What size hex driver?

[pmdevlin]

Hi all
I have a single cylinder laser 150 engine in an rc plane. The inlet valve spring is broken, I need to get the head off. The manufacturer told me they are 4ba bolts, requiring a 3/32 hex driver. I don't want to damage the bolts as 4 of the 6 are recessed in the head. A 3/32 alken key won't fit unless I knock it on, and a metric one won't fit, so perhaps someone previously used different bolts. Is there another size I could try
Thanks!

 

[jack620]

For something that critical I would just grind/lap the tip of the next size up hex key until it was a snug fit. When the job is finished, you can grind off the reduced end and your hex key is back to nominal size.

 

[pmdevlin]

Good idea!

 

[Ken I]

Grinding an accurate key is difficult - try this method.

Grinding Hexagon Thingumybobs

The sharp corners and grinding pattern also help with torque.

Regards, Ken

 

[goldstar31]

In the last millenia- or Donkey's Years, I made up a pair of mild steel hexagon spanners using driven in hexagon key steel. Every decade- give or take the odd century or two, I regrind the ends again.

Far more recently, I copied the spider of FOUR Allen/Hex keys to the format of Eccentric Engineerings tool to adjust the little 'little nuts' for their Acute tool and cutter grinder. A bit of round, drilled four holes and silver soldered.

That's it.

Norman

 

[harborfreight8x12]

Grinding an accurate key is difficult - try this method.

Grinding Hexagon Thingumybobs

The sharp corners and grinding pattern also help with torque.

Regards, Ken

What a neat idea. I can see using silver steel (O1 steel) and heat treating it.

 

[Ken I]

Silver steel works - its easier if you harden and temper it - then grind it - always being careful not to "blue" it. Whatever - your choice.

Hardened drill blanks work best. Again don't overheat during grinding.

I also make them out of old broken (but decent quality) screwdrivers - if you can file it - its no good - too soft.

This is a 0.040" that I have used for years in my slotcar hobby which is hard on allen keys.

Obviously using my method, you can only get a limited parallel portion (relative to tangent to grinding wheel) but you can also make it oversize and/or deliberately use the taper (by working off-tangent) for a burst grubscrew extraction tool - I have a selection - in various stages of oversize and taper - again for the slotcar hobby - all the guys bring me their burst screws (4-40 UNC) for extraction - I have never failed to get a screw out.

Regards, Ken

 

[harborfreight8x12]

Awesome. I'm 71 and still learning something new everyday. Thanks for sharing. Kind regards, Al

 

[Asm109]

Useful tidbit of information. The dimension across the points of Torx bits is the same as the distance cross the corners of several hex recesses on bolts.
When you have a screw that is starting to strip the recess, you can often find a torx bit that will fit nice and tight in the corners and remove the screw.

Not every allen wrench size has a corresponding Torx bit but a lot of them do. Especially in the smaller # size screws.

 

[pmdevlin]

thanks for suggestions, I looked at the thinymybob, great idea, my engineering skills, well maybe a long way behind you guys! The situation has now got worst! I tried using a stone to get a 3/32nd allen key to fit, after 45 mins still not getting anywhere, so far, I have not damaged the head bolts, so though what are my other options? leaving the head in place perhaps as the bolts may actually be seized, and corrosion might be the issue to why the correct size allen key wont fit? not sure what the bolts will be made from, so the heads might be soft creating more issues. I removed the glow plug, and using an allen key managed to lock the valve in place from inside the cylinder via the glow plug hole to remove the collets and offending broken spring. Carefully putting them on the workbench as they are tiny, turned back, the allen key had moved, and now the valve has fallen into the cylinder bore, at this point I was ready to get all my rc planes on ebay and buy a train set!
I like the idea of the torx keys, but once I go this way there is no turning back as the bolts will no doubt get damaged in the process. Think I need to take up knitting...................

 

[pmdevlin]

meant to ask, so we have imperial and metric allen keys, is there another unit that might work, they are 4 ba bolts, with 3/32nd recesses for allen keys

many thanks

 

[goldstar31]

meant to ask, so we have imperial and metric allen keys, is there another unit that might work, they are 4 ba bolts, with 3/32nd recesses for allen keys

many thanks

Well, yes
However my homemade pair will fit both Imperial and appropriate metric sizes.
Humm- back to dear old Euclid and the adage:-
The son of the squaw on the hippopotamus hide is equal to the sons of the squaws other two hides

 

[gadabout]

Have you miked up your 3/32 Allen key to see exactly what size it is? Try a few other 3/32 keys to see how they mike up, maybe they are oversized?
Mark

 

[goldstar31]

Have you miked up your 3/32 Allen key to see exactly what size it is? Try a few other 3/32 keys to see how they mike up, maybe they are oversized?
Mark

I got a SMALL 4 jaw Myford Burnerd independent chuck-- minus the key and the size is 'sort of hidden.

So I stripped a piece of scrap wire, made a hook in the end and put the end into the recess-- and filled it with m y hot glue gun. I 'Miked it-- QED

 

[jack620]

I tried using a stone to get a 3/32nd allen key to fit, after 45 mins still not getting anywhere...

You must be using a very fine stone. I have a 600 grit diamond stone that would do the job in a couple of minutes.

 

[jack620]

A 3/32 alken key won't fit unless I knock it on...

So I’m confused. You can get the 3/32 allen key to fit the screw by knocking it in, but it won’t fit even after 45 minutes of stoning?

 

[pmdevlin]

thanks to all, its now sorted, using a t10 torx tapped lightly into the head bolts, and off it came. Fitted new bolts, and guess what, I borrowed a cheap 3/32nd allen key from a friend, it fitted the original bolts perfect, the brand new bondhus one, well thats now in the bin, it was obv over sized

(yes it was a very fine oil stone!)