All the jammers on my boat are old and slip a bit. ie. The sheet or halyard gets pulled on tight, then the jammer after locking lets 1 or 2 inches slip back.
Has anyone tried disassembling one and fixing?

Not knowing the brand of jammer, I can't assist. there's two items here, one is the rope the other is the cam inside the jammer.
If the rope is older than a couple of years, replace it. Check the cam for signs of wear.

Also check the line diameter suitable for the jammer.

If you are tensioning the line through a closed jammer, once you have desired tension on open and close jammer again.

if its a Ronstan jammer, the jaws are replaceable/consumable.
And they come in size ranges too, depending on halliard dimension.

And if that fails, you can always consider burying some additional core into your rope tails to give the clutches something bulkier to bite into.

A few suggestions which may (or may not) help you:
* When closing the jammer on the line, on many of the better types - you can push the cam down to engage the line before you fully push the jammer down. This gives quicker engagement.
* And more general good practice - before opening a jammer that has had a line under load - haul the line though the jammer a fraction, just to ease the cam. This reduces wear on the line, particularly if a halyard that is jammed in about the same spot every time, and also makes releasing the jammer much easier.
* If the jammer runs directly to a winch, and you don't need that winch after tightening the line, leave the line taught on that winch, so the jammer is only a back-up.

I have two jammers on my boat and they are for the two spinnaker halyards. The rest were ****binned and replaced with 150mm cleats.

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Ramona said..
I have two jammers on my boat and they are for the two spinnaker halyards. The rest were ****binned and replaced with 150mm cleats.

Hell of a lot cheaper solution too!

Same with me Ramona. But l have 7 winches on a 24 footer. Jammers are for fastening off in front of a winch. Different to do that with cleats. Especially with halyards .

Clutches , jammers , cam cleats , cleats (horn) . I discovered that my headsail halyard was slipping as not only were are the old metal cam cleats teeth worn and smooth but the old 6mm dyneema halyard had gone hard and smooth in that area so replacing both. I don't have a coachroof winch on that side but am seriously considering one just for this line and using a horn cleat.

Thanks all for suggestions. I think mine are 1985 Spinlock. I had some success increasing rope size from 10mm to 12mm but rope doesn't run though quickly when you want it to. I'm going investigate disassembling them. Regarding cleats there's not many reasons to use jammers actually, except saving space maybe, cleats would be fine.

I'm actually looking at jammers and doing away with the cleats, (I will leave them in situ but you know what I mean ) and be able to make more use of the winches. I don't have high tension lines but at times there can be a lot of lines. Perhaps I've been spoiled sailing on a vessel with clutches & self tailers

Jammers are fine in front of winches but a cleat or several cleats behind each winch will be handy WHEN the jammers start slipping. Particularly on the 3rd reef lines where the tension can be high. My boat came with winches on the deck behind the mast with cleats. Some of these cleats were worn at least third the way through! I have kept them and bought more. For the control lines under less load I use camcleats. Quality Chinese made stuff.