Oversized Mounting Holes?

Can you think of one good reason why almost every used RC Tower 40 I buy off Ebay has the mounting holes drilled oversized? Is there some unwritten law I don't know about? One would think that the guys that designed the engine would know the proper sized hole.

Richard

The artf generation has neither the skill, the experience nor the tools to cope with the chore of drilling accurate mounting holes so, often resorts to opening out the engine as a quick solution to a badly drilled engine mount. I have seen it often.
So many of the skills that those of us who have built models all our lives have learnt are now disappearing

This discussion reminds me of what is seen on some of the treasured old ST .46s and ST .60s, where the case has been mangled with crude egged out or even notched mounting holes. In spite of it, the engines are prized for classic CL stunt use and remain in service, often through multiple "reworks", some of which are also crude hacks.

It's not just the RC or ARF dolts doing this sort of thing. We need to take away their hacksaws, files, rasps, and hand drills.

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"Integrity is what we do, what we say, and what we say we do." -Don Galer

Ron B.
F4Fguy

From my cold dead hands!

Ron B.

Most of the older American engines had standard hole spacing that made them pretty much interchangable.

Some of the "new wave" foreign RC engines have a spacing that is slightly off. So if you have a Fox airplane and decide "upgrade" to an OS or Thunder Tigre, maybe enlarging the holes a bit is your best option.

While I would never do this myself, a couple of auction engines came to me that way, and it sure helped bolting them on my planes.

I like to drill the mounts for the next size larger bolt.
Any 6-32 is better than the best 4-40. Never mind price and availability. If the oversizing has been done well, you are in good shape.

>I like to drill the mounts for the next size larger bolt.
>Any 6-32 is better than the best 4-40. Never mind price and
>availability. If the oversizing has been done well, you are
>in good shape.

I'm seriously considering going to all metric hardware. The stock engine mounting holes in a typical .46 engine are too large for a 4-40 bolt. I find the 6-32 to be perhaps a bit too large but a 4mm bolt fits very snuggly.

My local hardware store carries a bountiful supply of metric fasteners - and for a lot cheaper than my LHS.

Hang on there Steve! A #6-32 screw is 0.138" dia and a 4mm is 0.157" dia. An M3.5 is only a poofteenth smaller than a #6 dia.

HH

>Hang on there Steve! A #6-32 screw is 0.138" dia and a
>4mm is 0.157" dia. An M3.5 is only a poofteenth smaller
>than a #6 dia.
>
>HH

Whoa... It appears I picked up an 8-32 fastener. By my Harbor Freight calipers, a 4-40 shows 0.108", a 6-32 is 0.136", a 4mm (actually measured at 3.8mm) is 0.150" and the 8-32 is 0.161"

Thought it would be a simple thing like that Steve. You can see that there is that bit of difference when you measure the OD of a screw between the nominal size and the ACTUAL size.

HH

Well I say why care? I don't think there is anything "collectable" about the Tower 40 engine.

It would be me guess that they are using sheet metal screws in the nylon mounts that come with the ARF R/C trainers.
And why would someone use a mounting bolt, i.e. a 6/32, that WILL break the engine in the event of a mishap (crash)? Arn't the bolts much cheaper than a crankcase???????????? Duh, that's a no brainer!
Jerry

Gerald
I have had more problems with the 4-40 bolts. The 4=40's have worked loose or broken more than once for me. We went to the 6-32 for Mark 3 Combat specials and K/B 5.8's and the problems went away.
The Bolts are the cheap part of this equation, but very important.
A loose engine runs poorly and can be catastophic to both the plane and people.

I would have never thought of screwing an engine to nylon mounts with sheet metal screws.

Ron B.
F4Fguy

You obviously don't fly RC nor hang out with those who do.

Ron B.

>Gerald
>I have had more problems with the 4-40 bolts. The 4=40's have
>worked loose or broken more than once for me. We went to the
>6-32 for Mark 3 Combat specials and K/B 5.8's and the problems
>went away.
>The Bolts are the cheap part of this equation, but very
>important.
>A loose engine runs poorly and can be catastophic to both the
>plane and people.
>
>I would have never thought of screwing an engine to nylon
>mounts with sheet metal screws.

