Rope-seal; JD17B; JD17B-1; cutting up a spare crankshaft

Hi folks

I am at the stage of rebuilding my V12 (1974) after taking 3 years to get 5 snapped head-studs out of the block.

My first challenge is to size the new rear-main seal.

I appreciate this will have been discussed numerous times before but I have a new idea…

I’m having trouble getting the tools. I’m in UK and there is a place which sells/hires them for a reasonable fee but they don’t have them in stock.

Being in a rush to get on with it and having a spare crankshaft at my disposal I was thinking I could just cut part of that off and use it in place of the tools (JD17B and adaptor for V12 JD17B-1).

Is this ridiculous or genius?

Why cut the crank? Why not use it whole?

because it’s quite heavy and from what I’ve heard the operation takes some time rolling the seal into the groove, checking fit etc then doing same with cap. would be easier to handle if it was cut down to minimal length

I had a CAD drawing of the tool somewhere… I’ll try to find it tomorrow and will share the dimensions.

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That would be great John_John1

I don’t have any access to lathe or similar engineering but at least can check the dimensions of the tool(s) against the spare crankshaft :+1:

That’s all I ever did, with rope-sealed cranks.

For the xk engines the tool is slightly larger than the crank, but a crank will do. I hope it’s not a good crank.

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I think the crank is very good. It came out of an engine with very clean oil and no corrosion anywhere but the cranks seem very hard to sell in the UK so if I can’t get the tool (and can cut the crank) I’ll do that.

Will also need to check if the tool is actually bigger than the crank like you point out (for XK)

crank came out of a very clean engine

In a discussion a few months back, someone pointed out that the tool is actually smaller than the crank. That made zero sense to me.

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I believe that is incorrect: I think the sizing tool is very slightly bigger.

That said, and I’ve said this before–and people have pooh-poohed it–but I’ve set hundreds of rope seals with nothing but the crank, on Jaguars to Austins to Fords.

Done properly, and that’s the key word, they work fine


Hey, I found it! Here, check out this thread:

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Interesting; perhaps my memory/knowledge is in error.

If indeed it is, my method of using the crank-as-sizing-tool (TBH, it wasn’t until i began hanging out on JL that I even knew there was a special tool for it!) is more justified.

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I’m sure the author of that thread (Rich_Mozettea) did plenty thinking on it but my 1st reaction is…

Is he including adaptor JD17B-1 in his plans?

afaik the tool is correct size for XJ6 but you need an adapter, to make it bigger, for the V12

:face_with_monocle: :thinking:

@DouglasBuchan posted the data and I think you’re not in error.

I remember thinking about my increased risk of overheating the seal after using the crank as sizing tool. Maybe the tools aren’t so precise, and precision is not so important?

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So long as you followed my procedure to the letter, you shouldn’t have any problem.

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how do you mean ‘to the letter’?

I can’t see any specific instructions, on this thread, other than the fact you always used the crank. :face_with_monocle:

He‘s just telling me not to worry (which I don’t).
Not in this thread, more like last winter when I put my seal in. I think a crank works well and I don’t like the copy of the sizing tool that we have. One thing about the sizing tool is that the seal can be cut with the tool in place.


I get what you mean :+1:

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Just a note, Rob Beer racing has the V12 seal sizing tool for 110 UK quid.

~Paul K.

Apologies for delay, here’s the all info I have:

Measurements I found on different forums taken from different tools:

  • Zelenda J17B measures 3.1194"
  • “factory” tool (polished) 3.123"
  • brand new, never used, de-waxed for measurement 3.11945"
  • another J17B tool 3.1252"
  • Churchill J17B tool 3.110’’
  • 3.1145"
  • 3.119"
  • 3.120"

probably the tool is 3.125’’ nominal, but there is no doubt that the tool is smaller or at least equal to the crank. In addition to sizing the seal the tool centers it, but it looks like the same thing can be achieved by using the crank.