I had a strange problem yesterday that hopefully I can get some clarification on. On my 4.2 6-cylinder engine one of the center head bolts on the passenger side started leaking coolant from around it. I tightened it down to 75 ft/lbs; it was loose. And the leaking stopped. So... two questions:

1. The service manual shows the head nuts to be 7/16" which we know is wrong. It's a 3/4" nut. And it says to tighten to 54 ft/lbs. Which I don't think is enough. What is the correct torque?

2. Why coolant leaking out of the nut even if it was loose? Is the washer under the nut supposed to prevent coolant from coming out?

Thanks, Bill

 

RE: #2: As the head bolts extend into the water jacket on these engines, or so I've heard from those who claim to know, it's not surprising that a loose nut would leak coolant.

I hope tightening the top nut well and truly fixed the problem and didn't merely hide it.
(';')

 

If the coolant level begins to drop after tightening the acorn nut, the head gasket has most likely failed.

 

7/16" refers to the diameter of the studs. Yes the nuts are 3/4 AF.
Yes the coolant will leak up the passage from the block water jacket and out the top. So a good seal is needed between the head/washer/nut face to ensure all is well.
I used just a tiny bit of sealant on each nut and washer when last fitting my cylinder head.
My head is only tightened to 45 ft/lbs. 75 seems very high. I believe 54 is correct.

 

54 is correct.

Elinor is spot on, thats why Jaguar used Bars Leaks in ALL the engines, fact.

 

Alright my well respected Colleagues - I said the head nut was loose, but I didn't really tell you how it got loose. So bear with me - I'm going to my first car show tomorrow (well actually a Cars and Coffee at the Elliot Museum in Stuart, FL) and I wanted to further dress my engine up. So... I got a kit from John Gordon in England with chrome head and cam cover nuts (and copper washers). And figured on removing one head nut at a time, putting the chrome one on, torqueing it down, then proceed on. And the first one I tried started leaking, hence my post. I know, pretty stupid. But... "stupid is as stupid does". It's not leaking and I'm not losing coolant so I'm figuring no harm, no foul.

My next course of action is that I have the project ahead of me to put a temperature sensor in the upper radiator hose and connect it to a digital gauge I bought so I can see the actual readout instead of just the OEM analog gauge. I'm mounting it in the console ashtray. That involves emptying the coolant out. But I wanted to clean and flush the coolant system as part of that project anyway. So I figure once the system is empty I can remove one head bolt at a time and change over to the chrome ones. Yes, I know that it is risky on a few accounts. I might actually be removing the stud with the current nut; I might break a stud (big time problem); some nut might leak when I'm done filling it back up and running it; I might disturb the head gasket, etc. I'm thinking I should heat each nut prior to cracking it loose. Any thoughts, opinions, slap up-side the head comments?

 

No one here understands Shiny Things more than I do, and I hope it works out for you, but I would have to Really want to impress people a Whole lot to risk what you're risking. In your place, if I didn't already have a problem, I would shine up what I had best I could and keep all those Very Nice Shiny pieces for when you DO have a problem.

Just sayin......
(';')

 

Yeah, I share Elinor's idea. Bling is neat. But, at what risk?? Were it my car, I would not risk a head to block seal!! Touchy in all iron engines and more so in alloy to iron...

Carl

 

Now I know, and the memory (nightmare) flooded back.

Waaaaaay back, when we had the Daimler S2 4.2, dress up was the rage, and we joined in.

Same thing, CHROME head nuts etc. whoopee.

Did 6, coolant was pre drained, and the #7 snapped the stud in the block, down the bottom, in hell's kitchen, OOPS.

Engine out, full rebuild and balance, and STANDARD nuts etc fitted, bugga the bling.

Never got into dress up of any sort since.

 

I used chrome head nuts once. Years and years ago. Can't remember where I got 'em but they were not quite right, despite being advertised for XK engines. The problem was they were not quite deep enough so, on torquing them down, they (just barely) bottomed-out on the stud before full clamping torque was reached. End result, water leaks. I ended up using double washers, backed-up with a dab of sealer, and all was well after that.

On the XK head gasket jobs I'e done I use a small bead of Teflon sealant around the stud, turning wet studs into dry studs.....for as along as the sealant stays intact, at least.

Cheers
DD

 

All great information - thank you. Grant - your story is an sobering eye opener! Guess I'll just do the cam covers and leave it at that. IF I ever have to take the head off for a head gasket change, that's when I'll entertain the shiny bling. And I'll keep in mind the nut bottoming out thing Doug. Thank you all for keeping a potential disaster from happening. I kinda guessed it but nothing like the voices of experience. Went to the Cars and Coffee thing this morning, and my XJ6C got all the attention she wanted despite silver painted head nuts.

 

Bill,

I said a few choice words, I am surprised you did not hear them, even up there.

I had been warned, HAHAHA, and then I found out.

V12 can do the same thing.

WELL DONE on the attention, calms the inner man.