Is there a published reference for tightening torque values for various capscrews and fittings on the XK engine? Specifically I am looking for torque value for the banjo fitting that conducts oil from the rear of the block to the supply line that feeds the camshafts.
Any help will be greatly appreciated.
Thanks,
Bob Schjerven
There may be one in the manuals for later models such as the XJ6.
As I recall all three banjo bolts are the same and 5/16-18 UNC thread, but there is a hole through them that weakens them there, due to reduced cross sectional area, so lacking anything else I would look up the standard torque for 5/16-18 UNC Grade 5 bolts and reduce it by 15%, so 17(.85) = 15 foot pounds.
1 Like
I would be exceptionally cautious about torquing any steel fastener that threads into an aluminum casting, especially in this location where repairing a stripped thread will be quite challenging. It only needs to be tight enough to seal a dead-soft copper washer.
A garden variety aluminum casting has nowhere near the shear and tensile strength of even the cheapest steel. You can see evidence of this in the common occurrence of stripped sump drains and sparkplug holes. A SAE Grade 5 capscrew has an allowable tensile stress of about 120,000 psi. A 6061 aluminum alloy capscrew is around 30,000 psi. An aluminum casting won’t be close to 6061 strong.
I once witnessed a spark plug shoot out of the cylinder head of an IMMACULATELY restored S model XK150, denting the bonnet. Because of that, if the cylinder head is off, I have Helicoil inserts installed in all of the sparkplug holes.
1 Like
The spark plug holes is an example where they used a fine thread in an aluminum casting, I suppose because M14x1.25 is a standard thread for spark plugs and they used it previously in their cast iron heads, but to compensate for reduced thread strength, the head is thick there, making the threaded holes deeper.
I agree that maybe Grade 5 for banjos is too much, so one could use Grade 2 with a reduction, making it 8-10 foot pounds, depending on your source of information, I found several varied values.
If you’re really intent on using a torque wrench there, maybe put a question on the XK engine forum and somebody with an XJ6 manual will look it up.
Agreed. I could have been more clear…the question emanated from a leaking banjo fitting that is threaded into the cast iron block, routing oil from the rear main to the overhead cam on the left side. So the application is the steel banjo bolt (Gr 2 or 5) threaded into cast iron. The bolt is 1/2” x 13NC but is drilled out along the centerline and has a crosshole drilled thru perpendicular to the bolt axis. I’ve no clue with all the material removed and stress risers likely to be present how much torque is appropriate. The car is a 56 XK140mc. And yes, helicoils in the head.
Thanks,
Bob
Thanks Bob. After sixty plus years of wrenching I have a pretty good “feel” for what works. However the extra holes makes this sorta unique. I have tried new annealed washers and tightened to what I feel is appropriate but still have the leak. The embossing on the first set of washers didn’t reveal any anomalies in the block counterbore surface and the banjo fitting (new) looks pristine. Could reef on the bolt a tad more but am leery.
Thanks for your response.
Bob Schjerven
There must be a defect or the bolt is bent if you consider tightening it so much that you are worried about the bolts integrity!
I didn’t tighten it very hard, not unlike the same bolts in the head, and they seal fine (original, annealed washers). Have done that with several of those banjos and they only needed retightening once which fixed it.
The problem nowadays is the quality of modern copper washers, which are bullet-hard compared with originals. I’ve had real difficulty getting these to seal on some applications, despite re-annealing. I think the alloy composition is different. Try to resist the temptation to over tighten!
1 Like
Well, my memory was a little off, I checked a C8610 block and the thread is 3/8-16 UNC.
Torque on a full undrilled Grade 2 bolt is 15 foot pounds, so a reduction of 15% would be 12-13 ft lbs.
Thanks Rob, that sounds about right. I appreciate the follow up.
Bob
Against my better judgement I gave the banjo bolt a very slight additional elbow grease and turned it perhaps 30 degrees. It seems to have sealed (after a very short warm up drive still dry.)
1 Like
Much better to err on the safe side; happy for you!