Hi ,

What is the best bolt size for mounting a modern .60 size engine (PA , or Ro-Jett) ?

4-40 , or 6-32 ?

Thanks ,

Rob K.

Hi ,

What is the best bolt size for mounting a modern .60 size engine (PA , or Ro-Jett) ?

4-40 , or 6-32 ?

Thanks ,

Rob K.

Hi Rob

Most people use 4-40 or 6-32, I use 4-40 on all my ships and have used them with motors up to 88 size.
One grade 8 4-40 will secure 100 pounds and I have never had a good alloy  4-40 break,
The biggest prolem I have seen is the blind nuts...NEVER use the cheap thin ones, I use the DuBro T Nuts, you can  see how much thicker they are at the flange.
B roken  T nuts is the only problem I have ever had with mounting motors with 4-40s

Always make sure your holes are straight and indexed correctly, use good mounting wood and use some type of motor pad, wether that is steel  alum.  or linen phenolic.

Regards
Randy

It is correct to use the smallest bolt that will do the job, Usually 4-40. Larger or really high rockwell grade bolts can guarantee you a broken case in the event of a crash if hit on hard surfaces. Of course the rest of the engine might be reduced to junk also.I use Great Plane bolts not because they are good but because they will shear or give way in the event of a mishap. The only cavet is exactly like Randy said never use cheap T bolts or blind nuts as your just asking for misery when the threads or the prongs give way.
In my R/C days I usually held 60 sized wings on my models with 8/32 nylon bolts when the standard was 1/4/ 20 Simply because in the event of a serious mishap they would sheer off and cause no damage to the structures.
Dennis

In the 4/40 size, the Allen head capscrews are not so much of a weak point.  It's the blind nust or threaded plates that cause the failures.  In a profile setup with 4/40 bolts running through to Nylocks, it's just fine.  But in a high value plane where you need to dig out the bad nut, a 3mm is a better bet.

Completely agree with Randy, 4-40 cap head screws are plenty strong for anything we can hold onto and you can buy Dubro blind nuts in an economy pack. The thin blind nuts I bought from Small Parts are only used for accessories and hatches.

    I still use 4-40s and clearly they are adequate as far as strength goes. I have also used a lot of the stamped - Carl Goldberg and other brands -  blind nuts with no issues.  Although after Curt Contrata mentioned his issue I switched to DuBro, and might go back to nut plates if I ever build another airplane.

     But we did find a good reason to use 6-32s - they are much easier to extract with a screw extractor if they break off.

      Brett

I use 4-40's up to 40 size engine.  Over 40 engines I go to 6-32's.  I agree that a good quality 4-40 bolt is strong enough, but the 6-32 blind nuts have a larger diameter flange which minimizes the crushing of the wood mount.  

The thin ones will gradually sink into the bearer, the flange stretches and bends.

I'm considering going to a 3mm bolt size.  I find the 4-40's are too small for the bore in the engine mounting flange.  If there is slop there, it is hard to get an accurate engine thrust setting.  I build mostly profiles and have had some unpleasant experience with some unintended up/down thrust issues.

I figure most engines are drilled for metric so why not use metric mounting hardware?

I've been using 6-40 screws to mount my .65

Allen,

I got the screws from either Micro Fasteners or Small Parts, don't remember which at the moment. I bought a box of a hundred for something like 10 bucks. I also ordered blind nuts to go with them, but truth is, I don't use them much. All my engines are mounted in  aluminium blocks so I had to pick up a 6-40 tap, but that's it. I chose 6-40 over 6-32 because the thread is obviously finer and it put more threads in the tapped hole. Makes it easier to snug up the bolt.