I have fought this same exact problem on multiple occasions and always relegated it to egged out holes and 4-40's. NOT SO.

I finally this year found that my poor workmanship in setting the mounting plates (alu) for my speed limit combat planes was the culprit.

Now I take the time to use calipers and accurate angle material to get the mounting block SQUARE and exactly the right spacing. All the sudden I get perfect motor runs and no more broken 4-40's.

This might not be your problem but I was grabbing 6-32's in an effort to bandaid my construction problems. YMMV

Lane

I haven't had any problems with mounts in a long time, but I can't go back.(to 4-40)

You are correct that our skills and tools can be improved. I pot my engine in epoxy on oak beams, them the final assembly gets the engine glued onto the mold. Changing engine is a real bear, but the plane is mostly smashed by then.

Now that I have a couple of belch-fire 90's I will probably go to 1/4-20's.

I agree with the suggestion that most of these engines were frequently the first engines their owners ever saw, being purchased as part of an 'ARF' package. And subjected to mishandling by their unskilled/indifferent/inexperienced owners.

One thing I leaned in A&P sheetmetal class is that a properly designed riveted joint is designed to fail before the parts that are riveted fail. Extrapolating a bit, I'd prefer to fail a mounting bolt before I'd fail a mounting lug.

And drilled-out mounting lugs might be drilled/bushed/drilled back to size on a good day.
Peter Havriluk

I think the first answer (Mike Nelson) pretty much nailed it.

Mike Keville

>I think the first answer (Mike Nelson) pretty much nailed
>it.
>
>Mike Keville



What he said! ..... I say Ditto!...................

"Well I say why care? I don't think there is anything "collectable" about the Tower 40 engine."

Well, I say a Tower 40 (a clone of an OS 40fp) built like the many I have copied from a Mr. Neumann example, is more than a match for any stock OS 40fp. Super low end power, never a run away and the most linear needle valve adjustment I've come across. Most of the oversized holes are .142 to .144 which is 9/64 plus a few thousandths. That kind of proves that even with 6-32 bolts the motors are still moving around. I do make bushings and bring the holes back to stock size.

Richard

I would rather expect a jobber length drill (of unknown quality, but probably cheap!), powered by a hand drill or even a drill press, would make a hole a bit oversize. Even stubby screw machine drills can drill an oversized hole, if they're not sharpened exactly right. Don't forget that lubricant makes a difference...dry will definitely drill bigger, but different lubes will consistently make different sized holes.

Regarding the 4-40 vs. 6-32 machine screws, I'd bet that they were used with #6 sheet metal screws. I'm not sure if they even make #4 SMS's, but I'm pretty sure I've never seen them, even at McLendon's.
Steve

Steve Helmick
Renton, Washington
USA

Have to agree with Mike and Steve on the butchery that many practice from both R/C and our control line modellers. There is nothing new about the R/Cers using the self tapper type screws in glass filled nylon or even just nylon mounts which were available in many balsa kits long before ARF was thought about. Some engines came with larger holes which allowed the use of 6-32 or even 5/32" (just that bit smaller than 8-32) with the slightest skim out of them.

Back to our beloved stunt, 4-40 is adequate for most applications up to a 60 but being a later "British colony" we are lucky to still have 1/8" BSW (5-40 diameter and pitch for the Americans) easily available even though we went metric back in the 70s. I know it's only 0.013" bigger but they do fill that hole that bit neater and are of course just that bit stronger in the core diameter. Any hardware store here seems to have these old imperial sizes in mild steel and engineers suppliers have the socket head 1/8" x 1" long high tensile ones.

I know some suppliers in America still have 5-40 socket heads up to about 1 1/2" long so if you need that bit more strength, why not try and source some of them.

HH

Mike Nelson and uncle mikey have it right, ham-handed guys who really don't know any better. Maybe if they stick with it long enough they'll realize the error of their ways.

I am fortunate to have a lathe. K&S metals has a nice 1/4 OD alum tube that is just great for making flanged bushings for oversize mounting holes.

www.microfasteners.com is a mecca for any bolt (5-40s included), nut, washer, or self-tapping screw junky. And they do have many sizes of those sheet metal screws even some with allen recess heads, so much better than those Phillips/JIS thingys! But, I've never used sheet metal screws for engine mounting anything other than .049s on plywood and plastic firewalls.

don Burke
Menifee, CA
Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways - body thoroughly used up, totally worn out and screaming, WOO HOO, What a Ride!

