Mounting a Cox .020

[Serge_Krauss]

When I started on the firewall for my current project, I discovered another thing I probably ignored as a kid. The #2 screws I have don't fit perpendicularly through the lug holes on the tank, because the holes are too close to the tank-back casting. I chucked up one of my screws in the drill press (poor-man's lathe) and filed about 7-8 thousandths off of the head diameter, but there is still way too much interference, and I'm afraid of getting it too small to avoid excessive pressure on the lug. The only thing I can think of is to file more off and make washers with material removed from the tank side.  I must have used wood screws back then and just angled them in, but I don't want to crack these old castings.

So, what did/do you guys do to mount the Cox .020? Am I missing something obvious here? Thanks.

[Ken Culbertson]

When I started on the firewall for my current project, I discovered another thing I probably ignored as a kid. The #2 screws I have don't fit perpendicularly through the lug holes on the tank, because the holes are too close to the tank-back casting. I chucked up one of my screws in the drill press (poor-man's lathe) and filed about 7-8 thousandths off of the head diameter, but there is still way too much interference, and I'm afraid of getting it too small to avoid excessive pressure on the lug. The only thing I can think of is to file more off and make washers with material removed from the tank side.  I must have used wood screws back then and just angled them in, but I don't want to crack these old castings.

So, what did/do you guys do to mount the Cox .020? Am I missing something obvious here? Thanks.

A 2/56 Hex Head.  Or use rubber bands.

Ken

[Brett Buck]

When I started on the firewall for my current project, I discovered another thing I probably ignored as a kid. The #2 screws I have don't fit perpendicularly through the lug holes on the tank, because the holes are too close to the tank-back casting. I chucked up one of my screws in the drill press (poor-man's lathe) and filed about 7-8 thousandths off of the head diameter, but there is still way too much interference, and I'm afraid of getting it too small to avoid excessive pressure on the lug. The only thing I can think of is to file more off and make washers with material removed from the tank side.  I must have used wood screws back then and just angled them in, but I don't want to crack these old castings.

So, what did/do you guys do to mount the Cox .020? Am I missing something obvious here? Thanks.

   2-56 Fillister head, same as the screws holding the backplate on the engine, I think. The head diameter is .140 for most of them I found in a quick search, see if there is more than .070 from the center to the gussets around the lugs.


    Brett

[Serge_Krauss]

Thanks! I'll check that out now.

[Brett Buck]

Thanks! I'll check that out now.

    However, *if it was me*, my need for instant gratification and my barnyard mechanic approach would lead me to continue filing down the screws until they fit.

    Brett

p.s.


https://www.mcmaster.com/machine-screws/system-of-measurement~inch/thread-size~2-56/drive-style~phillips/rounded-head-style~fillister/

https://www.mcmaster.com/machine-screws/system-of-measurement~inch/drive-style~phillips/rounded-head-style~fillister/thread-size~4-40/

   Note that the 4-40s are the stock screw for the Fox 35.

[944_Jim]

https://www.mcmaster.com/machine-screws/system-of-measurement~inch/drive-style~phillips/rounded-head-style~fillister/thread-size~4-40/

   Note that the 4-40s are the stock screw for the Fox 35.

Thanks much Mr. Buck,
I could have used this information last weekend as I investigated how to mount a Fox .35 to my UKIE .35.

Now I know what to order and install in place of poorly fitting 4-40 pan head screws, and just as important, what the best screws for my PeeWees will be.

Greatfully,

Jim in MS

[FLOYD CARTER]

've built a few little free flight models with the Cox 020.  I've used small wood screws from the LHS.  I think they are used to mount RC servos.  Plenty secure.

[John Park]

I had this problem last year.  What I did was to rummage through my little box of scrap odds and ends of tubing until I found a piece of thick-walled aluminium tube which (a) accepted the screws I wanted to use and (b) was of an outside diameter small enough to seat on the mounting lugs without fouling the backplate.  I cut four short (about 3/16") lengths, with a jeweller's piercing-saw blade in my electric fretsaw, and used them in lieu of washers under the screw-heads. (I think the tube dated back to the days in England when every half-pint can of KeilKraft or Mercury diesel fuel came with an aluminium spout two or three inches long!)

[Serge_Krauss]

    However, *if it was me*, my need for instant gratification and my barnyard mechanic approach would lead me to continue filing down the screws until they fit.

Me too. That's just what I did this morning. I found that anything less that 150 thousandths would clear them.

Floyd - I'm pretty sure I used wood screws 60 years ago. Maybe they have smaller diameter heads? I'll look.

Thanks, everyone!

SK

[Steve Helmick]

Why not just use 00-90 machine screws?    Steve

[Serge_Krauss]

The "dry eye" blurriness has struck. Today I found, with clearer sight, that it takes a head-diameter reduction to around .140" to allow almost straight-in mounting screws, but that even then, there were problems with the tiny cast gussets. It seems that the only way to get really good results is to create washers that can be filed to miss the gussets. This really has surprised me - a classic engine that cannot be mounted without stresses or such a radical prep.