the ongoing 25 deg. vs 35 deg. mag drive cup debate

[Anonymous-0]

Ive been following this ongoing discussion on mag timing here at ytmag and other JD forums for the last several months. I have spoken to my local magneto expert and he has checked all his sources for the 25 deg long lug cups with no luck. He refered me to Std. Magneto, who now owns Wico.

I called Std. Mag and talked to a man there about this. He made me sound as if the notion were the craziest thing he had ever heard. He said that cup was just made for one particular tractor and that he had 60 of those cups on the shelves for over 20 years before they all sold out. Said it wouldnt make any difference in how the tractor ran and unless you were pulling, or running high compression for some other reason, it wasnt advisable. He said they would have to make at least 500 of them to justify the effort and he said they'd never sell so he wasnt gonna have em made. I wish I would have gotten his name, but I did email Std. Mag a link to this forum and told them I thought they were making a huge marketing error.

Listen, I am a machinist. What you guys have been saying makes perfect sense to me. I run gasoline exclusively in my '39 B and I would like to try a 25 degree timing setup. I cant even rotate my mag enough to get it below 30 degrees or so. These 25 degree cups might be hard to find, but hey, its only a little piece of metal, right? Why would it not be possible to take a short lug 25 degree cup and weld up the lugs to the longer length, and then re-machine the flats on the side of the lugs to the correct width? Is the lug length the only difference between the two? I think I am understanding that the short lug was made for the X mag, correct? Will it still mount into the C, even tho it would be too short to work correctly with the slot in the governor? I have seen a grand total of one - count em, (1) magneto disassembled, so yes, I am talking from a position of ignorance here, but I fabricate metal parts for a living so this is just how I think.

As far as strength goes, I wouldnt judge this to be a highly stressed part, so welding wouldnt seem to be detrimental as long as it was done by a good welder with heat control in mind to keep it from warping all to hell.

If these parts do interchange, (please let me know thru this forum) I plan on trying this process and will surely let you guys know over the next few months how it goes.

Mr. Browning, I have one of your H resto books, and I look forward to hearing your opinion on this.

 

[Mike M]

I tend to agree with the guy at the mag. shop. Under "normal" running you won't know the differance. I think this problem showed up with extra high compression pistons used for tractor pulling.

Anyhow if you can get the short lug ones ? Then just pull the governor apart and change the drive hub on it. For some reason I think they will fit. I might of seen it before ?

 

[johndeereman]

i havent followed your story so i dont know what you are trying to do but i have a wico model x mag from a jd h that i put on my b it had the 25deg cup but my model c mag that came on my b had a 35 deg cup so i changed it my b still runs ok but i cant get it timed right its hard to start backfires through the intake im going to put the 25 deg back on it pulled alot better that way and started first cranck every time so i think you have to change more than the cup for it to work the book says the b came with either one 25 or 35 deg just my expierience

 

[Duane Larson]

JoeBob,
You likely spoke with either Paul or at Standard. And yes, they are aware of this issue and will make a run of the drive cups but it takes 250 of them, and no repair shop has bellied up to pay for that sort of order yet, and likely won't. I have the drive cup number in the shop - I've only seen one or two in 25 years of restoring magnetos, so they are fairly hard to find.

It is not uncommon to find a piece of 5/16" square stock welded to a short lug coupling to make a long lug coupling - however in my experience it is mostly not successful as there are rather stout impulse forces experienced by the drive cup as the magneto impulses resulting from the impulse spring unwinding. Most, if not all, extensions are broken off at least one lug if not both. Not saying it can't be done, but the drive cup is hardened (a file won't touch it) to resist wear both on the drive lugs and at the junction of the drive cup and impulse stop plate which stops the drive cup during impulse.

Unfortunately, making a 25 deg drive cup from a 35 deg cup requires adding material to the drive cup where it strikes the stop plate - if it were a matter of removing it that could be done with a grinder.

This topic comes up for discussion periodically and I guess the best answer would be to agree to purchase 250 drive cups at about $15-$20 - an outlay I'm not willing to make. But if Standard would simply replace the 3744 35 deg drive cup with the 25 deg I know of no reason anyone would buy a 35 deg cup, given the propensity to run the early Deeres taking the long lug cup on gasoline.

Finally, my experience on dynamometers has been mixed, but in general it is not obviously better to use a 25 deg cup.

FWIW.

 

[Anonymous-0]

The 25 degree cup is part number 7596, I believe, Duane. Thanks for the correction on Std. Mag. Maybe he exaggerated the order requirement for my sake, but yes, either way it does appear that the chances of them offering them again are nil.

That the cup is hardened is something that I strongly suspected, but hadnt verified with a file yet as I havent had a reason to pull my mag since I started thinking on this. I never thought about using square stock as a build up measure, but I think a logical explanation for the high failure rate that Mr. Larson points out is that generally the small square stock that tends to be found lying around most peoples shops is just cold rolled. Key stock - whose purpose IS to break.

Perhaps I underestimated the forces that act upon the cup. I agree that the cyclic impact of impulse would be a concern, even tho it's not an issue above 250 rpm. Ive heard mine make some God-awful noises when she's shutting down and gets down around 75-100 rpm and then just happens to kick one more time. It puts it into that in-between mode where it's trying to impulse, but its still running way too fast and the stop cant get latched completely. At least thats what I picture happening.

Bottom line here as I see it is that this HAS been tried before, but maybe the method wasn't the best. Changing the coupler in the gov. to match a short lug cup seems also to be a viable option. It might even be simpler to turn down a new coupler out of 4140 and harden it. I dont relish the thought of tearing down the gov to change it, and offhand I dont think it can be changed thru the mag hole, but it IS an option. Seems to me, and correct me if Im wrong here, that if one were to go to that measure, then he could also change the angle of the slots in the coupler by 10 degrees and use the existing 35 degree drive cup.

