New Tailwheel Leaf Springs too thick

How to keep the Cessna 170 flying and airworthy.

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Larry E
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Joined: Mon Sep 30, 2019 1:06 pm

New Tailwheel Leaf Springs too thick

Post by Larry E »

Good day everyone.

We joined this group in the fall when we got #25749. In the late 80's we owned #25770 and were members of the 170 group, but foolishly sold the bird in 1994. The knowledge of the experts was tremendously helpful back then, and I see that tradition is being carried on (and even improved!) by the current guru's. Thank you very much for sharing your wisdom and experience.

I have replaced the worn-out Scott tailwheel with a new one, and am now installing new Univair PMA leaf springs (from Aircraft Spruce). There is an extra shim included in the package that I'm assuming is not needed. The 2 long leafs are the same thickness as the ones that were on the plane. The 2 shorter ones, however, are each thicker by 0.015 to 0.020 inches - depending on where the measurement is made. The forward part of the assembled group fits snuggly into the front space (where the AN6-21 bolt goes through) but as one moves aft, the new leaf spring group is too thick to fit in the space available. The plate that is held in place with 10 AN3-4 bolts is now too high and cannot be forced down enough to allow the 6 most-aft 3/16" holes to align. I hope the picture is attached...
IMG_1750.jpg
I'm probably not the first to have this problem and am wondering if anyone has an idea or 2 about what can be done. We could use the original 2 short leafs but I'd like to use the new springs as the old ones are pretty corroded.

Larry
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Bruce Fenstermacher
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Re: New Tailwheel Leaf Springs too thick

Post by Bruce Fenstermacher »

Hi Larry. No you are not the first to have this issue. Seems the exact measurements (thickness) of these springs has been lost to time and or convoluted with those of springs used with L-19 brackets made later. There are at least 3 styles of spring brackets made from the 170 to the L-19. The same springs may fit the next bracket as they are thought to be within the tolerance required. One thing I know, Univair is the only supplier, so you won't find springs thinner any other place.

Looking at your picture, look like your issue is at the aft end. Take out the springs and look at the bracket. Is there a small nearly square spacer riveted to it? If so remove the spacer. This spacer was eliminated in later brackets. If your springs fit, you are set. if they don't you have two choices. Use your old springs or figure out a way to make the new springs thin enough.
IMG_1750.jpg
As I have not had to tackle the problem of making the springs thinner, I can not suggest a method to do so but it might include a Bridgeport mill or maybe just a belt sander.

Good luck.
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IA DPE
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Re: New Tailwheel Leaf Springs too thick

Post by IA DPE »

Larry:

Welcome back to the wonderful world of -170s. Also, to the Association. It is true that the association and the vast experience on this forum is invaluable.

The search function (top right of page) will be your friend as you research care and feeding of your new friend. This thread http://www.cessna170.org/forums/viewtop ... ail+spring was just recent and may help you. There are others as well.

Welcome aboard!
1955 C170B N2993D s/n 26936
1986 DG-400 N9966C
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Larry E
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Re: New Tailwheel Leaf Springs too thick

Post by Larry E »

THANK YOU Bruce and IADPE!

There IS a small square spacer on the bracket. It's warm today so I'll be in the hangar removing it.

Computers and I do not play well together - I had tried the 'Search' function and spent hours down that wormhole... (actually learned a lot of other things)

Your quick replies have made my day!

Larry
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gfeher
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Re: New Tailwheel Leaf Springs too thick

Post by gfeher »

Hi Larry,

Welcome to the Association! One trick I've learned for searching answers on the forum: while the search function on the forum is adequate, if you are having trouble finding posts related to your question, try googling your question, starting with "Cessna 170". One of the responses will be Google's search of the forum - with better results. Google has a much better search engine.
Gene Feher
Argyle (1C3), NY
'52 170B N2315D s/n 20467 C-145-2
Experimental J3 Cub Copy N7GW O-200
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c170b53
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Re: New Tailwheel Leaf Springs too thick

Post by c170b53 »

Seldom do I find myself disagreeing with Bruce....but I would not remove that little spacer. As posted before the purpose of that spacer is to elevate the springs side radius above the inside radius of the tail wheel fitting. If the two radius contact, likely it will crack.
I'd be curious as to how well the spring pack fits at the back when the front of the spring pack is free to move. The pack rests on the internal fitting (retained by the two bolts running perpendicular to the longitudinal axis) do they rest properly on that pad. Lastly you could replace the top plate.New, it comes blank and you can drill the holes true, albeit the holes will be close to the side radius of the plates flanges as well.
If you try to reduce the thickness of the springs, be vary careful of generating heat in the metal.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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GAHorn
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Re: New Tailwheel Leaf Springs too thick

Post by GAHorn »

I’m not a fan of modifying expensive OEM parts so they will fit cheap PMA parts. There have been several instances of late where the Univair leaf springs have been suspect with regard to fitment and I’m not confident we’ve gotten to the bottom of their recent-production differences.
When I replaced my mainspring about ten years ago it fit perfectly, but communications from recent buyers have raised questions about the present product.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention.
An originality nut (mostly) for the right reasons. ;)
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cfzxo
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Re: New Tailwheel Leaf Springs too thick

Post by cfzxo »

This is a good argument for replacing those old springs with, The T3 tailwheel setup :D looks like a nice design and working great in the uncertified category.and there are good reports from the few field approvals.
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Bruce Fenstermacher
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Re: New Tailwheel Leaf Springs too thick

Post by Bruce Fenstermacher »

Folks, Jim Dyer owner of Univair just happened to check in with me today to follow up on a conversation I had with him last June regarding these springs. He wanted to know if I or the 170 group had come to a consensus of the thickness the springs should be for a 170. I can not say he will monitor this thread but I gave him a link to it and he does, as of this morning have a forum account.

