A puzzling question about magneto gap adjustment

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Model T Ford Forum: Forum 2012: A puzzling question about magneto gap adjustment
Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Robinson Salty Bottom, AL on Wednesday, July 11, 2012 - 09:25 pm:

A puzzling question about magneto gap adjustment

A friend and I just went through several meticulous hours of setting and re-setting the gap, over and over a number of times, of the magneto so the gap would equal .025 (or close) when the engine is horizontal running position.


Q. Why do we stand the engine on it’s nose and adjust the gap ideally at .025 at the top nearest the mag pickup and .040 at the bottom?

A. (in my eyes, at least) We adjust this way so that when the engine is rotated to the horizontal running position, gravity will pull the transmission down, thereby closing the bottom gap and opening the top gap, and ideally ending up with an equal gap all around and hopefully ending up with a gap that will give good engine performance while running on mag.


Then, the next step is to install the pan & hogshead, rotate the engine back on its nose, install the 4th main, glue it up real good with gasket material & RTV, bolt it up so that the 4th main & sealant keep the crank straight and prevent flexing of the crank.

Here’s my puzzling question:
Am I wrong or does the magneto gap revert back to the .025/.040 setting while in the vertical position being glued, then when turned horizontal gravity can’t close the gap because everything is glued thereby missing the original goal of equal gap for 360 degrees?

Just wondering.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Glen Chaffin on Wednesday, July 11, 2012 - 09:55 pm:

Bill, I use 0.025 at the top and 0.030 at the bottom. Since the height of the magnets vary a little bit. You can never get them all exactly the same. You should always find the highest magnet and make your clearance adjustments using that magnet, That is worse case. A babbitt 4th main will wear with time allowing the rear of the transmission to sag downward. This will increase the gap at the top and reduce the gap at the bottom. This is why you give the gap more clearance at the bottom. You won't have that problem with a ball bearing 4th main.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John Doolittle on Wednesday, July 11, 2012 - 10:03 pm:

So, Glen, are you saying that if the ball bearing 4th main is used, and the .025/.040 rule is followed, as stated in the electrical manual, then the gap has been set wrong?


Top of pagePrevious messageNext messageBottom of page Link to this message  By R.V. Anderson on Wednesday, July 11, 2012 - 10:20 pm:

The reason for the larger clearance at the bottom is due to the vibration, while running, of the lower (unsupported) section of the field coil.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kerry van Ekeren (Australia) on Wednesday, July 11, 2012 - 10:22 pm:

Why a .025/.040 rule?, the Ford service book quotes the gap to be only 6 to 10 thou difference from top to bottom.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Robinson Salty Bottom, AL on Wednesday, July 11, 2012 - 10:27 pm:

Glen, our group has found that setting the magnet height is the easy part, providing we don't get in a hurry and accept "close enough". The difficult part has been adjusting the coil ring, because as R.V. stated "the lower section is unsupported".

Re-gap the top a little, somewhere else the gap changes more.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Glen Chaffin on Thursday, July 12, 2012 - 12:02 am:

John, No, What I am saying is that if you use a ball bearing forth main the bearing will not wear like a babbitt bearing. But a babbitt bearing will wear causing the gap at the bottom to get smaller. RV's comment about vibration is also valid.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould on Thursday, July 12, 2012 - 12:24 am:

The Ford manual says to fit the lower half of the magneto coil support 6 to 10 thou greater clearance than the upper half "because the weight of the flywheel and transmission tends to decrease the clearance between the magnet clamps and the lower half of the magneto coil support".
That is the reason.
I fit and assemble the flywheel, transmission shaft and coil support horizontally before attaching the transmission. The weight of the transmission does cause the gap to close somewhat at the bottom. I've measured it.
As R.V. says, there may be more vibration at the bottom of the coil support when the car is running because it is bolted only at the top of the engine block. If so, that is an additional reason for the additional gap at the bottom.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Bill Robinson Salty Bottom, AL on Thursday, July 12, 2012 - 07:44 am:

Thanks all! On this engine, we ended up at plus or minus .030 in the horizontal position. I just got to thinking about how we ended up gap-wize once the hogshead was installed with the ball bearing end cap. At this point there is no way to measure.

