size of fasteners to mount OS .25

[Wayne Collier]

I've never used an OS .25, but have one and am getting ready to give it a try.  What size bolts/screws do you guys use to mount them?  A #4 machine screw looks very wimpy.  A #6 won't fit through the mounting holes.  I generally like for the screw to completely pass through the mount and use a nut to retain it.  Do I go with the #4 screws/bolts or is there a better size?  I'm ok with metric sizes if needed.

[Dwayne]

4-40 socket head bolts and lock nuts with #4 washers.

[Hoss Cain]

I've never used an OS .25, but have one and am getting ready to give it a try.  What size bolts/screws do you guys use to mount them?  A #4 machine screw looks very wimpy.  A #6 won't fit through the mounting holes.  I generally like for the screw to completely pass through the mount and use a nut to retain it.  Do I go with the #4 screws/bolts or is there a better size?  I'm ok with metric sizes if needed.

What are you mounting the engine on?  Typical wood profile, 4-40 will work fine. I prefer "T" blind mounting nuts. I also prefer socket head bolts. A wood mount will continue to compress over time. Socket head and Blind Nuts will be easier to get to, to tighten up prior to a flying session. On the resin mounts, you can tap into them and use sheet metal screws, #6 or larger, however it is better to tap for the bolt and back up with a lock-nut. I am not afraid to drill out an engine's mounting holes to assure a well mounted engine.

4-40 will adequately handle up to a .35. From the .35 to a .75, I use 6-32, and above that 8-24 or 8-32.  Once I hit the 26cc, I go for a #10 bolt preferably black, but in actual, whatever is handy. I always use T-nuts when the nut is not handy on the flying field. Nuts and bolts will always loosen for the darndest reason at the darndest time.    More so on wood and wood firewalls as the wood compresses, even when prior epoxy soaking is accomplished.

[Bill Little]

I know that Randy Smith (and many others) use 4-40 socket head bolts up to and including the PA .61.   A 4-40 is plenty for an OS 25FP.  A 4-40 is not going to break on these size engines. 

I am talking about socket head machine screws.  I get them in bulk from micro fasteners.

Big Bear

[Richard Entwhistle 823412]

Why do we think we know more than the engineers who built the motor?  If it needed a "10" sized fastener why wouldn't the mounting hole be sized for it?  If you drill out the mounting hole and use a larger fastener and tighten to the point were the wooden mount is going into a slight "crush", since the larger fastener has not stretched as much as a 4-40 as soon as the wood crushes a little more from engine vibtation all your clamping force is lost and the motor really get loose. The problem is not with the fastener but what the motor is clamped on.  Bigger is not always better.  I turned nut and bolts for forty years at my auto repair shop and although I didn't design fasteners I do have a "feel" for size.  It often crossed my mind that VW air cooled motors had much too big of a head stud (10 mm) that didn't stretch much at all for the 23lbs head torque.  The heads came loose after 40,000 to 60,000 miles in Fairbanks Alaska like clock work.  After VW switched to a smaller (8 mm) stud that was torqued to 18lbs the loose head problem went away for the most part.  A 36% reduction in cross section in area with a 22% reduction in stud torque resulted in heads that didn't work loose.
 
If the OS engineers are smart enough to keep the head on their motors with four 3mm head bolts I think they are smart enough to size the mounting holes correctly.

Later
Richard

[Garf]

The trick is to use high quality screws. I started with "perfect" brand screws. The would regularly stretch and break. Then I found stainless steel screws for my combat engines. Today I use socket head cap screws for up to my Super Tigre G21 46. No problems ever.

[Brett Buck]

I've never used an OS .25, but have one and am getting ready to give it a try.  What size bolts/screws do you guys use to mount them?  A #4 machine screw looks very wimpy.  A #6 won't fit through the mounting holes.  I generally like for the screw to completely pass through the mount and use a nut to retain it.  Do I go with the #4 screws/bolts or is there a better size? 

   4-40's are fine for this. That's what I use on 61's. I only use hardened socket cap screws anymore, but I wouldn't worry about plain old steel screws. The socket cap screws are a lot easier to screw in from an angle because you can use a ball driver.

      Brett

[Bill Heher]

I just mounted an OS .25 on a repaired Ringmaster front end that has a C/L Central U -plate to reinforce the battered and broken mount beams. 4-40 socket head cap screws with lock washers for security and flat washers to spread the clamping loads on both sides. Solid as a rock, and easy to check / re-torque with a ball end driver like Brett said.

I also like to bush the bolt holes in the mount beams with pieces of brass tubing that are just a hair shorter than the total length of the mount beams and ply doublers. This help prevent the "crushing" that comes from tightening the hardware repeatedly.  I suppose carbon fibre tube of the appropriate size could also be used.

Accurate marking and drilling of the mounting holes so that everything lines up without having to hog out the holes also pays big dividends in security and makes life easier when removing and installing the engine. On profiles you can use a drill press and get accurate / aligned holes before you assemble anything or start finishing.

[Wayne Collier]

Thanks for the encouragement guys.  I'll go with the number four screws/bolts.  I think I will look for stainless or socket head.  If all you guys are using 4-40 with no problem, they must be fine. 

[Dwayne]

Thanks for the encouragement guys.  I'll go with the number four screws/bolts.  I think I will look for stainless or socket head.  If all you guys are using 4-40 with no problem, they must be fine.  

Wayne
4-40x1 1/4 inch "bolts" and "#4" washers.

[john e. holliday]

You can get top quality machine screws from RTL Fasteners.  What you pay for a set of four, you can get bulk quantities.