I'm in the process of replacing a triple gear pin due to damage from a frozen bushing. I've never done this before and don't know how much of an interference fit is acceptable. The old pin was tight, but came out cleanly with a press. There is no damage to the flywheel, the hole looks good. The best I can measure the hole using a telescoping gauge and mic, it is .6825" at both the motor and transmission side of the flywheel. In other words, it is a straight hole.
The new pin will slip into the hole but stops (using only finger pressure) with .696" protruding. The diameter of the new pin from Lang's is .6825" where the shoulder that seats in the hole begins and is .683" at the end of the shoulder area. So there is only about .0005" difference in the area that should be slightly tapered.



The old pin was .6785”-.679” at the beginning of the shoulder area and .683” at the end, a difference of about .004”. So is this pin OK, or should it have more taper? I certainly don’t want to crack the flywheel.

Steve Blancard:

Triple gear pins are TAPERED. You can easily crack a fly wheel if you drive it in from the wrong end. It must come in and out from the magnet side of the fly wheel.

Dave - yes I understand that. I pressed the old one out towards the magnets and the new one is setting in the hole in the photo, inserted from the magnet side.

Actually, the pins are not tapered but stepped, unless some one has improved the finish on the new replacement ones, it's best to spin them in a lathe and clean them up with a very fine paper.

Steve, you pointed out something I never noticed before. I had two sets of those original pins here that I removed from flywheels used on Hand Crank Coil Testers. I just measured some with my Digital Chinese Caliper. They all measured .679 or .6795, except for that small area on the end, which is about 1/16th of an inch long and it measured .687 on all of them. That slightly larger section that you have shown in your photo is not tapered on any of them. On other pins that I have replaced in the past that smaller area was tapered from the smaller size to the max size at a 45 degree angle.

That pin will not come out the way it went in with the magnet bolted in place. That slightly larger area is also probably large enough to keep the pin from working through the hole.

Well it does appear to be slightly stepped.

Jim - I failed to mention that last 1/16" inch, it measures .691". So it sounds like this pin will work ok. I think I will polish the pin a bit. Its not as smooth as I'd like.

Steve,

In my view, the main body of the pin will have too light a press fit at .0005 interference. While on the other hand, the .691 portion will be way too big to press into the .6825 hole. In fact, it won't even start into the hole.

Not sure what the ultimate solution will be but I don't think that pin is going to do it.

I'd love to step up to the plate here and help with advice...but the boo-birds will shoot me down as they always do when I talk tranny I'll do it anyway as someday someone might actual listen and motivate me to get back into the tranny reverse engineering project

1- T 715 was in fact a 3 step shaft. T 715 for as simple as it is is almost one of the most complex Ford drawings I have ever worked with!

2- I firmly believe that there was also an undocumented T 715 AR that also called for a re-ream of the flywheel bores when used or just might have fit the hole 'right' when you somehow managed to pull the pin from the wrong side . But I can't prove it but those pencil numbers on the drawing just won't work in new flywheel tolerances, you'd shatter the flywheel!

3- As near as I can figure that final small head end (magnet side)was not meant to be bashed into the hole but what do I know...I could probably equally argue it was the final seating but at that diameter most flywheels of that material should then eventually spider crack in use! The drawing does not show a chamfer on the underside as good as my eyes can see it.

4- T 715 also allows for pins to come up short in overall length but doesn't say where the short applies.

5- I still don't have an actual drawing for the flywheel and am having difficulty getting someone to send me a cropped area just showing the bores, but one gentle soul read me the bore dimensional numbers. (someday I'll pour yet another 50 bucks at this thing and then another and then another...it costs a fortune to do real research )

6- Calculate the minimum fit shown between the 2 drawings and you get 0.003" and let's not even talk about the more that is possible. Let's just say that Fordism allowed then to hold 4th decimal place and they took advantage of it. At 3 thou you are looking at a shrink and not a press fit and that 3 thou would actually scare me in the flywheel material used. I'd like to think the flywheel drawing lies and manufacturing never told engineering what they really did

7- The step between the bush area and flywheel area on the pin is so small it is almost a why even bother and I'll leave it to others to even attempt to explain why.

