Distributor advance springs puzzle

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david.dickson
Posts: 5
Joined: Thu Dec 19, 2013 9:56 am
Zip Code: 39574
Tractors Owned: 1953 Cub
1953 Super A
1947 BN
Location: Mississippi, Harrison County

Distributor advance springs puzzle

Postby david.dickson » Mon Sep 28, 2015 6:42 pm

I have been a largely silent reader of the forums here for several years, having discovered long ago that the best way to learn is to be silent when wise people speak (and boy, have I learned a lot).
However now I have encountered a puzzle that requires asking a question of the assembled experts so here goes: Having progressed with the continual, never-ending improvement program with my Cub ("Baby") to the distributor, I began assembling all of the necessary parts. Now, my policy is to never put back a spring, gasket, seal or piece of hardware once removed. Replace with new when at all possible.
Which brings me to the advance springs. From the available parts books I determine that my 1953 cub requires p/n 358 107 R91 so, I ordered a set from my friendly local CNH dealer. Came in a clear plastic envelope clearly marked 358107R91 but inside, another plastic envelope labelled 358108R91. Begins the puzzle, according to the parts books these springs are appropriate for a Cub with a much later serial number than mine. Can anyone tell me anything reassuring about this before I tear into the distributor? Is the -108 p/n a current approved substitute for the specified -107? Will I still obtain the proper advance characteristic. Any idea what the reason for using these -108 springs in later models was? OK, that's not just one question but I'm trying to reassure myself before proceeding since the -107 spring appears to be un-obtanium.
Any help, advice or guidance from wiser heads would be greatly appreciated. If I have missed some obvious fact somewhere, my apologies, chalk it up to being low on the self-teaching curve re: tractor mechanics.
Dave
1953 Cub
1953 Super A
Member IHCC #31

Eugene
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Re: Distributor advance springs puzzle

Postby Eugene » Mon Sep 28, 2015 7:20 pm

(Bates Corp.) 358108R91 Distributor Spring Set $37.38 New Parts ***NEW*** Distributor Spring Set for 200-240, 300, 350, 400, Super C, Super H and Super M tractors. ***Kit contains 2 springs.

Don't think the springs will work in a Cub. Listed tractors have a higher/larger advance than a Cub.
I have an excuse. CRS.

Jim Becker
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Re: Distributor advance springs puzzle

Postby Jim Becker » Mon Sep 28, 2015 7:59 pm

I think that over time they have simply reduced the number of different springs that are still available. As far as I know, the 107 spring was correct for all Cubs. It should advance at:
0 degrees @ 400
5 degrees @ 800
12 degrees @ 1200
16 degrees at 1600

The 108 springs are weaker and thus advance quicker:
0 degrees @ 400
9 degrees @ 800
24 degrees @ 1200
30 or 32 degrees at 1600 depending on the distributor shaft.

If the 108 springs are put in a Cub distributor, the shaft will limit the advance to 16 degrees, making the curve:
0 degrees @ 400
9 degrees @ 800
16 degrees @ 1200
16 degrees at 1600 depending on the distributor shaft.

So if you are replacing the springs because you think they have gotten weak, your newer ones are probably weaker yet.

Try hooking one of your old springs to the end of a new spring and stretch them a bit. See which one stretches first. I'll bet the new one is the weaker of the two.

The 108 springs will give you more advance at intermediate engine speeds, won't make any difference at low or high ends. I doubt that would be an improvement. I'd use the old ones.

david.dickson
Posts: 5
Joined: Thu Dec 19, 2013 9:56 am
Zip Code: 39574
Tractors Owned: 1953 Cub
1953 Super A
1947 BN
Location: Mississippi, Harrison County

Re: Distributor advance springs puzzle

Postby david.dickson » Wed Sep 30, 2015 11:57 am

Jim, Eugene Thanks for the information. I got the original springs out of the distributor, here is a photo comparing the original and the -108 springs received from CNH, upper = original.
SDC12780 (Large).JPG

The original spring looks stretched to me but the tractor runs fine, seems to have plenty of power. Although, i had not checked the advance before i took it out.
I made these measurements of the two springs. I am assuming that the original spring is stretched due to age/use so the coil length was measured with it compressed.
Description Coil O.D. Coil Length Turns Wire Dia.
Original 0.244 0.254 5-1/2 0.040
358 108 R91 0.266 0.248 5-1/2 0.041
The differences seem slight to me but they are not identical. I will proceed to do the following:
1. Re-assemble the distributor with original springs and check the advance (which I should have done before becoming over-eager and taking it apart). BTW, it was clean and had some evidence lubrication but now will be really clean and re-lubricated.
2. Test the springs as Eugene suggests. Plus i have a digital balance scale and will test the strength of each by measuring pull vs elongation.
Thanks for your replies and great information. Being a retired engineer who still misses practicing the trade, I have my teeth into this problem and want to understand it and perhaps be helpful to others in the future.
i am now thinking that if it advances per the information Eugene provides, leave it be. I'll just return the springs to CNH as being the wrong part and get my $48.50 back.
Dave
1953 Cub
1953 Super A
Member IHCC #31


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