The International Cessna 170 Association

My mechanic and I recently installed a Univair tail spring on my 170B. The only way we could do it was to remove the top leaf.
With just 3 leaves it's a perfect fit into the available space. I'm thinking Univair must have used thicker steel for the leaves than
the originals, and I was wondering if anyone else has encountered this problem. They unfortunately don't radius off the ends of
the leaves so they don't gouge a groove in the lower spring during flexing. I had to accomplish that with grinder and file.

I've installed a new Univair main spring leaf in my airplane twice (most recently Dec '18) and have never had the problem you describe.

You're talking about a p/n U0542106, right? Did you clamp the spring pack together before inserting it into the bracket? http://cessna170.org/forums/viewtopic.php?f=24&t=9605

Del Lehmann at Mountain Airframe in Mena has been trying to replace the lower tailwheel spring on my 170. He has bought two of them so far from Univair and they are both twisted to the point the tailwheel runs crooked on the side of the tire.

The old TW spring has 500 hours on it and I was hoping replacement would be no big deal but after two tries, I told Del to put the old one back on.

I'm talking about the spring set Univair sells for $235.01, and another $24 for shipping. It consist of 4 leaves, and there's no way
all 4 could be installed. With the 8 AN3 side bolts in the plate above removed, the assembly could of course be slid into place.
But it was too thick to allow the holes for those 8 bolts to line up with all 4 leaves in place, even though I used a large C clamp.
At least I had no problem with a twisted leaf.

My mechanic agreed that those 3 leaves would be enough, especially since they entirely filled the space available.

Buckeyejim wrote:I'm talking about the spring set Univair sells for $235.01, and another $24 for shipping. It consist of 4 leaves, and there's no way all 4 could be installed.

No doubt you have the later "fish mount" style tail wheel bracket. Univair sells the same spring pack for all models of 170. Those who still have the early steal tailwheel bracket must use all 4 springs as that is what is intended and why 4 springs are included. Those with the later "fish mount" style only use the bottom 3 springs and that is all that is intended.

Likely your mechanic referenced the IPC before agreeing to only use the required amount of springs.

Correction: I'm wrong above. There should be 4 springs.
So in short, '48 with steel bracket, 5 springs, all other years no matter the bracket type, or a '48 with "fish mouth" bracket, 4 springs.

hilltop170 wrote:Del Lehmann at Mountain Airframe in Mena has been trying to replace the lower tailwheel spring on my 170. He has bought two of them so far from Univair and they are both twisted to the point the tailwheel runs crooked on the side of the tire

I just got of the phone with Jim Dyer, President of Univair. I know JIm from many years visiting with him at the Sentimental Journey Fly-in at Lockhaven Pa. and thought I ought to give him a call regarding the twisted springs and another report within the year of someone getting a main spring that was not bent correctly.

I told him what has been reported recently. He said what happens is the springs warp if they are not quenched correctly for hardening. He said of course the springs are inspected but obviously they, Univair, needs to step up their game in this department. He knows they've got to be straight and made with the correct angles and does not want bad parts getting out. He appreciated my call.

On another note Jim also said they recently got the necessary paperwork to approve the installation of the thicker L-19 main spring. He knows they've been used on 170s by some folks for years but Univair wanted to get the necessary approval for it. I told him, while I don't understand it as the L-19 and the 170 use the same "fish mouth" mount, that using the thicker spring makes the installation extremely tight. At least that is what I've found.

We also briefly discussed the availability of the "fish mouth" mount, and I wondered how many Air Repair may have to support the 170. Jim said there is an extremely large surplus of those beyond what Air Repair has. Bringing them to market when Air Repair runs out he thought would not be a problem. Good news.

I thanked him and Univair for supporting our Cessna aircraft. We'd all really be stuck for some parts if they did not.

Bruce, that's good news. But you must have a different parts manual than I have.
Mine, on page 54, shows the tailwheel spring for planes serial numbers 20806 and
on. My plane's serial number is 25174, so that picture should apply. It shows 4 leaves.

Buckeyejim wrote:Bruce, that's good news. But you must have a different parts manual than I have.
Mine, on page 54, shows the tailwheel spring for planes serial numbers 20806 and
on. My plane's serial number is 25174, so that picture should apply. It shows 4 leaves.

Per page 53 of the IPC, service kit SK70A52-4 can be used to update earlier model airplanes (20806 and earlier) to the thicker aluminum bracket from the steel one. The IPC shows four springs for both, but Bruce is saying that three is acceptable for the later style of bracket.

Buckeyejim wrote:Bruce, that's good news. But you must have a different parts manual than I have.
Mine, on page 54, shows the tailwheel spring for planes serial numbers 20806 and
on. My plane's serial number is 25174, so that picture should apply. It shows 4 leaves.