So Larry, if you still could, could you take precise measurements of the thickness of your old springs that do not match the Univair springs. And we'd also like anyone else to do the same with any springs they feel may be original and report back here. Hopefully we can get 10 or 15 data points as to what thicknesses will work.
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Bruce Fenstermacher
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Re: New Tailwheel Leaf Springs too thick

Post by Bruce Fenstermacher »

c170b53 wrote:Seldom do I find myself disagreeing with Bruce....but I would not remove that little spacer.
Jim, certainly what you describe if in fact one finds to be the case with their spring and bracket, I might feel the same. However I will point out, if I'm not mistaken, later L-19 brackets don't have the shim. And when weight is on the wheel the springs rest against the top bracket, not the bottom where the shim is. Just this past fall I replaced my main spring and had to remove the shim and didn't think a thing of it. The springs edges are rounded as is the bottom of the bracket.

I happen to have a brand new old stock Cessna produced repair kit for the L-19 which consist of the fish mount bracket, a set of springs and all hardware required to mount the springs. It is interesting that when a somewhat accurate thickness measurement is taken of all the NOS springs, they will not fit in the bracket. The opening on the bracket above the shim is .935" and the spring pack is 1.051" thick. Hmmm :?
IMG_0615D.jpeg
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Larry E
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Re: New Tailwheel Leaf Springs too thick

Post by Larry E »

Gentlemen
I managed to install the new springs today by removing the riveted shim (it's thickness is 0.063"). I measured the new springs (before installation) in the area that's enclosed by the bracket:

All 4 are 1.510" wide.

Longest : 0.320" thick U0542106
Next : 0.265" U0542105
Next : 0.210" U0442108-2
Shortest : 0.210 U0442109-2

Shim included with the set but not used is 0.105" thick U0442107

Originals are all 1.520" wide

Longest : 0.320" thick
Next : 0.260"
Next : 0.195"
Shortest : 0.193"



It's beyond my ability to measure distance from the bushing hole to each spring's bend axis, but I think this could also affect the total thickness dimension if the bends are not precisely located. This, perhaps, explains why by removing the square shim and increasing the space available by 0.063", allows the new spring group to fit quite snugly - despite the fact it is only about 0.038' thicker than the original spring group. Needless to say, we'll be monitoring the bracket as Jim mentioned the possibility of cracks.

Again, thanks to all for the help.

Larry
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c170b53
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Re: New Tailwheel Leaf Springs too thick

Post by c170b53 »

Bruce , like you say there’s differences with the L-19 assembly. However I have zero disassembly experience with that plane. I’ve only looked at them and wondered why the top plate’s aft differs from our planes. The L-19 looks beefed up, possibly to prevent cracks at the top aft. Just a total WAG on my part.
Get what you are saying about the load path but as it’s cyclical, again just my opinion, direct steel to aluminium, radius contact was possibly undesirable to Cessna. The shim solves that problem cheaply. Or maybe Cessna just needed to get rid of some excess aluminum...cheaply....what do I know. :oops:
Back to seriousness, any relative movement between these parts is not good.
Jim McIntosh..
1953 C170B S/N 25656
02 K1200RS
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n2582d
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Re: New Tailwheel Leaf Springs too thick

Post by n2582d »

L-19 IPC Fig. 54 (Click to Enlarge)
L-19 IPC Fig. 54 (Click to Enlarge)
Strange that your spare L-19 bracket doesn't have the spacer Bruce. Must be a SUP. :wink: The notes associated with fig. 54-34 say this tailwheel bracket spacer is included with spare bracket assemblies p/n 0642140-3 (for the O-1A and TO-1A) and p/n 0642150-10 (for the later models).
Gary
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Bruce Fenstermacher
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Re: New Tailwheel Leaf Springs too thick

Post by Bruce Fenstermacher »

Gary, I said I thought we established that later L-19 brackets did not have the shim. I did not say my L-19 bracket doesn't have the shim, it does. But the L-19 springs, ALL Cessna NOS will not fit my L-19 bracket as I measure it. 8O So Cessna

As for the differences in the later L-19 bracket I believe the Military probably experienced the cracking and missing pieces we see in this bracket at a higher rate than we see and Cessna basically removed the parts that where breaking off since they don't seem to be necessary. I've seen many 170 brackets with missing parts the owners with no clue.
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Bruce Fenstermacher
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Re: New Tailwheel Leaf Springs too thick

Post by Bruce Fenstermacher »

Larry I had a thought this AM. Your springs look so close to fitting and you say the bottom two new springs are the same thickness as the old. The top two new may be slightly thicker than the old.

But we have not established that your old bottom two springs where correct, they may be replacements that are too thick. You say you got your Univair spring pack through Aircraft Spruce. AC sold you inventory they have from Univair that may have two bottom springs, at least the bottom spring, that is too thick. Your two new upper springs may be slightly thicker, but within tolerance and it is a main spring that is your issue. Jim mentioned to me that since our last June conversation Univair no longer includes the thicker L-19 spring with their spring pack, they do still sell the L-19 spring separate. But perhaps Spruce has older inventory. I would measure your main spring and call Univair and compare what you have with what they have for the 170 main spring thickness. You might solve the issue.
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