Replace main brg cap bolts?

I am rebuilding my 3.8 engine. I’m wondering if most people are reusing their main cap bolts as opposed to using new ones (at $18-25 per bolt)?

“I hope to…” he said, with two hands tightly on his wallet…

No prob reusing…but at that price, can’t you get ARP studs?

You can certainly reuse undamaged main cap bolts on any street engine. You can make an argument about con rod bolts either way. Ring gear and flywheel bolts are prudently one-and-done.

I am not aware of an ARP XK main stud kit, I’ve never found one in their extensive catalog. If you are resourceful, however, you might find that the ARP main studs for certain big block V8 Chevrolets might be of some interest.

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I reused flywheels always, on street cars: never experienced a failure.

I would always replace the single gland but on VWs and Porkers, however.

Hi Mike,

I agree that con rod bolts should be replaced. I suppose I can get grade 8 main bolts locally if they are a standard fine thread. I’ll check on that.

Thanks for your input,

Al

Do NOT - EVER- use SAE grade 8 bolts for connecting rods. First, they will not come in the required configuration. Second, even if they did, no one is actually checking them to see that they actually conform to the SAE spec. Buy specialized bolts designed for connecting rods. The later 4.2L S3 XJ6 bolts will be a good upgrade (if that’s not what you have already).

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Hi Mike,

I’m using the ARP rod bolts XKS sells for this engine. I was talking about Grade 8’s for the main bearing caps.

Thx

Is there a problem with using off-the-shelf Grade 8 bolts for the main bearing caps?

As Mike mentioned, there are a lot of “bad” Grade 8 bolts out there, being made overseas, to who-knows-what spec. The problem is so prolific that Caterpilllar actually started making their own bolts, to ensure they aren’t bitten by a “bad batch”. Where I work, we aren’t allowed to use fasteners from places like McMaster-Carr, or similar suppliers…only ones that are “qualified”, and can demonstrate traceability in their supply chain.

All that being said, there’s a retired GM engineer on my favorite MG forum, who builds some very successful race engines, and he often advocates for using Grade-8 bolts for main caps and flywheels.

I’ve bought main cap and flywheel bolts from well-respected suppliers like Moss Motors and MiniSpares, and had them come with Grade 5 markings, or no markings whatsoever. Compared to those, I’d feel better using Grade-8’s from a trusted source.

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I believe ARP makes a set of studs & nuts for this engine. Perhaps that’s the way to go.

Thanks for your input.

Be aware of the rod bolt cut, and chamfer: ensure that the rod bolt head both sits flat on the rod surface, and that the one side that is flat fits in the rod area that ensures the bolt doesn’t rotate.

A real SAE Grade 8 bolt might be perfectly fine on the main caps. Maybe.

Here’s the problem, you will need a bolt with exactly the correct length of threads. I doubt you will find that in an off the shelf SAE bolt.

Some things you need the remember about bolts:

First, the ultimate tensile strength of a bolt is dictated by its diameter at the root of the threads, not the shank diameter. This is the location of it’s smallest cross section. Those thread roots are also stress risers where fatigue stress cracks can occur. The more excess threads you have, the stretchier the bolt is and the more stress risers are exposed to the load. Unless you buy your grade 8 bolts long and cut off the excess threads, that bolt will have an inferior fatigue life and be statistically less reliable.

I’ve never felt compelled to experiment with grade 8 bolts on the main caps. Personally, I would stick to the original Jaguar supplied bolts, which I have never seen fail or loosen, or I would use ARP studs, washers and 12 point nuts. I don’t trust anything in between the two.

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Agreed: again, this smacks of JL wheezy overthink.

The factory-supplied bolts are of sufficient quality, to be safely reused a number of times, and are exactly the correct dimensions.

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Are OE Jaguar main bolts still available? If so, then this is a no brainer.

Why don’t you want to use your old ones?

Just FYI, you addressed this to the wrong person.

As recently as perhaps last year it was reported on here that ARP stated that they do not make a rod bolt for any Jaguar. They also told me that when I contacted them way back when I rebuilt my own 3.8 engine

Here is one of the ARP rod bolts that I was sold by a major (at the time) supplier when I rebuilt my engine, pictured against an original Jaguar rod bolt.
DSCF00242

Notice the chamfer under the head of the Jaguar bolt. That is missing on the ARP bolt which instead has a sharp 90 deg corner. When the bolt was torqued up, this corner bit into the radius machined into the rod preventing the head from sitting down properly. You cannot see this but you can measure it by inserting a feeler under the head where it’s supposed to be making contact. I measured an .008"gap under the head, which couldn’t be seen by eye.

What I did was to file a similar chamfer on one side of each bolt head, being very careful not to knick the shank and then assembled them in the rods with the chamfers all oriented inwards toward the rod. Worked for me.

Re the original question, I don’t know why anyone would want to replace the original mains cap bolts unless they were damaged. They are already pretty hefty.

BTW, the mains cap bolts will have a “grade” letter on the head, denoting the strength, maybe S, T or higher. (Flywheels use an ‘X’). If anyone knows what it is, mayeb they could say…

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It’s a puzzlement to me, too: I rebuilt hundreds of XK engines, and never once saw a mains bolt come loose, or have an issue. Ditto with all other streeters I built.

Try this;

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