8- FORD DRAFTING ROOM STANDARDS used in the era would say the following. For a shaft > 0.500 and < 0.999 then a press fit would be 0.001" and a shrink fit 0.0015" so the drawing shows double that using the mins. You decide. (Somehow Humber Car Co. UK wound up with a set of Ford in-house design standards, and now what Humber had, I own)

No conclusions to offer here, just sharing what I have found for posterity. Hint...if someone has the flywheel drawing and would be willing to send me the partial area...I'd be happy to go through all my work and correct if the bore is different than what I was told

George, the T-715 AR that you mention is in the Ford Service Manual, page 79, paragraph 289, it says that .003" oversize shafts were available, P/N T-3315-T-751-AR

Ken,

Aha...well thanks for that. I miss some of the very basics <grin>

I never could find a T-715AR print, but the regular T-715 drawing has pencil notes on it with only 2-steps in the pin (instead of 3) and a bigger end for the flywheel. I'll look later to see if the pencil number is the 0.003" difference.

Ken and others,

I looked at the pencil markings on the T 715 drawing and YES...the flywheel 'barrel' end in pencil is exactly 0.003" larger than the original ink underneath. No 3rd step at the magnet end either.

Guess that answers why I've never been able to find a T 715 AR drawing.

Thanks for that 'clue'.

I wonder next if they expected a new ream or simply use as is in a hole where a pin had already spun. Ream would make sense but who knows? Standard pin spins, say up to 0.001" of clearance in the spin, leaves 0.002" for a new press/shrink fit? Take a ream at 0.0015 oversize and still leaves 0.0015" interference?

Has anyone used a line to line fit with the modern Locktite used to mount shafts in bores. Seems to me that this would eliminate any cracked flywheels due to interference fit and still hold the shaft (pin) securely.

Neil

My new pin is .691" on that last 1/16" end. The hole has a corresponding area at the end of the hole that is about .690". So there would be just a slight interference fit. I too was thinking about using some bearing retaining loctite for extra security.

Steve, do you have a knurling tool for your lathe. If you knurl that small larger area, that might help keep the peg in place.

Steve,

It's not that end that holds so much, it's the barrel part of the flywheel contact pressing on the pin. "My new pin is..." is an oddball size to me, even for the back lip, just asking.

I was hoping someone would answer your Loctite question as I just don't use the stuff other than thread locker so have no experience.

I'm a little confused (easy to do). Since the original pin needed a press and came out nice, what was the dimension on the flywheel part of the pin? That should be your guiding direction I would think.

If the number you find is different from your new Lang's pin check around as there are several pin suppliers presently and there might be differences, I don't know. I am told that Chaffin's make their own.

The previous post is a good suggestion if you can do it if you come up light. Some may say that it is a crutch, but in the recent forensic study of pins and bushes of days gone by, there was a Ford original 'N' steel pin that had a knurled flywheel end barrel. It apparently went in and seated, it stayed, even though one of the other 3 let go in that same transmission.

FWIW, Mac's list their T-3315 pins as being .003" oversize. I would expect that to be on the flywheel end.

The old pin was .6785”-.679” at the beginning of the area that fits in the hole and .683” at the end, except for the last 1/16" at the very end which is .687". I remeasured the very forward end of the hole as best I could. I'm getting .6875", not .690" I mentioned earlier. There may be a little error in my measurements. It seems the hole matches the old pin pretty closely. I have used the liquid bearing retainer before and it works very well. I will probably use it on this new pin and see how it feels being pressed in. If it goes in too easily, I'll look for a larger pin.

I just got off the phone with Dave at Chaffin's. I told him about my pin situation and asked his advice. He was very helpful and took the time to explain the history of the reproduction pins being made today and in the past. Since my flywheel ha been rebuilt before with oversize pins, using a new pin now will not provide enough interference fit (.0005") to be safe. He recommended that I look for another flywheel that still has its original pins, and rebuild that with new pins. So it looks like that is the way ahead.

I have to say I really appreciated Dave taking the time to share his knowledge and experience me on this. Same goes for all of you here on the forum too. Its reassuring to know there are so many folks willing to help a guy out.