Main Tailwheel Leaf Spring Comparison

How to keep the Cessna 170 flying and airworthy.

Moderators: GAHorn, Karl Towle, Bruce Fenstermacher

User avatar
GAHorn
Posts: 20968
Joined: Fri Apr 12, 2002 8:45 pm

Main Tailwheel Leaf Spring Comparison

Post by GAHorn »

After reading the posts regarding “too thick” mail tailwheel springs from Univair, I decided to run a personal review and ordered a fresh, current-production additional spare from Univair to add to the one I ordered as a spare ten years ago. I compared them to what is believed to be the Original Cessna Spring on a 170B sent to me by a Member a few years ago (for inspection purposes) as well as the one which was on my restored 170B when I purchased it. (I had replaced this one at 500 hours per our own recommendations.)

The Cessna springs were painted with what is believed to be polyurethane. The Univair springs are as received, painted in what appears to be black enamel. Therefore, although not “excessive” …. paint thickness is included in measurements and likely influences results.

The springs were measured where paint was unmarred as near to the upper and lower mounting holes as possible without engaging any “cut/punched” edges, I.E., on smooth, un-worn surfaces.

Here are the results using a digital caliper:

OEM Cessna Springs:

#1 Upper: 0.3165”
#1 Lower: 0.3295”

#2 Upper: 0.3210”
#2 Lower: 0.3160”

New/Unused Univair:

#1 Upper: 0.3180”
#1 Lower: 0.3230”

#2 Upper: 0.3225”
#2 Lower: 0.3175”

With these results I would not expect any difficulty in replacing springs… However i noted the used springs had taken a “set”. I.E., the new springs had sharper, more acute angles at bends …whereas the “original” springs had a very slight but noticeable “flatness” … they likely had “relaxed” slightly over time. When “nested” there existed about .020 space between the new and the “original” springs at mid-point. This shouldn’t post a problem.

Hope this helps.
'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. ;)
User avatar
Hawkeyenfo
Posts: 266
Joined: Tue Aug 14, 2007 4:25 am

Re: Main Tailwheel Leaf Spring Comparison

Post by Hawkeyenfo »

Thanks for the research!
Fly Navy !!!!

1941 Boeing PT-13D Stearman
1952 Cessna 170B
1960 Piper Aztec (PA23-250)
User avatar
Bruce Fenstermacher
Posts: 10313
Joined: Tue Apr 23, 2002 11:24 am

Re: Main Tailwheel Leaf Spring Comparison

Post by Bruce Fenstermacher »

This thread is what we found a few years ago:
viewtopic.php?f=24&t=15043
CAUTION - My forum posts may be worth what you paid for them!

Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
User avatar
GAHorn
Posts: 20968
Joined: Fri Apr 12, 2002 8:45 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by GAHorn »

Just a great piece of discovery by Del Lehmann (quoted from the other thread to consolidate data):
wingnut wrote:I have a complete new old stock tail cone assembly for a 170B. It still had the original Cessna pick tag wired to it when I bought it, and also included the leaf springs.
I have reason to believe the springs have been with the tail cone assembly since it left Cessna.
The springs measure as follows;
#1 (bottom main leaf) is .312 thick, forward bend is 10.8 degrees, aft bend is 9.3 degrees
#2 is .250 thick, bend is 11.7 degrees
#3 is .195 thick, bend is 13.9 degrees
#4 (top) is .185 thick, bend is 14.7 degrees
The averaged measurements from the data I posted to begin this thread works out to be a (painted) main spring average thickness of 0.320
This would imply a paint thickness of .008

If you find the main spring you are trying to install is greater than that…you likely have an L-19/O-1 BirdDog spring …not a 170 spring.
'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. ;)
User avatar
Bruce Fenstermacher
Posts: 10313
Joined: Tue Apr 23, 2002 11:24 am

Re: Main Tailwheel Leaf Spring Comparison

Post by Bruce Fenstermacher »

GAHorn wrote:
The averaged measurements from the data I posted to begin this thread works out to be a (painted) main spring average thickness of 0.320
This would imply a paint thickness of .008

If you find the main spring you are trying to install is greater than that…you likely have an L-19/O-1 BirdDog spring …not a 170 spring.
That or a spring made out of the closest thickness material available today for the spring manufacturer (Univair) to use. It appears that knowledge of the correct thickness may have been lost by Univair and over time the thickness of available material got thicker to the point the springs they were making wouldn't fit depending on the thickness of the other springs in the pack.
CAUTION - My forum posts may be worth what you paid for them!

Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
User avatar
GAHorn
Posts: 20968
Joined: Fri Apr 12, 2002 8:45 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by GAHorn »

Bruce Fenstermacher wrote:
GAHorn wrote:
The averaged measurements from the data I posted to begin this thread works out to be a (painted) main spring average thickness of 0.320
This would imply a paint thickness of .008

If you find the main spring you are trying to install is greater than that…you likely have an L-19/O-1 BirdDog spring …not a 170 spring.
That or a spring made out of the closest thickness material available today for the spring manufacturer (Univair) to use. It appears that knowledge of the correct thickness may have been lost by Univair and over time the thickness of available material got thicker to the point the springs they were making wouldn't fit depending on the thickness of the other springs in the pack.
If (the above is) true….then Univair will not be in compliance with their FAA-PMA. The reason I started this thread was to confirm that currently-ordered/received Univair product conforms to original and previous products that fit.

At any rate, anyone ordering a Univair spring (either direct from Univair or from one of the distributors who offer the Univair product such as Spruce, etc.) might wish to confirm spring thickness upon receipt and Before any attempt at installation…. in order to return it for replacement.
'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. ;)
User avatar
interstellardust
Posts: 70
Joined: Sat Aug 28, 2010 11:36 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by interstellardust »

I was getting ready to replace the tailwheel leaf springs in my 1955 170B per the 500 hour recommendation but after reading through three different vintages of discussion about that process here I am a bit nervous. The various issues about thickness being a problem with fit may or may not be resolved. When I looked at the Univair web site today, the photo of the sprink package actually shows the bend in the main leaf to be at a different angle thaan the other 3 leaves. The phot makes it look like this makes the package thinker in the middle which would lead to the installation issues discribed in the various threads. Some other phots posed here don't look like that, What's the latest? reusing the other three leaves wouldn't solve the problem is the angle of bend is the issue.
Attachments
u170AB__98357.jpg
Bill Garnett
1955 Cessna 170B N2974D
bill@interstellardust.com
User avatar
redacted
Posts: 96
Joined: Wed Jun 15, 2022 4:31 am

Re: Main Tailwheel Leaf Spring Comparison

Post by redacted »

I’ll repost, my new spring pack from Univair bolted up no problem. Took a month to get cuz they had to be made. Also ordered a spare main spring right away cuz why not.

Has anyone is the past 3 months got a spring pack from Univair that didn’t fit? Fly bleep in the pepper. Buy a set and install. This is getting way too deep in the weeds.
Nathaniel Perlman
1952 170B
N2282D S/N 20434
User avatar
dstates
Posts: 472
Joined: Sun Jun 30, 2019 5:50 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by dstates »

Bill,

I got a full set for my 170A with the original steel tailwheel bracket (not the fishmouth) and it fit up fine. Depending on the vintage of your 170 and if you have the later model fishmouth bracket that could determine fitup as well.

Wouldn't hurt to take a caliper to your current main spring and compare the thickness to what George posted above for the parts he just received.

Doug
N1235D - 1951 170A - SN: 20118
User avatar
interstellardust
Posts: 70
Joined: Sat Aug 28, 2010 11:36 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by interstellardust »

Thanks Nat and Doug. It sounds like Univair has gotten past any previous fitment issues. I'll go ahead and order a set. It's almost annual time.
Bill Garnett
1955 Cessna 170B N2974D
bill@interstellardust.com
User avatar
interstellardust
Posts: 70
Joined: Sat Aug 28, 2010 11:36 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by interstellardust »

I need to update my experience with Univair and tail spring fit and ask a question...

I ordered a full new of tail springs (their part number U170AB) and the tailwheel bushing they say is a match (their part number U3241A-002) from Univair on November 5th, 2022 after the discussion above. No news, but I waited until after the holidays and called them in January. I was told they were having trouble getting the required material to make the springs so it might be another couple of months. A few month later I called again. They were still waiting on spring material. I called again at the end of September. The springs were sitting there supposedly waiting on the bushing to ship. They went in he back and looked and the bushing was also in stock so the shipped both on September 29th. I'm not sure why parts were just sitting, but OK.