Many say that it isnt worth the effort to change to the 25 degree setup, but there are also many in this forum who have discussed it at length in the past and seem to be looking for a way to make the change, not being able to find the long lug cup. Do a search for "25 degree drive cup" and you will see. Seems like there must be something to it.

Thank you all for your thoughts!

 

[Mike M]

I think everyone is missing the point here. Those couplings retard the timing for starting and after the engine gets to maybe 200 rpm the mag. is direct drive. So you can just set the timing with the engine running for best HP on a dyno and not worry about where the starting timing is as long as it is starts and doesn't kick back.

 

[Mike Aylward]

I agree with Mike. The 35 degree coupling is for a hand crank tractor so it doesn't kick back when cranking. When the tractor is started this figure has no bearing on the running timing. Mike

 

[Anonymous-0]

Mike and Mike,
Respectfully, please check out the following links:

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=jd&th=249755
http://www.johnnypopper.com/forums/index.php?showtopic=14235&hl
http://www.ytmag.com/cgi-bin/viewit.cgi?bd=jd&th=208996

Reading these was what got me onto this "thing".
The object here is to be able to retard the running timing from 35 to 25 degrees without changing the start timing. Start timing should always be very near to TDC. Run timing is determined largely by the drive cup used. With the 35 degree cup, retarding to 25 degrees (running) is "out of range" of being able to get there with mag rotation alone. I have seen this by using a timing light. The 25 degree cup gets you close enough that you can then fine tune the relationship between start and run timing by using a timing light and making stop plate adjustments along with mag rotation. Also, Mr. Larson has an excellent post on this at this link:

http://www.ytmag.com/cgi-bin/viewit.cgi?bd=jd&th=196242

Someone also posted a method of setting both of these timings by using a bit of a backwards trial and error method, which I think has much merit.

JoeBob

 

[Mike M]

JoeBob; I think you say you have a 1939 B model and running it on gas. Are you having any troubles with it if you set the starting timing to the TDC mark and just let the running timing fall wherever ? I have a 1936 B and this is how I set it and forget it gas or kerosene and it runs fine. Are you trying to fix a problem that really isn't ?????

Sometimes I think many try to over think this stuff. They want to apply smallblock chevy racing technology to an engine that can't use it and doesn't need it.

Now if you are running a modified 2cyl. for tractor pulls then you may be having troubles with detonation and trying to get max. HP. If this is the case a timing light and timing specs. are for the most part useless. Every engine is going to be different. You NEED to set your timing on a dyno while running for max. HP and make sure you are running the same fuel mixture you are going to pull with as any change in octane will make a BIG effect on these modified engines. After set a timing light may be good to referance. Really if your pulling a dist. will be a better choice so you get a timing curve.

 

[Anonymous-0]

Oooops, gotta correct myself. Changing the drive coupler slot angle in the governor would be equivalent to rotating the mag in its housing. The lag angle would not change and impulse would also be retarded. Still, a coupler designed to work with the short lugs would solve it.

 

[Earl - IL]

JoeBob, My Allen mag machine I can check the lag on the coupler .I find that most USED 35deg couplers will be 37-40 deg lag depending on the wear in the cup & the driven flange. The USED 25 deg will test out at 27-30 deg. lag.So maybe your coupler is worn worse than this. Remember these mags have wear. You can correct this if you have the time & are a good welder. You can hard surface spray the contact area and get it perfect if you want to take the time & do it. But as they have touched on this before ,the running timing is what counts.Make sure your TDC is true TDC. Worn flywheel splines & stroker cranks can be off at TDC.Do you have a good marker when checking timing? Thats a long distance from the tractor case to the flywheel. To make a short coupler longer. Long keystock across both lugs clamped & weld in place on the ends. Then cut in the middle and clamp & weld middle 2 side of the ears. This will run true and last forever parading & tractor pulling.Let us know how everything works out.

 

[Anonymous-0]

The honest answer to your question, Mike, about trying to solve a problem that isnt, is - YES. That is EXACTLY what I am doing. My tractor is 100% stock and I have strived to restore it to a condition as it came from the factory. Cheated here & there, but the basic motor configuration is stock B. I dont pull (although I dearly love the sport) and will never work this tractor, I have another for that. My resto doesnt even have the hood on it yet, but I have rigged a gallon can which hangs by a wire from my steering shaft so I can run it some and start getting her broke in while finishing the hood.

Two months ago, I had a starting problem which turned out to be just dirt in the carb (from running it out of the gallon can with no filter, of course). I discovered this forum and started reading about both mag and carb problems, cures, etc. and ran across a thread where the guys were talking about the 25 degree drive cup. Their logic about running 25 degrees run timing when on gas seemed sound to me. Well, I had been playing with my mag a bit and noticed how it seemed to run a bit less lumpy as I retarded it. Noticing that, I wanted to "super-retard" it momentarily just for kicks to see if things got even smoother, but couldnt because the mag wouldnt rotate far enough. When I measured off my wheel and made the appropriate marks, then put the timing light on it, things started coming into focus for me. After re-reading the forum articles, I started seeing the relationship between run & start timing and I then knew that a 25 degree cup would be necessary to make the switch. Now, Im seeing that 25 degree cups are fairly rare, so Im just looking for a cheap way to "produce" one.

Thats all I am doing here is trying to figure out how I can experiment with the 25 degree system without a huge outlay in cash or effort. I'm tinkering is all. Hell, who out there that owns a JD tractor isnt a bit of a tinkerer? Thanks to all you guys, Im learning a lot too. Cheers!