This is likely a subject which will haunt us for the rest of our lives... (I took the liberty of bold/italicizing your comment above)... to drive home the point:

We DO NOT HAVE A PARTS MANUAL for this airplane. We have a Parts CATALOG.

What is the difference between a MANUAL and a CATALOG?

A MANUAL is traditionally an approved document*, is instructive and useful for correctly assembling and servicing parts.

A CATALOG is NOT APPROVED, and only useful for ORDERING parts. DO NOT RELY UPON THE 170 PARTS CATALOG FOR ASSEMBLY because there are numerous errors throughout.

Some of the better known examples are:
The FUEL SELECTOR VALVE which will not be capable of shutting off fuel if assembled as depicted in the catalog.
The elevator cable flight control circuit which will result in REVERSED ELEVATOR CONTROL if routed as depicted in the catalog.

[The comments I made subsequently to the above was nonsense and Bruce kindly points that out below. I've deleted them to avoid confusing anyone further.)
Moderators note: George was no doubt lead by on by my earlier incorrect info when stating only 3 springs are used. All 170A abd B models used 4 springs regardless of the bracket mount style. - Bruce

Buckeyejim wrote:Bruce, that's good news. But you must have a different parts manual than I have.
Mine, on page 54, shows the tailwheel spring for planes serial numbers 20806 and
on. My plane's serial number is 25174, so that picture should apply. It shows 4 leaves.

Yep, I can't count to 4 when I expect to see 3.

Even just looked at the service kit and it pictures 4. The first 3 pages of instructions are missing. Would be interesting if they list 3 or 4 springs in the parts on page 1 of the SK

Bruce Fenstermacher wrote:Even just looked at the service kit and it pictures 4. The first 3 pages of instructions are missing. Would be interesting if they list 3 or 4 springs in the parts on page 1 of the SK

All seven pages are shown in the Maintenance Library. It doesn’t list the springs as the four old springs are reused. My new main spring is not twisted but it has a slight lateral curve to it.

The last time I changed the main spring I was able to draw the whole spring pack together with the U-bolt and install the 8 bolts without drama!

Maybe it's my imagination, but looking at your tailspring picture those leaves look thinner
than the ones Univair sent me. Did those 4 leaves come from Univair? Your coil springs look
like the ones I bought from Univair.

Buckeyejim wrote:Maybe it's my imagination, but looking at your tailspring picture those leaves look thinner
than the ones Univair sent me. Did those 4 leaves come from Univair? Your coil springs look
like the ones I bought from Univair.

Buck,
I have used springs from both Uni-ver and Air-repair, I am unsure which set is in the picture, and both companies spring sets have worked for me. I have not tried to use the L-19 spring, I have it because I have a new L-19 tailwheel spring mounting bracket I may someday put on the airplane.

For reference I have attached photos of a used 170 spring, a new 170 spring and a L-19 spring, hard to tell from the picture the thickness of the springs and my recall may not be the best but I think the L-19 spring is about .060" thicker than the 170 spring. The l-19 spring does have a slightly different bend angle at the end that the tailwheel mounts on.

Some of the confusion might arise from a difference in the thickness of the springs used in the 170B versus the L19/O-1 Birddog.
The PNs for the leaf springs for a Birddog are (from top to bottom) 0642104, 0642103, 0642102, 0642101... those are for the early O-1A and TO1A, and are mated to the bracket 0642140-3.

If that's not confusing enough, Later 0-1B and subsequent use bracket 0642150-10 and are mated to leaf spring PNs 0642157, 0642156, 0642154, 0642160.

None of those compare exactly to the PNs for the 170, which uses bracket 0512000-75 and which the IPC shows uses leafsprings 0442109-2, 0442108-2, 0542105, 0542106.

The SK 70A52 lists the parts to be discarded when modifying to the later aluminum bracket 0512000-75 and all 4 leaf springs are to be re-installed.

The problem we're facing is that when using the O1/L19 bracket (which happens to be a different PN than that listed in the SK70A52) ... those O1/L19 brackets are to be used with the O1/L19 leafsprings which are different PNs. Perhaps the problem involves a different thickness of the leafsprings, as it is well known that the Birddog main leafspring is definitely thicker than the original 170 main leafspring (and the others might be as well, frankly I have not had a chance to make a comparison in-person.)

As for the present, .... I suspect the thickness of the leafsprings have not been well sorted out due to the use of O1/L19 parts, especially since the bracket PN in the SK70A52 is so expensive and backordered at Cessna...that O1-L19 parts have been substituted in lieu of 170 brackets. This is all very speculative at this point, but may be a possible explanation.

[deleted line of bran-phartt commentary]

John, the two leaf springs you display are both 064XXXX Part numbers which indicate they are not 170, but O1/L19 parts.