John_M_

That is pretty neat... the buda in the example has a very smooth print to it to begin with, its just not glossy... the acetone vapor shines it up nicely, clever

Cat 1

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Bearings are in and will soon start putting the Boxer back together. What are people using for torquing these tiny fasteners? I thought I was covered only to find my Wera in my Box "at work" only goes down to 5 in/Lbs.. The matching low scale version is a bit pricy so Im looking for options as I spent all my "Fun money"... One of our local modellers passed away in February and his widow call us up to take a look at his "Kits" - He was a great fellow and not really a flyer but a great builder AND APPARENTLY a Kit Hoarder.. About 30 Kits 1960's to 1990's most BNIB. Berkeley/MK/Marutaka/Royal/Topflite/Stafford/Pica. Me and a Cohort turned our pockets inside out and made his wife an offer she was very happy with - And relieved her of the burden of not knowing what to do with the Stash. We will recover our investment with sales and have a few very nice planes for our collections - but its a bunch of work... I now have some very nice future airframe prospects for some gasser conversions though!!!!

Glowgeek

If the rod cap bolts are M2.5 socket head cap screws the torque charts vary from 1.05 n/m to 1.4 n/m. I've been torqueing them to 1.2 nm with no issues. About 10.6 in/lb.

Cat 1

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The 160's are M2... And my numbers were off by bad conversion - I have a 11-29 inch pound driver and the lower one is 2.5 to 11.5 - I should have one but haven't had a need (Until now) - the rule of tools - Need it - Buy it.

Glowgeek

So are the rod cap bolts for OS120 & 160 Gemini. M2 socket head cap screw charts show 0.50 n/m - 0.65 n/m (4.42 in/lb - 5.75 in/lb).

Buy it or borrow it. One of my flying buddies has a Metrology Lab. That's not only handy, I can count on the instruments or tools I borrow to be calibrated.

John_M_

What the ASP FT160?... the rod cap screws are 2 mm?... wow, that is small... whats the thread fit like when you thread the screws into the big end... I would forgo oiling the threads during assembly... I'd clean / decrease the threads, and use blue loctite ... And the fact they are just standard socket head screws concerns me as for their quality... the actual rods themselves are not an issue, its those tiny cap screws.

Glowgeek

As I understand it, you won't achieve an accurate torque value using loctite. ???
I use a bit of 100% castor as a torqueing oil, in hopes that it will gum and semi solidify with heat over time. "Nature's loctite".

Raleighcopter

the downside is larger part tolerances when you do that but the finish is really nice

Glowgeek

Quite a bit of detail lost on Yoda with vapor smoothing too.

500 years off his appearance it took.......mmmmmm?

1967brutus

Yet in reality they hold up just fine...

Glowgeek

The ASP version perhaps, but the two early OS FT120's had rod problems. I can't remember which ones had problems, they made machined aluminum, cast aluminum and cast bronze rods IIRC. None of the failures were rod cap bolt related from what I remember.

Raleighcopter

i thought the problem with the os rods was that they split, not that the bolts sheared

John_M_

They do the job, but with concerns always on ones mind regarding peak rpm,

The rod issues with the early Gemini's was due to the rods being cast bronze then cast aluminum... There is more than enough materiel in the OS ( most like with the ASP as well ) rod big end to go up to a 3 mm, possibly even a 3.5 mm screw, with a low profile socket head, or button head machine screw... the issue would be the screw head to case clearance... I wouldn't even be concerned about the added reciprocating weight.... If I could find the right screw / nut combination, I might even considers a close tolerance pass through captured screw and loc-nut ( again whether clearances allowing is unknown ).

John_M_

It would have no adverse effect on those small torque valves, the loctite would fill the clearances, and prevent the cap screws from losing their torque from the rod end caps shifting side to side... those rods caps have no means of alignment to the big end... and if the caps shift on the fastener hole tolerances, the wear under the head of the screws against the rod cap would have an adverse effect on the torque values... it would be just a matter of time before the cap screws loosened and the rod cap shifted side to side hammering on the cap screws until they fatigued.

Glowgeek

I was going to use blue (242) removable loctite on my first OS twin rebuild but was instructed not to use loctite at all by a well respected model engine fellow. He recommended 100% Castor only, so that's what I have used on 6 twins, OS and Saito, and they are still running today. I'm not saying loctite is a good idea or a bad idea on these tiny bolts, I have no way to know. I have a gut feeling that proper torque is the most important thing.

I have not seen any signs of thread locker during disassembly of twins in the past. When loosened there is a audible cracking sound and the bolts have just spun out of the threads with ease. Same with my recently acquired 200ti.

John_M_

Well go over to loctites website, they claim, no adjustments in torque specifications are required with the use of their thread locker.

Thats friction loc on the threads, and as long as the thread and hole tolerances are a good class fit then there is no issue with that means of tightening a fastener, but like I stated, any movement of the rod cap under dynamic loads, 4 in/lbs is nothing on those cap screws... 2-56, or 2 mm screws are marginal in that particular installation... poor thread tolerances, is the primary issue, and those cap screws are just mild steel.

Glowgeek

There's always a worst case scenario to consider, BUT

7 twins disassembled, no visible thread locker, screws crack when loosened and freely spin out of their threaded bores, no signs of the cap and rod abrading. There would be black colored aluminum oxide present on the surfaces if the cap was shifting around. I think the system works, tiny fasteners or not.

