Unusual prop shaft size

[Frank Clyma]

I have an MRC .15 glow engine.  The propshaft is larger than I usually see on this size engine, it appears to be either 6mm or #12, with a fine thread.  I found 6.1 and 12-24 samples, the diameters are about right but the thread is not. Does anyone happen to know the spec on this propshaft?

[Crist Rigotti]

Probably 6x1mm.  Might get a better response if this were posted in the engines section.

[Andrew Hathaway]

Is it the MRC .15 that looks like a clone of an older (pre-FP) OS Max .15?  If so the older OS .15 has an unusual prop nut thread (Might be 7/32-32?).  It would probably be easier to find an OS prop nut, since there are more of them out there.  Assuming that the crankshaft thread is the same as the OS... 

[Dane Martin]

i think Christ is right. 6mm x 1.0
at the .35 size, they go up to a 7mm x 1.0

[Frank Clyma]

Thanks for the replies----and thank you Robert for moving this to a more appropriate forum.

I found a "Thread Identification Chart" by Maryland Metrics on the web this AM.  I measured per their instructions, and get .021 inches OD on the thread, and 28 thread count per inch.  The closest spec in their chart is #12-28.  6mm is a tad larger, which means a Fox spinner nut is not available for this propshaft.   The engine is hard to start by hand, and I don't like using the electric starter without a spinner nut---too hard on needles when I slip.

[Frank Clyma]

Make that 0.21 inches----

[Dane Martin]

you could take it down to a hardware store to verify the size. then check out this spinner nut ( i hope i got the link right!)

http://www.truturn.com/af.htm
and if you like this style, tru-turn will tap it to the size you specify.

[Carl Cisneros]

it is a 6 x  1mm thread size.

here ya go.

http://tech.flygsw.org/enginepropshaft.htm

Carl

[Dane Martin]

Is it the MRC .15 that looks like a clone of an older (pre-FP) OS Max .15?  If so the older OS .15 has an unusual prop nut thread (Might be 7/32-32?).  It would probably be easier to find an OS prop nut, since there are more of them out there.  Assuming that the crankshaft thread is the same as the OS... 

i missed this comment. Andrew you may be spot on. if frank is measuring .21" and 7/32 is .21875, then the os nut might be the easiest. still, tru-turn might be the easiest way to get a spinner nut. a lot of engines were 5mmx 0.8 also. that's kinda close. something to look at.

[Steve Helmick]

I did some machine work for Paul's  "909" B-17 stunter project in Y2K, and researched the prop nut situation. The OS .15's up through the .15FP used the "popular" (being cynical) 7/32"-32tpi. It appears that the .15LA doesn't. My recollection (IIRC) is that there was no source for 7/32 x 32 thread spinner nuts other than OS, and I can't find a current source (not Tower, anyway), unless you get lucky with finding some NOS parts in the LHS. A tap would have been something outrageous, like $40, probably plus shipping, and time was kinda short right then. Paul got the OS prop nuts, I machined away everything that didn't look like a B-17 part, and they flew in France.

I've used my "electric finger" on bare shafts a fair amount, with no problem. You are turning the rubber cup around backwards, right? It helps a bunch, but still isn't really wonderful.   Steve  

[Akihiro Danjo]

This nut is usable for OS FP15. So, maybe yours, too.

http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXBCZF&P=7

I cannot find OS spinner nut for FP15 here in Japan, too.

Aki

[Frank Clyma]

MRC stands for Model Rectifier Corp----evidently they used to import Enya engines, so what I have may be similar to an Enya.  Fortunately, I have the original prop nut---it was a better way to use an electric starter that I was after. I'm going to try a plastic spinner from DuBro, hopefully that will work. Turning the cup around didn't help much, nor did anything else I've tried so far.

The number of threads is exactly 28/inch----the diameter may be 7/32, or 5.5mm, or #12----without a known test sample, I can't be sure.  Those sizes with a 28TPI are not to be found in hardware stores, at least in the four I've tried.

I sure appreciate all the replies to my question!


 

[Andrew Hathaway]

Does the crankcase have a back cover plate?  Or does the front half of the case bolt to the rear half of the case?

From what I've seen... MRC imported Enyas but they were actual Enyas.  What I see when I do I google image search for an MRC 15 engine is a spitting image clone of an OS MAX .15 from the 80's or older.  The only Enya 15 in my stash has a 6mm standard prop nut.  The older OS 15s have a more unusual thread.

MRC 15s






Enya 15s





OS Max 15s

[Jim Kraft]

I have an MRC 15 also, and it looks like the OS. It has the back plate screwed on instead of the front end like the Enya. It says made in Taiwan on the back plate and the carb looks just like an OS only has MRC on it in raised letters. It also has a very long shaft for a a 15.

[Frank Clyma]

Yes, mine has a backplate.  It looks exactly like the one in the pictures with the red plastic spinner, and it also  matches Jim Kraft's description exactly.  So maybe a nut for an older OS15 would fit.

However, the threaded portion of the propshaft is exactly 3/4 of an inch long, and has exactly 21 threads, which would equate to 28 threads per inch.  Which doesn't quite match Steve's info on the older OS's being 7/32 X 32.  Maybe the MRC is a real oddball even compared to the older OS motors.

[Tim Wescott]

So, have you tried just getting your hands on a 12-28 nut and putting it on?  It sure sounds like the right size.

[Steve Helmick]

So, have you tried just getting your hands on a 12-28 nut and putting it on?  It sure sounds like the right size.

