The International Cessna 170 Association

Hi all,

I have reached the point of reattaching the engine to EI-AEN ('48 ragwing) and I'm confused (again).

Based on the 48 and 170A IPCs the required bolts are:
Bolts - (Attach Engine Mount - Upper Firewall) are AN6-22A with 2 washers - 1 of AN970-6 (big wide one) and 1 of AN960-616 ("regular" one).
Bolts - (Attach Engine Mount - Lower Firewall) are AN6-32A with 2 washers - 1 of AN970-6 (big wide one) and 1 of AN960-616 ("regular" one).

Note: reason for looking at the A model is the '48 IPC neglected the lower ones, the upper ones are identical in both. The A model IPC specifies this setup for (Serials 18003 to 19200) which includes mine.

The question: While the IPC calls for a single small washer, it looks like I'm going to need about 5 on each bolt to make this work (see picture). Am I missing something?

Thanks,

Gareth.

Gareth, Looking at the “A” parts book, the A uses the same bolt length for the mount but after s/n 19200 uses a rubber bushing arrangement and a large square washer to sandwich the rubber against the mount. The rubber isolator installation means there’s a larger diameter hole through the firewall and through the engine mount fuselage stringer, so its unlikely that was the arrangement in early models. Engine mount’s are the same p/n so I’m wondering whether the stringer mount fitting is thicker ?

Just looking at all the IPC’s, its a head scratcher. On your model the bolt is orientated, head forward, all other models after the addition of the rubber isolator, head is the other way around. What is underneath your bolt head ? If you look at the “A” model it uses a square spacer ( pad) p/n 0550154 to trap the rubber bushing on the fuselage side of the mount. The “B” uses a round pad to do the same thing.
Guess we need a true early model expert.

c170b53 wrote:Gareth, Looking at the “A” parts book, the A uses the same bolt length for the mount but after s/n 19200 uses a rubber bushing arrangement and a large square washer to sandwich the rubber against the mount. The rubber isolator installation means there’s a larger diameter hole through the firewall and through the engine mount fuselage stringer, so its unlikely that was the arrangement in early models. Engine mount’s are the same p/n so I’m wondering whether the stringer mount fitting is thicker ?

It might be - but it would have to be a full 1/4 inch for these bolts to work with only 1 or 2 washers.

P.S. The plot thickens!

It looks like the bolts I'm replacing had their threads extended. I'm guessing someone ran into this before and cut longer threads on all 4 bolts!?

Has anyone seen this on a ragwing?

So, two things. One you have the bolt, protruding out the firewall which is not as depicted in the two parts manuals. It doesn’t matter really but what is underneath the head ? The IPC also suggests and granted its an artists rendition the depicted washers sandwich the firewall but I think common dog suggests there should also be a washer under the head and the nut.
The 170A with the change in mount hardware; the spacer which I mention in the previous post (its from my memory, guesstimate) is about .100 to .165 thick. I can measure one tomorrow. Maybe a pic of the stringer side of the firewall mount might give up some info.

It’s my belief that aircraft hardware (such as bolts) are NOT “cut threads”.... but are “rolled threads” ....and therefore it’s not “kosher” to use a tap/die to extend threads. Cutting threads can create “stress-risers” which may lead to cracks.

Gareth,
I only have a B-model parts book does your 170 not use the rubber bushings on the firewall to engine mount bolts?

No, the '48 Ragwing does not use the rubber bushings.

I don't believe this is the only omission or error that you will run across in the '48 IPC.
Why not measure the grip length that you need, and acquire the appropriate length bolt?

It seems the bolt length remains constant in all models which suggests to me there was something taking up the gap, likely on the inside on the 48. We know on the late 170 and 170A model change in stringer mount design that a square spacer was used, on the 170B a round spacer is used. So its just a guess that the solution to understanding whats going on is to look at that early 48 stringer engine mount to see the difference.

Thanks for the feedback. I wanted to make sure that nothing important and non-obvious was missing and I'm happy that it isn't.

Agreed on the rolled vs cut threads - not an option to extend the threads.

Good catch on the back-to-front bolts issue. I installed them reversed so I could measure in comfort rather than crawling around inside the cabin! They have now gone in properly.

There are definitely no rubber bushings or pads on the 1948 ragwing or early A models 18003-19200.

I considered using AN6-20A on top and AN6-30A on the bottom removing the need for so many washers. However, I couldn't find them in stock in Europe and it didn't fancy spending $50 shipping and a week's delay for $7 worth of bolts. I'm using washers on the inside to pack out the length adequately before the nut goes on. I used 3/8in washers with 15/16in OD to spread the load on the inside. This is about the biggest washer that will fit inside the stringer and lie flat against the firewall. The result is a large bearing surface on the inside and outside of the firewall and enough but not too much thread showing through the nut.

Cheers,

Gareth.

The IPC [illustrated parts catalogue ] is NOT a authorised maintenance manual. It has a number of mistakes in it and you will get caught out. I know it’s a head scratching situation but with critical assembly [and some times some cost] contact Cessna and get a reference from their technical department . IF the original bolts were fitted correctly measure and order as such. You can’t go stacking washers on a bolt to get the correct grip length either. I would “NEVER” cut a thread on a rolled threaded bolt. This would induce stress raisers internally on the bolt. The manufacturing process of rolling a thread on a bolt increases its strength in the thread and also internally.
Note:: obtaining the correct bolts can be a very frustrating thing . Twenty years ago , I was trying to obtain the special NAS bolts to attach the tail plane on a Cessna 170a and couldn’t obtain them “anywhere” . I had professional “parts chases ‘ from a major airline on the job. Nothing anywhere . I will not use second hand bolts or nuts . Cessna had 7 of them in stock [2 different sizes] and they wanted $800 each. Yes eight hundred dollars US . I told the guy at Cessna I didn’t want them wrapped in gold just put them in a card board box . He hung up in my ear . Maybe I deserved it. 7 months later a friend found a box of them here in Australia at only 9 dollars each with paper work .

Gareth,
I have just reread your second last paragraph and feel what you are doing isn’t correct. and from memory [its fading ,I am 69 years old] that the fitting inside the cockpit against the fire wall and stringer has a slightly curved surface and then a small machined flat for the bolt hole for the washer and bolt. . I would only use the correct washer specified . I hope I have explained this clearly. This area of the fitting is prone to cracking . Even late model Cessna 172 ,s have had this issue.

I’m pretty sure Gareth knows there’s no maintenance manual period.

When my ‘53B model was restored from wreckage ....(run out of gas over “Cut and Shoot”, Texas)* .....it was found to have one of its’ bolts to have been assembled with a brass nut. (I’m not certain but that there may be several nuts with brass in Cut and Shoot.)

* https://en.wikipedia.org/wiki/Cut_and_Shoot,_Texas

Miss America 1983 was from Cut and Shoot, but having failed several times to win Miss Texas won the Miss America pageant by representing California.

You can’t make this stuff up.

Was at the hangar and remembered I was going to look into the 170A fuselage side of the engine mount attach hardware, specifically the square washer. It measures 1 x 1.125 and is . 100 thick. I measured the “B” round washer and surprise, surprise its also .100 thick.
Sorry Gareth I took so long to reply.