AN509 - Screw head size

[JanRV6UK]

I am just about to bolt on he tank and have noticed a inconsistency in the size between a old AN509 screw (supplied by Vans 15 years ago !!) ? and its current replacement MS24694. The MS head is slightly bigger, not a lot ? 0.016 ? but as you can see from the photos below .. on the CS it does make a difference ? I CS all the holes for the AN509 ? (I bought a bag of MS24694 some time ago from a reputable supplier just to have) ?.

Have anyone else experienced this ? Currently the CS is cut to a knife edge ? and the original AN509 is a good fit ?. I am reluctant to CS any more .. just for the MS24694 ? The dimple is fine for both as it is deeper ?.

Have anyone else experienced this ? I know many people replace the AN509 for stainless ?. What is the head dia on the stainless ?? The AN509 is 0.299 The MS24694 is 0.315

On most tank installation ..is the CS bolt head dead flush ..??

I have plenty of the original AN509 ?so not an issue as it is ? just curious what others have done here ? Cut deeper to get is dead flush ? or leave as is ?

 

[ChiefPilot]

Thanks for the heads up - I have lots of CS holes and a few dimples for the AN509 screws but haven't installed them yet (tank attach, emp. fairing, wing root fairing, etc.). I am definitely going to check on this today when I'm at the hanger!

 

[GAHco]

The specs were made to....

MS24694 or NASM24694

On the latest official spec, the head height for the 8-32 is .068 +.015 -.000

So because of its size the percentage of variation is i big percentage in my opinion.

The good new is, if they are all from the same lot, they should be fairly consistent within the lot.

If the dimples are set to the deep tolerance, they look and feel better if you screw heads al slightly under vs protruding. IMHO.

Best wishes.

 

[az_gila]

Could something else...

...be going on here?

The fillet radius at the base of the head has a tolerance of 0.005 to 0.015 inch.

Try a test piece with the screw hole enlarged so that the fillet is not resting on the bottom of the skin countersink. Perhaps this fillet is the difference?

The AN509 spec was made obsolete in 1959, so I'm not sure if any new parts are really made to that actual specification any more. Even the govt. doesn't archive a copy on-line of the original AN509 spec...

 

[JanRV6UK]

Thanks for the feedback Tom and Gil .. I will do a bit more investigation tomorrow after work - The material one have to play with here is the back baffle plate (0.050 from memory + 0.032 for the tank seal + Proseal .001 or .002) .. That adds up to .083 or 0.084

 

[JanRV6UK]

Well I spent some time measuring holes and screws …

The tank skin and the rear back baffle is .032 + .032 + thickness of the pro-seal

I measure a total thickness of .069 to .072 … obviously due to change in pro-seal ..

Now my “old” screws …

Head dia = 0.302
Head height = 0.069
The hole dia in the Alu today is 0.065

My “new” screw

Head dia 0.315
Head height = 0.076

So clearly the old screw will fit the CS hole the best ….. or I have to cut another .005” to get a good flush surface ….

The dimpled holes are better … could use the “new” screw in this location ….

As we say in Norway … “you can always cut a sausage that is to long … but hard to stick it back on” … loosely translated

I think I leave the CS holes as is …and use the “old” screw … and the new in the dimple holes .. I can always open the hole if needed later on ….

 

[az_gila]

The question is...

Well I spent some time measuring holes and screws …

The tank skin and the rear back baffle is .032 + .032 + thickness of the pro-seal

I measure a total thickness of .069 to .072 … obviously due to change in pro-seal ..

Now my “old” screws …

Head dia = 0.302
Head height = 0.069
The hole dia in the Alu today is 0.065

My “new” screw

Head dia 0.315
Head height = 0.076

So clearly the old screw will fit the CS hole the best ….. or I have to cut another .005” to get a good flush surface ….

The dimpled holes are better … could use the “new” screw in this location ….

As we say in Norway … “you can always cut a sausage that is to long … but hard to stick it back on” … loosely translated

I think I leave the CS holes as is …and use the “old” screw … and the new in the dimple holes .. I can always open the hole if needed later on ….

...how did you measure the head height when there is a variable radius at the base of the head?

Try a new screw in an old size countersink, but with a hole that is 0.025 or so larger than a #8 screw - a test piece of course - a drill size of #22 should do it.

Your dimpled holes are actually larger than the #19 drill you probably used for the countersunk holes, so that gives a hint...

 

[JanRV6UK]

The joy of measing screws

I tried to measure from the top of the head to the point were the small internal radius start ? I was carefully sliding the jaw of my vernier down the screw shank ?trying not to ride up the small internal radius ..

Well done a bit more screw checking this evening

1 ? I took a 1/8 thick strip and CS using my original setting. The OLD screw did go in fine..but the NEW screw sat high

2 ? I then re-adjusted my CS cutter ? + 5 clicks (I believe that is 0.005)

3 ? When re-cut ? the NEW screw sits fine ?.

4 ? The OLD screw is sitting to deep.

As my test strip is 1/8 thick ? no knife edge ? The hole dia is 0.165 ? same as the tank hole ? in the CS cut hole ? Yes the Dimpled hole is bigger but so is the dimple it self.

I also tried to increase the hole dia .. to see if the NEW screw in the original CS hole would do the trick ? Drilled #13 .. (bigger hole than the original #19) ?Hole is now 0.185 ? made no difference ? NEW screw still riding high ?



Well I think I have spent enough time measuring screws ? better get some building done ?.