The International Cessna 170 Association

We are looking for data points to try to determine the thickness of each spring in the tailwheel spring pack. If you suspect your 170 has some old perhaps original springs. Please try to measure them as close as you can and report back with their thicknesses. We will use share results of our data collection with Univair in hopes they can support us with a better fit spring pack.

We are looking for all data taken. From very early indications it appears the top spring should be under .200", we have samples of .180", 190" and .196". The second spring down should be about .180" to .190". The third spring should be between .235" and .254". The main spring should be .312". We are looking for the total thickness of the spring pack to be very close to .951"

What do you have. Thanks for the effort.

Two old main springs: both .312

Currently installed:
top: .170
second: .170
third: .233
main: .322 bought new from Univair in 2018

All measured with a digital calipers. Main springs measured in caliper mode, upper springs measured using the extendo-thingy on the end.

The first time I replaced the main spring, everything went back together exactly as it came out. In 2018 it was a tighter fit and I had to adjust the shimming to make it work. Now I see that's because the new spring is thicker. I'm referring to the forward shim that sits on the top of the spring pack, parts catalog figure/index 29-10, p/n 0510000-30, "shim". The aft shim fixed to the bottom of the bracket is present.

The logs have no entries to indicate that the second, third, and fourth springs have ever been replaced.

EDIT: See revised measurements in post below.

DaveF wrote:Two old main springs: both .312

Currently installed:
top: .170
second: .170
third: .233
main: .322 bought new from Univair in 2018

That totals .895 which quite a bit less then the .951 that Bruce was looking for. Seems like it would be a lot easier to install.

Here are the thicknesses of the springs I took off my 170A. It has ~1300 hrs total time and the plane was in storage from the mid 1960’s till 2003 so I am confident they are old.

Measurements taken with digital calipers.

Top: 0.190
Second: 0.190”
Third: 0.250”
Main: 0.322”

I purchased a new set from Univair. I can measure those this weekend. The new stack is thicker. You can see that in the second picture.

Doug

DaveF wrote:Two old main springs: both .312

Currently installed:
top: .170
second: .170
third: .233
main: .322 bought new from Univair in 2018

All measured with a digital calipers. Main springs measured in caliper mode, upper springs measured using the extendo-thingy on the end.

The first time I replaced the main spring, everything went back together exactly as it came out. In 2018 it was a tighter fit and I had to adjust the shimming to make it work. Now I see that's because the new spring is thicker. I'm referring to the forward shim that sits on the top of the spring pack, parts catalog figure/index 29-10, p/n 0510000-30, "shim". The aft shim fixed to the bottom of the bracket is present.

The logs have no entries to indicate that the second, third, and fourth springs have ever been replaced.

Re-measure only using the normal caliper function, not the depth probe. Measured with the spring pack on the airplane, which is not the best way to do it.

Results, top leaf to main leaf:
0.193
0.192
0.233
0.322
total: 0.940
This agrees a lot better with what we expect

I was unable to directly measure the full thickness because the (new) main leaf has the wrong bend and doesn't nest with the other springs. Between the thickness and poor bend, it was hard to get the pack into the bracket. That's the best one I could find, even sorting through Univair's box of finished goods springs in November 2018. I hope they get their act together on this.
Method, in case you're interested:
Measure main leaf directly: 0.322
Measure sum of main and second: 0.555
Measure sum of main, second, and third: 0.747
Measure second, third, fourth: 0.618

Main leaf = 0.322
Second leaf = .555-.322 = 0.233
Third leaf = .747-.555 = 0.192
Top leaf = .618-.233-.192 = 0.193

Thanks Doug and Dave. Doug, you old pack would fit my new bracket. Dave, yours will not.

Bruce Fenstermacher wrote:Thanks Doug and Dave. Doug, you old pack would fit my new bracket. Dave, yours will not.

Why? Our packs are about the same -- Doug's is 0.952, mine is 0.940. The difference is within the tolerance of the paint thicknesses. When I installed the new thicker spring, I had to use a 0.025" forward shim instead of the previous .032".

DaveF wrote: Why? Our packs are about the same -- Doug's is 0.952, mine is 0.940. The difference is within the tolerance of the paint thicknesses. When I installed the new thicker spring, I had to use a 0.025" forward shim instead of the previous .032".

Cause I can't operate a calculator I guess.

Thanks Dave.

Well, I didn't help by posting totally wrong numbers in my first post!

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

One other interesting tidbit; all of these Cessna NOS leaf springs were already chamfered/beveled, so as not to wear into the springs below them. I've not seen this with any aftermarket springs.

I'm guessing these are the only full set of OEM new springs in the wild. Maybe someone else has some, but these are definitely better quality than anything I've seen before.

Good stuff Del. Really appreciate it.

Checked the old springs, mainspring wasn’t old, spring above main .245, above that .190 lost the one above that. The springs were particle blasted so I guess with primer and paint to both sides might add .005 to the dimensions.

I just removed mine that have been on this aircraft for 520 hr. (2005) I measured old and the new Univair that I have had for two years on the shelf. It took longer to reach 500 hrs than I thought. Also my new springs where not fitting and I was trying to figure out what to do when I discovered this in the retaining channel new one on order. The thickness of the old springs starting with the top going down to the longest:
1- .195
2- .193
3- .254
4- .323
New Univair Springs :
1- .223
2- .225
3- .273
4- .330
My understanding is the new channel will not be drilled so I should be able to fit the new springs with a custom drilled channel. (I hope)
I hope you can see the crack in the corner.

I would like to reemphasize that replacing ALL the springs is not particularly beneficial IMO.
I recommend only to replace the lower, main spring. It’s the one that breaks, not the others, and that will likely avoid the thickness problems of the new spring-stacks.