Don't bother looking for a 12-28 nut...it won't be easy to come up with one, and it won't fit. You can check the thread pitch with a long reach glowplug, to see if the two would "mesh", or a 10-32 machine screw and have a bit more thread to engage. Then, you need to caliper or mike the major diameter of the crankshaft thread...should be somewhere just a bit under .218", say .210". If those two things happen (as I expect), it's a 7/32 x 32 thread.  Steve

[Tim Wescott]

Don't bother looking for a 12-28 nut...it won't be easy to come up with one, and it won't fit. You can check the thread pitch with a long reach glowplug, to see if the two would "mesh", or a 10-32 machine screw and have a bit more thread to engage. Then, you need to caliper or mike the major diameter of the crankshaft thread...should be somewhere just a bit under .218", say .210". If those two things happen (as I expect), it's a 7/32 x 32 thread.  Steve

He reports exactly 21 threads in exactly 3/4 inch.  That's 28 TPI.  32 TPI would be 24 threads.  Granted, he could be miscounting or mis-measuring, but he'd have to be off by 3/32 of an inch for it to be a measurement error.

[Steve Helmick]

Well, then, get yourself a 1/4-28 machine screw or the crank of about 80% of the world's supply of glow engines, and see if the threads "mesh" when held side-by-side. No need to count anything. That, or pop for a thread pitch gauge...or two, if you want both SAE and Metric. I still doubt if it's likely they'll have a 12-28 nut at the local Ace Hardware or even a good industrial fastener store. The latter may be able to order one in for you, however!   

[John Kelly]

   ... eleven cents at FASTENAL. RC Fasteners & Components Inc. , and AMERICAN FASTENERS TECHNOLOGIES CORP. have them also. ( 12 - 28 NUTS ) ( corrected from 1/4 -28 brain fahrt )

[Frank Clyma]

And I thought I was asking a simple question------.

I didn't find a machine screw in my stuff, but I do have a couple of aircraft bolts, which are fine thread.  The 10-32 (AN-3) meshes nicely with the long glow plug, but not with the MRC propshaft.  The 1/4-28 (AN-4) meshes nicely with a larger glow engine shaft, and also with the MRC. (And of course, the two propshafts mesh easily together.)

Maybe the best thing I've learned from all this is that you can check thread pitch with something that is not the same diameter as the item in question----the threads will still mesh----I assume there are practical limits to how much different the diameters are, but it sure works if there isn't too much difference.  Thank you, Steve!

It would be interesting to see if Jim's engine checks out the same as mine.

[Steve Helmick]

  ... eleven cents at FASTENAL. RC Fasteners & Components Inc. , and AMERICAN FASTENERS TECHNOLOGIES CORP. have them also. ( 1/4 - 28 NUTS )

Yep, 1/4-28 nuts (please get grade 8 nuts for prop nuts) are available easily. The problem was 12-28 nuts, and I think you'll find those pretty rare.

As for Frank's diameter/pitch testing...it's been 35 years since I had to do much threading on a lathe, but the diameter has nothing to do with it, except for (perhaps?) tolerances. I recall having to come up with pitch diameter for a thread that wasn't in my machinist's handbook, and asked my leadman. He said to look up the dimensions of the size that had the right pitch and add whatever needed to get the diameter. That was probably about 1/8" (.1250!!!) or so. I worked in an experimental shop at Boeing in '80/'81 as a (manual) lathe operator. Did some weird parts, low volume, crappy old lathes, great experience.  Steve

[Jim Kraft]

Even though a 1/4-28 will thread on loosely, it is still the wrong thread. I measured the shaft at 5mm, or approx .207". I could not find another nut off of any of my engines that would fit. So, it is a reall odd ball in my book. When comparing a 1/4-28 shaft to it, the 1/4-28 in finer thread. It could be wentworth. If not that it is some metric size.

[Tim Wescott]

Even though a 1/4-28 will thread on loosely, it is still the wrong thread. I measured the shaft at 5mm, or approx .207". I could not find another nut off of any of my engines that would fit. So, it is a reall odd ball in my book. When comparing a 1/4-28 shaft to it, the 1/4-28 in finer thread. It could be wentworth. If not that it is some metric size.

Standard 5mm diameter threads are either 1mm spacing or 0.75mm spacing.  1mm is 25.4 TPI, so it should be pretty close to a 24 TPI 'merican screw (or really really close to a 1mm pitch metric screw!)

[John Kelly]

   ...OOPS!  Meant 12-28, that's what I searched and found. ( not enough coffee) Sorry!

[Tim Wescott]

Maybe the best thing I've learned from all this is that you can check thread pitch with something that is not the same diameter as the item in question----the threads will still mesh----I assume there are practical limits to how much different the diameters are, but it sure works if there isn't too much difference.  Thank you, Steve!

I thought everyone knew that!

I don't think there's any real practical limit -- you can check a Cox cylinder thread against a 4-40 screw if you don't believe me.

[Dane Martin]

sounds like it's a pretty positive match for 12x28. it's not a very common size, but easy to order.
if you call tru-turn, I'm certain they will tap out a spinner nut to your size. they just take a 10x28 spinner and re-drill and tap it. cheap easy fix to the situation. 

[bill bischoff]

You can buy an entire box of 100 12-28 nuts from Mcmaster-Carr for less than $4, so don't get too carried away with alternate solutions.

[Dane Martin]

the original request was identifying the shaft for the purpose of buying a spinner nut. as far as 12x28, there's not a lot available. but, tru-turn can drill and tap one to the size you need, if it's not in stock. so, not an alternative solution, but specifically the solution for the purpose of the thread 

[Frank Clyma]

I'm going to try the DuBro plastic spinner when I get back to messing with this engine. I much prefer the aluminum spinner nut, but don't want to add a lot of cost to this project.

Thanks to everyone for the postings!