Henry Nelson sold the combat 36 with "oversize" holes so it could be bolted into the same mount as a Fox. The Fox had .125 bolt holes, the Nelson .140 The extra .030 was just enough clearance to allow for the different hole spacing. I wouldn't get too anal about engine mounting holes. They don't affect how the motor runs one whit. Really hogged out holes don't look so good when you come to sell the motor, but a good washer under the bolts covers a multitude of sins.

>Can you think of one good reason why almost every used RC
>Tower 40 I buy off Ebay has the mounting holes drilled
>oversized? It there some unwritten law I don't know about?
>One would think that the guys that designed the engine would
>know the proper sized hole.
>
>Richard

Are they really drilled that way on purpose or perhaps a wandering electric drill while using the engine as a "locater" for an aluminum mount?

George

Some of the Chinese motors eg ASP 46 already have the holes drilled to 4.25mm. Saves the butcher boys doing a bad job with the wobbly drill or even worse, a round file [photo not recovered: sipcof.gif]

HH

Having seen enough of the rather dumb things that C/L flyers do in building a rather simple model I'm always suprised at the alleged superior attitude exhibited by Joe Bellcrank. Seriously, if you think that flying around in a circle requires superior skills then perhaps you would show me how well you fly a jet at over 200 MPH, or fly the R/C precision turnaround pattern, or fly , much less set up a helicopter to even so much as hover. or even posess the skills and attitude needed to even build one of these creations much less fly it safely
If you think that these aspects of modelling tolerate sloppy habits then your sadly mistaken. Granted most of the rather crude things you see are done by the unskilled ,please don't assume that everyone besides yourself is slightly left of a Chimpanze in their developement or machine skills.
I have flown all aspects of the hobby and I have met more then my share of both skilled and sloppy modellers. Lets stop this innate jabbing at the other aspects of the hobby it's demeaning and shows a lack of maturity.
Dennis

It isn't just the ARF guys drilling oversized holes. You guys know I am a Fox collector and flier. I have seen myriads of engines produced long before ARFs with butchered mounting holes. Some guys just cannot produce a mounting pattern on the plane to match the engine.

Having seen enough of the rather dumb things that C/L flyers do in building a rather simple model I'm always suprised at the alleged superior attitude exhibited by Joe Bellcrank.
Seriously, if you think that flying around in a circle requires superior skills then perhaps you would show me how well you fly a jet at over 200 MPH, or fly the R/C precision turnaround pattern, or fly , much less set up a helicopter to even so much as hover. or even posess the skills and attitude needed to even build one of these creations much less fly it safely
If you think that these aspects of modelling tolerate sloppy habits then your sadly mistaken. Granted most of the rather crude things you see are done by the unskilled ,please don't assume that everyone besides yourself is slightly left of a Chimpanze in their developement or machine skills.
I have flown all aspects of the hobby and I have met more then my share of both skilled and sloppy modellers. Lets stop this innate jabbing at the other aspects of the hobby it's demeaning and shows a lack of maturity.
Dennis

Dennis,

What you say is true, but most who are at the top of their game in any discipline would PROBABLY not still be using a Tower .40 RC.

What flyers of ALL of these disciplines have in common are that they were once newbies. Many newbies do not have the skills, knowledge nor tools to accomplish some tasks. I believe a lot of newbies buy used equipment to start. That may include a plane (sometimes ARF) and engine from different sources. Perhaps the former owner of the plane used a different engine, etc.

I don't believe the intention was to put down RC fliers as much as understand why many drill the holes bigger.

IMHO, only someone who has flown only one type of model would put down another type. Any form of modeling can be as simple or as complex as we'd like to delve into it.

And, of course, this is also just one opinion.

George

I agree with the sentiment about not stereotypically putting down members of another discipline.

However, I have re-read the posts on this thread and have not found that prejudice against RC modelers. There were remarks made about approaches to modeling, and the original engines were R/C engines, but remarks did not seem to me to single out and stereotype RC modelers or flyers. 'my perspective.

