqued to spec, and a check with PlastiGage shows proper bearing clearances, I, personally, might try it.

VW connecting rods have no positive locating schema, and properly fitted, work in race engines up to 7000 rpm. Cams see FAR less stress.

They were only what, a thou or two short of fitting? I’d be tempted to make a “butterfly”, a shaft with a split in it into which a strip of sandpaper is inserted. Chuck it up in a drill and run it through that hole a couple of times. See if that opens it just enough to assemble.

I do agree that a slight enlargement of the holes isn’t a big deal. It’s just odd that it’s necessary at all.

I think it equally odd too.
Not sure I have a rational explanation, but there was no avoiding it.
The tappet studs are long ones and I had no problem taking them out of the old heads using a locknut.
I think a couple were pretty stiff but still came out just using a locknut.
No reason any of them were damaged or bent.
There were 2 or 3 short studs in the heads that needed removal with a stud extractor, and that tends to somewhat gouge the stud but they cleaned up OK.

I could float an idea. Those tappet studs are long and I assume the caps are line bored with the tappet block fitted to the head it will eventually be used on. If the studs are transferred to another head, and being fairly long to boot, they may not line up exactly the way they were on the original head.
They could be 5 or 10 thou out, depending on the clearance in the tappet block and perhaps slight deviation in stud concentricity So when you clamp the caps do they tend to clamp to follow the studs, or do they clamp to follow the camshaft ?

I think on the heads I have the caps followed the studs and that was a problem. By drilling the cap stud holes oversize it seems they can clamp to follow the camshaft

I will bet those new studs are M8, all the studs in my 6.0L are M8 but the 5.3L are 5/16.