Which type of engine mount bolts are the strongest?
 

Occasionally my 6-32 socket head engine mount bolts will break a head off of one. I'm running a Saito 82 on a profile plane. The prop is balanced well but the profile fuselage planes do tend to vibrate. I have been using black alloy steel bolts from RTL fasteners. My question is what type of 6-32 bolts would be stronger? Stainless steel? I don't see any other types at RTL. I don't really want to drill out the engine mounting lugs for #8 bolts, would rather use them on the larger Saito. Thanks!

Stainless is softer. Im not 100% sure about its sheer strength qualities. I suppose this could potentially be beneficial if the vibration is breaking the brittle bolts your using now but I would think either should be plenty adequate.......in all cases you have to be careful not to over torque anything in these small sizes. The threads are strong enough to stay together while stretching the shank and this could cause your weak link.

yes making them to tight will cause them to break. You have some issues if breaking 6-32. i have used 4-40 on Saito 125s

The softer steel bolts/screws including stainless stretch. Some stretch more than others and this causes stress hardening which leads to making them more brittle. I use cheapie Dubro 6-32 cap screws most of the time and have only had one head break off due to.

Bolts come in different strengths. The bolts sold for hobby use are probably the weakest, cheapest grade. You can get high strength bolts from an industrial supplier like McMaster Carr. They will be considerably more expensive. High strength bolts are available in both steel and stainless steel. Steel is less expensive.

Check out your closest fastner type store........I would stay away from SS just too soft. There are many grades of bolts and those that deal in them will be your best source.

Don't think I ever broke a mount bolt in all my years? How tight are you pulling them down, over tightning can cause undue stress and failure?

Thanks for the good replies guys. It is possible I might be over tightening them. All I'm using to tighten them though is a nut driver with a screwdriver like handle. Still, #6 is not that large and I could be over tightening them. Hey, an idea though - when I get home and am cleaning the plane up I usually check the engine mount bolts for tightness and find they usually need about a 1/16th turn or so after flying. Maybe I should just leave them alone. Another thought is that I am not putting a washer under the head of the socket head bolt where it goes into the Saito mounting lug. A washer might soften the vibes on the socket head. There might not be room for a washer though, will have to take a peek.

The washer is not a bad idea, I suggest it also be under a ny-lock nut on the back side. These nuts are designed not to move unless you use a wrench. Just hold your allen head stationary while tightning and you will not gall the engine mount lug under the bolt.

Whether a nyloc or a taper lock I would definitely use some form of lock nut. Otherwise your gonna be subject to over tightening and they are still goin to vibrate loose and/or break. The self lockers allow you to not have to "stand on them" when you tighten, a little passed snug and they'll stay forever.

I try to use the black mounts from Great Planes. After countersinking a small hole, I use the cheap 6/32" socket head screws that the hobby shop carries. They will never vibrate out, and it's a light and easy assembly. I also use a small washer under the screws, but I never really pay attention to grade, etc. I've had a couple of crashes and the mount always came free first, usually taking the firewall with it. The screws never broke and one crash was right on the spinner doing 70 or 80 mph. They have bent some, but have never broke.

Honestly, I wouldn't worry about this part of the mount too much.

I don't like the cheap blind nuts. I do check the threading on those and the bolts I use to get a good fit. I also swab some epoxy into the threads of the blind nut when I'm using thinned epoxy to fuel proof the area. I've not had one vibrate loose since doing it this way. I have had problems here in the past, but not with engine mounting screws.

My first thoughts was the screws are being overtightened causing the heads to stretch and break off after a while. The next thing to cause it is excessive vibration that can stress the screws and cause them to break. Now the OP stated the engine was in a profile plane, so right off the bat we have excessive vibration then. But with a Saito 82 I would think that a 8/32 screw would fit. But then if the mounting holes are a little too tight for 8/32 then there is a metric size a little larger than 6/32 that would work too. Also using a screw with a shoulder on it (the unthreaded portion next to the head), as the shoulder would strengthen that head end of the screw more and maybe that would help solve the problem. One can always cut the excess screw length off as needed too.

Lots of good ideas here guys, thanks a bunch! Yes, I'm using nylon lock nuts on the back side with washers. Just no washers (yet) between the socket heads and engine lugs. An 8-32 will not go through the Saito 82 lugs without drilling them larger. Just didn't seem like I should have to do that. I'll try not snugging the 6-32's down all the time after flying and we'll see how that does...... will keep ya posted.....:rolleyes: PS - I agree with BLW about the black nylon mounts working out nicely when not on a profile plane. I have a lot of fun drilling the holes a bit undersized then putting a 6-32 (or whatever size) tap in the chuck on my little electric screwdriver. The screws never even loosen up a bit in that nylon. Anyone else have a hex drive chuck that pops into your little battery powered screwdriver? Very handy. I even put the Great Planes Dead Center Hole locater in that chuck instead of spinning it by hand. Works like magic. http://www3.towerhobbies.com/cgi-bin...&I=LXK378&P=ML Earlwb, my 6-32 bolts do have a shoulder on them - good idea.

The snugging down the screws after flying could be the cause of it, as that is overtightening the screws. You need to do the same thing they do when re-torque'ing the bolts on a engine. You loosen the bolts and then re-torque them to spec. Trying to snug them down to check tighteness just results in them getting too tight. Also if you are using a electric screwdriver, it can over tighten the screws too. Using washers under the heads of the screws is needed as the engine lugs are aluminum which is softer than steel and the washer helps spread the stress of the screw out over the lug itself. The engine vibration can easily wear the aluminum away from the screw head making it appear to have become more loose.

I missed the part about the model being a profile.

Another thing is to make sure the bolts are straight - if the holes are drilled at an angle and you crank the head up against the mounting lug, you are placing tremendous bending forces on the junction between the head and the bolt shank and could induce a shear failure under the heavy vibration. I'd be more worried when using a tapped metal mount though. On a profile I would be tempted to fit some aluminum plates under the engine lugs. What do the lugs sit on, plywood? That's a big shaky engine to rest itself on two littl metal lugs on wood.

A single cheap grade #2 6-32 bolt has a maximum clamping strength of about 375 pounds. For grade 8 up around 800lb.

Stay away from SS. It has no value as a motor mounting fastener except for corrosion resistance, bling, or for a weak link. Jett uses SS bolts on their mufflers so they shear off in a crash.

Great idea about how to re-torque the mounting bolts. I will do that. No, don't use the electric screwdriver for tightening them, just for the initial installation. I will try washers under the heads also. Only problem there is that sometimes you have to grind one side off of the washer so it does not hit the crankcase..... but easy enough to do.

All good points, thanks a bunch. The engine does not sit directly onto the hardwood but rather, onto 2 degree nylon right thrust plates that go between the engine lugs and the hardwood. So the holes are drilled perpendicular to the 2 degree plates. These plates could be a source of vibration but that is the common way of mounting to a profile fuselage. The bolt heads sit correctly onto the engine this way, but on the back side of the profile plane the lock nuts do sit at an angle. The washers there seem to dig into the wood just fine. It is the head of the bolts that are occasionally breaking, not the lock nut side anyway. Hopefully I'll be okay by not over tightening them.