SK

I'm with those that don't see it as a problem or issue, I always use a flat washer and lock washer on the screw head so doesn't matter what size the holes are unless they are larger than the OD of the flat washer. Also 4-40's are plenty large enough for anything we might bolt into a stunt ship, 6-32's just add unneeded weight and are totally unnecessary.

So OK you have spent 3 or 4 weeks finishing your stunt ship and in the trimming process you discover you need a little more (or less) engine offset. You used blind nuts and the only way to change the mounting holes is to cut into the top block. What do you do? I know what I would do without hesitation and have done it without a thread of guilt.

I have to line up with Mr Reeves here. This is a lot of do-do over nothing. I suppose it's a matter of degrees and workmanship but the typical .125 (1/8) holes found in a lot of mounting lugs are simply too small. A 4-40 screw has a major diameter of .118 and a typical industry standard clearance hole for that is .030 over that or .148.
Yes I know that's not a standard drill size found in most folks tool kits but even .156 is really not too big.
The tolerance of location on mating holes is determined by the size of the clearance hole and when the mating part (engine mounts) has tapped holes (blind nuts) that tolerance must be divided between the two mating parts. In this case only .015 diametrical tolerance is allowed on each. (With .030 oversize holes). Drilling holes in engine mounts is not an exacting science for most of us and .015 (+/-.0075) isn't that easy to do. With .125 holes that becomes +/- .0035 which isn't easy for anyone to do even with NC equipment. Hence most folks enlarge the holes a little. Good practice demands using flat washers and lock washers anyway.

Randy C [photo not recovered: sipcof.gif] [photo not recovered: sipcof.gif] [photo not recovered: sipcof.gif]

I have to agree with Steve Scott back in reply #29. My dial and digital calipers both show a 4-40 diameter as .108" which will have plenty of clearance in a 1/8" (.125") hole. I just finished building a new tank for Snaggletooth. While I was at it, I made a small steel brake for bending tank tinstock. I wanted the two screw holes to be tight for alignment of the brake. A 4-40 screw was measured at .108" and the brake drilled .110" for a very nice fit.

[photo not recovered: 29574.jpg]

Like others here, I have been modeling actively for over forty years and the thought of drilling mounting lugs is abhorrent. I've never found the need to do it.

I do agree that 4-40s are plenty strong and use them to mount all of my two stroke engines including the Ro-Jett .76s in the Snaggletooth and Millennium Cavalier and the .46 in the Mustunt IV. I've never broken a socket head screw of any diameter or had them loosen when used with a thread lock liquid material. I do use 6-32s for my Satio .72 and .91 four stroke engines as their lug holes accommodate them without modification. Strengthwise, I'm confident that 4-40s would suffice.

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I also agree that accurate mounting of our engines does require fairly exacting mounting holes and fasteners. All of my airplanes, since I found out about laser cutting, have been built with a plywood plate in the engine compartment which is predrilled (laser cut) with 1/8" engine mounting holes for the specific engine intended for that particular application. My engine mounts begin with the plywood plate and maple engine bearers are simply glued to the back of the plywood plate carefully aligned with the edges of the plywood. the laser cut 1/8" holes are used as a drill guide for drilling through the maple mounts. after drilling, the assembly is inverted and back drilled from the bottom of the maple mounts and blind mounting hardware installed. With this done the mount can be turned upright and the engine bolted into place with no measuring ever have been involved in the precise fitting. This is a quick and easy way to build accurate engine mounts into my airplanes.

Al

Attachment #1, (jpg file)
Attachment #2, (jpg file)

Al,
Yes!
In my previous statement about 4-40s I said that the nominal diameter was .118....just faulty memory. The actual nominal diameter is listed as .112. That is a Max dimension 4-40UNC machine screws.
However the little analysis that I showed for locational tolerance still holds but with different numbers. The point being that to insure being able to mount an engine or different engines of the same type, tolerances of location must be considered. The accepted method for gaining some relief on that locational tolerance is to use oversize clearance holes. .030 oversize is a standard for #4 size screws.

