Bicycle Mechanics - Loose headset

A couple of months ago I had to replace the headset on one of my bikes. During the process, I discovered that the fork has non-standard Raleigh threads (the bike is a late-70's Raleigh Grand Prix) , so it can't take new headsets.
Fortunately, the mechanics at my wonderful LBS rooted around in their back room and came up with a pile of used Raleigh headsets. With a little trial and error, we were able to mix and match parts and come up with a complete headset.

All was well for about a month, when I noticed the headset was loose again. I tightened it up, but a week later it was knocking once more. Since then I have to tighten it about once a week. Last time I regreased the threads and cranked the locknut down as tight as I could, but it still came loose again.

What could be causing this, and how can I fix it?

Despite the headset's questionable pedigree, it works great when it's tight, and everything seems to fit correctly. If the headset is somehow defective, my LBS can probably come up with another used Raleigh headset, but a new headset isn't an option unless I replace the fork.

Are we sure the various locknuts and washers (don't know the exact config for your headset) are in the right order. I've had similar problems with older bikes I've acquired --and re-ordered things such that one threaded nut tightens the headset appropriately and another locks tight against that nut to keep both from ever moving.

Above the head tube, there's only the threaded upper bearing race, a couple of spacers, then the locknut. I don't see how anything could be out of order there.

I had to tighten it up this morning, and the locknut didn't seem loose at all. Also, once I got it tight enough to eliminate play, it felt noticeable rougher than when I first installed the headset, and the handlebars don't turn quite as freely (though it's still good enough to be rideable).

I'm worried that the headset is installed incorrectly, and is somehow causing excessive wear on the bearings and/or races. When I tighten the headset, it clamps down on the worn bearings and gets rid of the play -- but only until the bearings wear down a little more. Is it possible for bearings to wear enough to cause noticeable play in a week (about 50 miles riding)?

I should probably head down to the bike shop at some point and overhaul it.

Either that or that headset is worn out.

The stack height of the replacement headset may be slightly lower than the original headset's stack height. If so, you may be tightening the locking nut against the top of the steerer tube rather than against the spacer and upper bearing race. In cases like this, it takes a few rides for the upper bearing race to work its way loose again, which would explain what's been happening to your bike.

Simple fix: add headset spacers until there's sufficient clearance between the top of the steerer tube and the inner lip of the locking nut.

yeah, it's time for an overhaul/pitting inspection. Once everything gets all flintstones in there it's tough to say what's causing what. You get one messed up bearing that likes to sit a specific way in one bad spot, and it feels one way for a while, then the bearings move a bit and everything changes, etc...

i supsect you're lacking a washer in the assembly, as trakhak suggests. the raleigh headsets aren't exactly the smoothest, but generally hold up well with a little maintenance.

i supsect you're lacking a washer in the assembly, as trakhak suggests. the raleigh headsets aren't exactly the smoothest, but generally hold up well with a little maintenance.

I checked for that when I first installed the headset. I'm pretty sure there are enough washers to get a tight fit with the locknut.

I took it in to the bike shop yesterday and talked to the mechanic who helped me install it. She thinks that the mismatched parts are causing the problem, and that the best option is to replace the fork and install a new headset.
For now, I'm going to just keep adjusting it as needed, and look around for a used fork with the right amount of rake.

I checked for that when I first installed the headset. I'm pretty sure there are enough washers to get a tight fit with the locknut.

I took it in to the bike shop yesterday and talked to the mechanic who helped me install it. She thinks that the mismatched parts are causing the problem, and that the best option is to replace the fork and install a new headset.
For now, I'm going to just keep adjusting it as needed, and look around for a used fork with the right amount of rake.

Did the mechanic pull the stem out yesterday? If not, pull the stem out and check for a gap (or lack thereof) at the top of the steerer tube. Also make sure that the steerer was cut flat. I've never seen an English Raleigh fork that wasn't cut correctly flat, but you never know. If it all looks good, then the mechanic is probably right and the mismatched parts are the root of the problem.

If the bottom cone on the steerer or either of the cups in the head-tube weren't seated fully & evenly, they will continue to seat as you ride (until bottomed-out), leaving the headset loose. Not a common problem, but it can happen.