John_M_

Yes I understand what you're saying, and there are always things to consider ... like Bert said, they work and I agree with that, BUT those fasteners are on the small size for that application... we can agree to disagree, and that's fine... if I ever have to go into the OS boxer to that extent, I will be taking a closer look at those rod cap screws... David just had some bad luck with his ASP, possibly just a bad spec cap screw, or he did something incorrectly reassembling his engine... you know the saying "if it ain't broke, don't fix it"... don't dismantle a NIB engine just to have a look inside, Murphy may be looking over your shoulder... the real twist to his dilemma, would that rod still have failed if he hadn't dismantled his engine... those engines are not that fragile, or every single one would have thrown a rod when it hit 9500 rpm.

As another example of possibly over analyzing things... the wing strut attachment clevises on the PA-18 are made of aluminum, 4mm .7 thread... they only thread on maybe 5/16" of an inch, and there's a jam lock nut to secure the clevis in position... there's quite a few of those flying, and I've only seen one fail on a youtube video, and mined you, they don't fly very well without the wing struts attached... the operator either got a bad spec'd clevis end, he over tightened it, or forgot to tighten it... after seeing that video, I took a closer look at mine and felt the threads were a bit sloppy, so I replaced mine with stainless steel ones that thread all the way down the threaded end... So was it overkill for me to take those measures, to prevent something from occurring that may not have ever happened, or at least it hasn't happened to anyone else's that I'm aware of... I'll never have to worry about those clevises ever failing... You take precautions when you see something you don't like.

1967brutus

A statement like that cannot be made, too many "minute variations" that can either all line up towards "good" or "bad", being the difference why one engine fails at 9500 and the other doesn't.
Having said that: I have always been amazed that these rods do not have some sort of alignment, like the big engines. Big engines usually have either serrations on the faces, or locating pins or what not to make sure the cap and rod form a pervect circular bearing, these things seem to have nothing like the sorts.
Having said that, I think such measures are to locate the cap, not to prevent the cap sliding around under load, simply because the sideways forces are not that big.
I have no idea why those bolts are threaded all the way, but I don't think that is an issue, as bolts are not supposed to be loaded sideways anyway, and I don't believe the caps are moving: That would immediately lead to a "ridge" in the bearing at the landing face, which would scrape off the oil film and lead to immediate destruction of the bearing.

Glowgeek

Don't get me wrong John, I'm not disagreeing with you, just sharing imperical data. There's always room for improvement. I am pleased that Saito chose to use M2.5 cap screws instead of M2 on the 200ti.

BTW, Loctite claims the torque values will not be affected with the use of their threadlocking compounds, however in the case of their 242 product they claim it is suitable for use with bolts that are 6mm and larger. Interesting, don't you think?

1967brutus

To the best of my knowledge that has to do with "undoing" of smaller bolts. The bond can be strong enough to either break the bolt or to pull the threads out of the hole, especially with non-steel threaded holes. When I was still mainly involved with RC helicopters, I would, wherever a bolt needed securing in an aluminium part, be extremely frugal with 242 otherwise threads would strip on dismantling, even when the loctite was heated.
I would put a single drop of product on a piece of plastic (Loctite does not dry in open air, so that drop would remain usable for the whole day) and only dip the last single thread in it, carefully wiping the product off the flat tip of the bolt. That would hold the bolt in place without stripping the threads on dismantling.

It is not that 242 won't hold or secure smaller bolts, it is that it holds and secures them too well...

John_M_

Thats why I made that statement, there are many variations that could have contributed to that failure... there are radial loads on the big end, the side to side movement of the end cap depends on the tolerances of the threads in the big end and the hole tolerances around the heads of the screw in the end cap... if they were pined with shouldered fasteners the shear loads would be on the shoulder, not on the treads... there are journal / rod bearing clearances that keep the radial loads central to the big end, that all goes away when the fasteners stretch, when the rpm reaches a point where the inertia loads are beyond what the fasteners can handle... that's the fault of an undersized fastener not the rod material... those bronze rods can handle 10k rpm, its fasteners that can't... they're not even identified as to their grade because they are just common machine cap screws, most likely rolled threads, but nobody knows the quality... it wouldn't hurt to investigate it further if one had the engine apart... that's the only point I'm making.

John_M_

well I'm in deep. if I ever have to remove all the screws from my 1/10 rc buggy chassis, which I haven't run for years... and that came with a small tube of blue thread locker... blue isn't going to hurt anything... if your that concerned use the 222 purple low strength... or just stay with your castor oil regiment, its to your discretion, nobodies forcing your hand.

1967brutus

Not really... the shear forces will ALWAYS be on the threaded part. The shouldered part is resting flat against the inside of the bore, but the shearforces act on the exact spot where the bolt leaves the rod and enters the cap, and that exact location is by definition still threaded.
I know what you mean with the radial forces on the rods, but they should be relatively minor, since the largest lateral acceleration of the bigend is slightly after 90 deg before TDC and slightly before 90 deg after TDC, and that still is "rod territory" when it comes to the forces of the crankpin in the bigend bearing.
The inertial stretching forces IMHO are the only real problems: If those overstress the bolts, the cap will come loose from the rod, but it won't slip under the bolt heads, the cap will start to slide around on its mating face with the rod. These forces can be calculated fairly easily.
In all fairlness, if I take a look at what M2 bolts can hold in a helicopter rotorhead, I am not too worried.

But I do have always wondered why there is no torqueing instructions on these caps from OS or ASP.
To my amazement, there also are no instructions on that included with that V8 I am on the verge of assembling...

Cat 1

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What cheapskates do when they need a torque wrench for a single use... Calibrated before and after use and very repeatable...