Today is the day the engine gets cranked on the test stand and tested with the Fun Projects St Louis Meter.

The last one our group rebuilt about a month ago runs great and measures high on the meter.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Steve in Tennessee on Thursday, July 12, 2012 - 09:20 am:

Glen,

Bill and I discussed this at some length last night and I offered (I think I offered) the same observation (or thought rather) - that if using an original 4th main (babbit) your wider gap at the bottom is in anticipation of babbitt wear in the 4th main.

*IF* that is true then would it not seem logical to NOT provide the wider gap at the bottom when using a ball bearing 4th main? It will not be wearing.

Have I interpreted you correctly?

These views assume of course that the pan is a rigid body and will show no deformation. It is after all supported, hanging in space, by the arms.

I was fortunate to be with the group of 5 that did the first mag set that Bill speaks of and it was quite the rewarding experience. Not something for those with patience issues to attempt but highly rewarding. With some help from an experienced hand at this - Keith Barrier - I think we taught ourselves that really anyone can do this job if they have a little help and the willingness to go slow.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Allan Richard Bennett on Thursday, July 12, 2012 - 09:39 am:

Fellows, setting the magnet heights all the same can take up much time. Once I had to rebuild a flywheel when the balancing guys beat the dowels out the wrong way. I did not have a gap gauge at the time. After getting them close, but not as close as I would like, I took the flywheel down to the local machine shop for a bit of fat chewing. Their obvious solution was to put it on their flywheel facer and grind the magnet clamps. As I was chasing only .001"-.004", facing the clamps removed very little material, not even enough to contact the brass mag screws. The end result was a set of magnet clamps at exactly the same height, which were absolutely true to the mounting face at the crank flange.

Anthony and I have done this twice more on our own cars. So little material is taken off if the heights are close to start with, that the static balance I do on knife edges is not upset enough to warrant any correction. I have not offered to do this when building a motor for others. Is it a bit too far out to be considered good practise?

Allan from down under.


Top of pagePrevious messageNext messageBottom of page Link to this message  By R.V. Anderson on Thursday, July 12, 2012 - 08:32 pm:

It can certainly be done that way, but the end result in my experience doesn't justify the extra effort. The usable output is, in practical terms, the same whether all magnets are indeed exactly the same or within 2-3 thousandths of being the same.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Larry Smith on Thursday, July 12, 2012 - 08:46 pm:

Back when I was much younger than I am now, numerous people used to have their flanges on the crankshafts break off using ball bearing 4th mains. I still have the rebuilt 4th main in my '13 that I put in over 50,000 miles ago. No problems. It is a bronze bushed ball cap. I have a NOS 4th main in my new '25, and haven't had any problems with it either, but I only have 800 miles on it.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Allan Richard Bennett on Thursday, July 12, 2012 - 08:47 pm:

Thanks, RV. I did not expect grinding the keepers to have any effect on the mag. output. It just makes setting the gap much less of a headache and saves time on the job. Son Anthony tends to be a perfectionist and seeks to get things as well fitted as possible. He is used to working with more modern machinery and translates this to working on his T's.

Allan from down under.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Anthonie Boer on Friday, July 13, 2012 - 03:22 am:

Here are some pictures how I do it.
Toon
154R
155R
538R


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Patrick on Friday, July 13, 2012 - 10:12 am:

George King III of "Connecticutt Antique Engine Restoration, LLC" (www.enginerestoration.com), did a great job in re-pouring the babbit in my 4th bearing ball cap in 2010. This was after I was cautioned by several on the Forum about using the new ball bearing ball cap because they were subject to failure. Also, at that time, in order to fit the ball bearing ball cap, one was required to cut 3/8" from around the drive shaft bell, so I elected not to use it, instead, deciding on the original babbited 4th bearing ball cap.

If you want him to do yours, he requires you send him your ball cap and your drive plate so he can precisely machine the the babbited bearing to the drive plate tail shaft. Not only does it provide my transmission with plenty of support, the fit is so precise that my engine does not leak out the rear of the engine. Jim Patrick


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