Randy C

Randy,

I don't understand why there would be a nominal dimension listed for a 4-40 at .113". All of my 4-40s will slide through .110" holes drilled with a #35 bit. I figure that the .017" between the measured diameter of my 4-40s give me enough operating clearance for normal machining tolerances. Using my laser cut engine mounting plate for a drill guide, my engines typically mount easily, without adjustments, to the mount or lugs, without opening the mounting lug hole diameter. As for abrasion of the mounting screws, I have two thoughts. First, what abrasion? the whole idea is that there be no movement between the lugs and mounting screws once the engine is properly tightened. Second, If abrasion does occur, considering the difference in hardness of aluminum case lugs and hardened steel socket head screws, I think there would likely no no detectable wear on the screws. I'm no engineer and usually defer to learned brethren, but in this case, I'll muddle through based on red neck logic and experience.

I have a farther argument against butchering the engine mounting lugs. Our engines are subject to very high loads in our operating environment. There is unavoidable vibration from all single cylinder engines which is impossible to eliminate with any degree of balancing. The engines are also subject to the twisting loads of torque and gyroscopic precession from our abrupt maneuvers which are greater than some would suspect. The more incidental contact we have between our mounting screws and the engine lugs the more stable the engine will be in its ability to resist loads and vibration which would tend to loosen the mounting screws.

It is one thing to perhaps use a file to slightly add clearance to a difficult mounting screw. It is an altogether thing to automatically open the mounting lug holes to get clearance that may not be necessary.

Al

Al,
Well there are a couple of things to say and several different ways to look at this problem (as there are with most stress situations).
Marks Handbook for Engineers and the Machinery Handbook both list the Maximum major diameter for 4-40 screws as .1120, and the Tolerance on the major diameter for class 2A and 3A (Socket head cap screws are usually class 3A)as .0051. This means the Minimum Major diameter would be .1069.
Now to the question of whether or not we want the screws to contact the engine mounting lugs or not. While it's probably not possible to achieve from a practical point of view it would be best for the strongest mounting situation and least fatigue on the screws if they did not contact the engine lugs on the inside of the holes. As you pointed out there is a significant amount of vibration inherent in our single cylinder model engines. Screws in contact with the engine mounting holes will be subject to a lot of abrasion in the are of this contact. This abrasion can eventually weaken the screw and result in breakage. Yes, I've seen this happen. Probably the ideal mounting method (although a lot of trouble) would be to line the mounting holes with nylon (or some other tough elastomer) and let the sctews contact this bushing. The second best would be to not allow the screws to contact the engine lugs except on top and have the retaining force to hold the engine in be a result of the clamping force from the torquing of the screws only.
My humble engineering opinion!
Locational tolerances on patterns of screw holes is a well known engineering fact. If one expects to change from one engine to another (of the same type) it is likely that holes larger than the screw Major diameter will be necessary to compensate for maching tolerances between the two engines. For most folks it's not possible to hold "match drill" tolerances when drilling holes in engine mounts. Larger mounting holes in the engine lugs are the answer (within reason of course), to not winding up with a half a hole showing through the engine mounting hole when the engine is put in place.
Finally....Full scale Space craft and Aircraft are built using these principles do our models need to be different.

Randy C

Me?, I leave the micrometer in the toolbox, just bolt the engine on and go fly If the washer covers the
hole in the mounting lug, you will be the only person that knows the holes are wallowed.

Nice to see some common sense Bob.

Even back in high school, I had figured out that to get the hole pattern correct, you just need to locate the first hole, mount the engine and get it snugged down then mark and/or if possible, drill the other three holes.

I have a couple reasons for drilling out the holes on mounting lugs. A, I have had numerous cases of landing planes and finding the heads of softer 4-40 bolts missing, especially for engines over .29, so I re-drill for 6-32s, and B, either some re-drilling to enlarge or just using a cutter bit on a Dremel to elongate holes when changing one engine with another engine of a diferent brand with a different hole spacing.

I once had an (I Apologize for this, but I fly anything, including hand launch glider)RC plane with a Fox BB .15 lose the engine in flight. Plane came down with 4 headless 4-40s sticking out of the aluminum mount. Somehow I managed to bring the plane down in one piece, then find the engine in a weed patch after only about an hour of searching.I still use 4-40s on my baffle 15s, but drill for 6-32 on the Schnuerle .15s and .25s.

In fact, I hd to do the same with my old O.K. Cub .19s, whether I used the beam or radial mount method.