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M24 Hop up adjustment set screw

3884 Views 10 Replies 5 Participants Last post by  josephus
What size is the set screw in the hop up adjustment? I've found it takes an .050 inch Allen wrench to turn and the diameter of the set screw is about 2mm. English and metric in the same piece of equipment? I guess so.

More to the point, is there a particular number/description of this set screw, such as a part number? I've banged up mine pretty good and am looking for a replacement. The smallest set screw I've been able to find is 3mm in diameter. Of course, I'm here in the US and metric parts are harder to find. I imagine in the rest of the world such parts are much easier to locate and purchase.

I know I can ebay or google for parts but I don't want to buy an entire hop up unit (ebay) or search through 28,000 some odd hits (google).

Any suggestions, guidance or words of wisdom?

Thank you, in advance, for your courtesies,


Edited for punctuation and clarity
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Is it the stock one or the PDI set?

If it's PDI you can order hop up sets from their site. Shipping sucks, but you don't need to get the whole unit. You can also just buy a ton of em at once, they're cheap.
PDI Chamber Repair kit, $6.99 USD:

I have used my stock CA set screws in my PDI hopup, they're not mutually exclusive. I believe they're 1mm set screws although when you hold it in your hand it looks a little bigger. The stock CA set screws may be a little bigger than the PDI ones but I experienced no stripped threading.

EDIT: If you have any RC hobby shops around you it wouldn't be a bad idea to check there. A lot of RC planes and cars use small set screws.
Embed said:
although when you hold it in your hand it looks a little bigger.
Ahh, so true ;) pity the wife doesn't have the same opinion
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Man, I just keep discovering more and more reasons to love this site! Thank you for all the information, folks.

My hop up unit is the stock CA one. Will the PDI screws work in it?

There are a few RC stores in my area. I didn't even think to check them. I want to support locally-owned businesses whenever I can so I'll first look around here. If they don't have what I need then I'll expand my search to the virtual realm.


PS--Vindicareassassin, I have the same problem. LOL.

Edited for grammar
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M3 is the smallest you will find commercially and I'm in a metric country (that's why you only get 1.5 allen keys and not 1). I have seen smaller on McMaster Carr I think though. I retapped my PDI hop for a single M3 grub screw and filled in the two little ones - it's much better now.

I'm finding the 3mm limit the hard way. I can't throw a rock without hitting a hardware store, etc that stocks 3mm set screws. 2 mm? No one, even a warehouse supplier, has even heard of them.

I've thought about tapping the hole out to 3 mm or even 4 mm. The hop up unit feels like brass--soft and easy to tap. Any thoughts?


I owe you a cold one. Link:

What length do I need? I dropped my only screw and now I can't find it to measure.



Edit: Corrected link
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Josephus, you need to utilize the "Modify" button located at the top right corner of every post you make. Instead of double posting, you can just modify your original post if you need to add more. That's only if you were the last person to post of course, otherwise it's a normal reply and not a double post.

Administrative duties aside...

Wow! That's a great link, I may even sticky that. Also, I never thought of tapping my hopup for a 3mm. I have the day off tomorrow, guess what I'll be doing!? I don't find the two-screw setup to be as effective, for my gun at least. I threw my stock CA hopup in a while ago for kicks and it performed admirably. Although recently I've come to question if it's the 1 hop-arm PDI hopup that was around before the Duralmin ones.
I'll measure my one tomorrow morning (in 12hours). Probably the longest one though (5mm).

Should be easy to tap because the hop unit is soft metal like aluminium (= duralim) or maybe brass. Drill a 2.5mm hole and get an intermediate M3 tap. Just use a small adjustable spanner rather than a tap handle (cheap). A cheap/free source of taps is machine shops where their machine taps (also a plus if you can get them because they have flutes like a drill bit so you can tap the hole is a second using a cordless drill) aren't sharp enough for steel but will still chew through soft metals - or get them to do it for you. You can also do an equivelant small size in imperial if that's easier where you are.

I prefer spending a few days fine tuning my hop by adding epoxy and filing it back and then having the simply up/down adjustment forever after. Nearly there with my Firefly bucking.. will post up what I did when it's done.
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I found the one I dropped. It measures 5 mm in length. Bonzo, your instincts were dead on.

As a side note, Embed, how did the tapping go?

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