The International Cessna 170 Association

What's the approval basis for using structural screws apposed to rivets?

Also, are the Mcfarlane seat rails already spot faced? I thought I read that somewhere but, I can't confirm it on their website.

The McFarlene rails do not come spot faced but they will sell you the Screw Kit ($136).

bagarre wrote:What's the approval basis for using structural screws apposed to rivets?

bagarre wrote:Also, are the Mcfarlane seat rails already spot faced? I thought I read that somewhere but, I can't confirm it on their website.

As has been mentioned, the rails are not already spotfaced. McFarlane sells the spotface tool although you might shop around to find it cheaper.

DaveF wrote:I just replaced my pilot side rails. ... I used the drilled template strips under the floor as doublers.

I think a lot of cracks in the seat rails come from the fact that us pilots are generally a little heavier they were in the '50s. In my case about 25% heavier the typical (mythical?) 170 lb. pilot. This extra weight causes flexing of these seat tracks which eventually cracks them.I like Dave's idea of using a doubler. I'm planning on using a section of the old seat track inverted as a doubler under the floorboard between the landing gear bulkhead and the bulkhead at station 44.

Would someone be able to measure the exact distance O.C. between the seat rails? The rollers on my seats measure 11" apart laterally in front and about 1/2" less in back. My project was missing the floorboard sheet when I bought it.

They measure 11" even but they are not square to the floor skin. If you measure along the floor skin, they will be 1/8th or so too narrow.
Let me know if you need any more measurements,I have mine on the bench about ready to rivet the rails in.

https://photos.app.goo.gl/yN7RJwQkhaBeHiBl2

Dave, Thanks for measuring that for me. Not sure I follow the "not square with the floor skin". Could you elaborate? Your work on the floor/seat rails looks fantastic! Makes sense to add the rails before riveting the floor skin in place.

If you look at the photo, the rails are not parallel to the edge of the floor skin. They are of course parallel to each other but the are on the floor at a slight angle.
If you need an old skin as a template, I still have mine. I could make a drawing to locate the rails and big holes if that would help.
I’d ship you the floor but I don’t think it’d be worth the cost unless you were to put it back in to save making a new one. It’s usable, just more banged up than I wanted and I’m getting pretty good at duplicating skins.

Dave, I thought that was what you were referring to but the pictures made it look like the rails are a lot more than an 1/8" out of parallel from the floor edge. Probably an optical illusion with the photo angle. That drawing of the floor sheet would really help. Thanks.

The seat rails are different by an inch or so. I'll get the measurements today and post here.

If you measure 11 inches along the edge of the skin to get your rail centers, you will be off by 1/8th or so because the rails are not perpendicular to the skin edge. That's what I meant by 1/8th.
The measurements will make more sense with a photo later today.

https://photos.app.goo.gl/yN7RJwQkhaBeHiBl2

I added more photos to the link that show more of the measurements.
The floor is 2024-T3 0.032 38" x 35" square. I put photos of the seat rails with a ruler on them. They center on the rivets so if you make sure the center of the rail is between the rivet holes, it's hard to mess things up.

Just remember to square the ruler to the RAIL when you measure 11 inches vs squaring the ruler to the skin. I know Im saying this a lot but it's because I nearly mis-drilled the whole thing twice while setting things up (measure twice, drill once)

Flanging the square hole was a PIA and didnt come out as nice as I'd like. To do it again, I woudlnt bother flanging it - I'd just put a doubler around it.

On another note - consider adding nut plates for the seat belt brackets to save some skin trying to get those nuts started.

Beautiful work, David!

Thanks for the sketch with the measurements. Could you add the other rectangle measurements and position and the position of the fuel valve circle? Again, beautiful work!

Wow. That sure looks a lot easier with it sitting on the bench. And less painful.

Nice work.

falco wrote:Wow. That sure looks a lot easier with it sitting on the bench. And less painful.

Nice work.

I confess to wondering if it would be easier to remove the floor skin, replace the rails on the bench, then reinstall the assembly.

While replacing the seat rails it’s also prudent to inspect and measure the seat retaining tangs. New tangs are obtainable from Mc Farlane . When I replaced 2 of my seat rails [should have done 4 ] , we replaced the rollers also so all assembled and shock when we saw what little was holding the seat to the seat rails . A AD was released about 3 months later advising the inspection of the seat tangs .