Now I have the parts but am not sure either fits as required. The angle of bend on the various leaves is not the same so you can see about 1/8" of light between each spring and the pack is thicker towards the middle. Two A&Ps on the field took a look and said with weight on the tail they MAY flatten out to match but they don't know for sure. I tied pitting the pack in a vise and as tight as I could get it there is still space between the leaves. Given the fitment issues I have read about in other threads, I'm a bit worried.

The bushing also looks way to thin to do it's job. It doesn't come close to filling the hole in the springs.

I was kind of waiting for annual inspection in January, but I'd like to know if it will be a problem before I take things apart
Attachments
Univair C170B Tailwheel spring bushing.jpeg
Univair U170 tailspring.jpeg
Bill Garnett
1955 Cessna 170B N2974D
bill@interstellardust.com
User avatar
Bruce Fenstermacher
Posts: 10313
Joined: Tue Apr 23, 2002 11:24 am

Re: Main Tailwheel Leaf Spring Comparison

Post by Bruce Fenstermacher »

From what I can see the bushing fits fine. It does not need to be a tight fit. Also from what I can see your springs fit as closely as any spring pack I've seen when not under compression and installed.

In order to install the pack you pile up the pack and place the bushing in place. The take a large C-clamp and compress the pack together just after the first top bend. Install all the bolts in the spring braked and tighten, then release the C-clamp. At this point there may be some separation between the springs. Once the tailwheel is install and on the ground most of it will disappear.
CAUTION - My forum posts may be worth what you paid for them!

Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
User avatar
n2582d
Posts: 2808
Joined: Mon Apr 29, 2002 4:58 am

Re: Main Tailwheel Leaf Spring Comparison

Post by n2582d »

Bill,
The U3241A-002 bushing is Univair's p/n for Scott p/n 3241-2 or Cessna p/n 0442011-2 as seen in fig. 94-3. This is the bushing that fits at the back end of the spring as shown here. I believe your photo is showing the forward end of the spring pack. There the bushing (p/n 0510000-36 in fig. 29-5) has an O.D. slightly smaller than 5/8" and an I.D. of roughly 3/8" as seen here.
Bushing 0510000-36.png
Univair only wants $931.70 for it. I'd suggest calling Air Repair or your local machine shop if you need that bushing. The L-19 p/n may be 0642135 but not sure as one version had a thicker main spring. Air Repair would know.
Gary
User avatar
interstellardust
Posts: 70
Joined: Sat Aug 28, 2010 11:36 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by interstellardust »

Thank for the reply Bruce, and N2582D,

Perhaps you are right and I will be able to compress the leaves with a C-clamp enough to get the bracket bolted back together. I put the pack in a vise and was able to compress it some but not all the way. Time will tell. I'll test with a C-clamp.

As I was about to hit enter, I got N2582D's response and photo. My picture of the U3241A-022 bushing was indeed in the holes in the front of all 4 springs and the hole is about 5/8" in diameter. 3/8" ID would be about right for the AN6-22A bolt. I hope he is kidding about the $931.70 price or I'll be inspecting and reusing the old one or going elsewhere to replace it. The Parts Catalog does show part number 0510000-36.

Having said that, the U3241A-002 bushing won't go through the hole in the back of the tail spring at all. The hole is about 7/16" and the bushing is 1/2" OD. The ID is about 7/16" and would fit the AN7 bolt that holds the tailwheel on. However, the Parts Catalog does not show a bushing at the tailwheel end.

The current tail spring is 25 years old and has about 900 hours on it, so it's due for replacement. The original main leaf lasted 43 years and 4595 hours before it broke but I don't want that to happen again.

None of the A&Ps on the filed know as much as I do about 170s which is why I'm asking here. I'll place a call to Univair to see if they have any other information about part 0510000-36.

Bill
Attachments
Tailwheel diagram Catalog pg54.jpeg
Bill Garnett
1955 Cessna 170B N2974D
bill@interstellardust.com
User avatar
sfarringer
Posts: 309
Joined: Wed Jan 12, 2005 10:49 pm

Re: Main Tailwheel Leaf Spring Comparison

Post by sfarringer »

Isn't that spacer to bush the hole in the Scott 3200 bracket and spring adapter (not the tailspring)?
The spacer which goes in the hole at the front end of the spring, and ties the spring leaves together, should be a fairly close fit to the spring stack and to the bolt.
Ragwing S/N 18073
Post Reply