Vintage M20 Upper Door Latch

[Guest Anonymous]

Has anyone had occasion to work on the upper cabin door latch, and can they provide any insights.  During a cross country last weekend, the upper door latch failed to open after shutdown.  It would "loosen" sufficiently that I could get a screw driver in front of the hook, and then pull it back to get the door open, but it would not move rearward on its own.

I have not had it apart yet, but looking at the parts manual, I suspect that the cotter pin holding up the bushings that run through the slot in the hook and underneath it broke off.  I am pretty certain that the back (outside) pin that fastens to the hook is OK because the door still latches correctly and the seal is tight once it is closed.  So, the push pull rod assembly must still be working, and also the linkage as far as the pivot arm and the rear (outside) linkage from the pivot arm to the back of the hook. 

Does anyone have any tips and tricks as to what to expect once the door liner is pulled and the upper door latch bracket assembly  is exposed.

Thanks,

RFB

[eaglebkh]

I only have 1 door latch - didn't know there was an upper latch on some models.

[Guest Anonymous]

My '68 has a pin that is at about the same altitude as the door handles that moves into the door frame when the handle is put in the closed and locked position.  It also has a "Hook"  near the center of the top of the door that moves forward to engage a strike pin in a slot in the airframe door frame.  The "hook" engages the striker and then as the handle moves to the closed and locked positiion, the "hook" is drawn back toward the door, thus drawing the door tighter against the airframe and compressing the door seal.  I did not know that earlier models had only the pin.

The parts manual I have shows the identical setup, with some minor differences by year, on C,E,F and G models, but I only have parts data for 66-69 models.

RFB

 

[eaglebkh]

Ah, I have the "hook" too.  I though you were talking about a separate latch handle for the top, like what Cherokees have.

[Guest Anonymous]

I finally got into it and took it apart this weekend.  It turns out that there is an overcenter mechanism that provides the "lock" for the door system (both pin and latch) in the upper latch assembly.  In mine, the cotter pins that hold the linkage clevis pins in the two rotating components had fatigued and bent, permitting the washers above them to back off and permitting the clevis pins to wobble excessively in the linkage, causing excessive play.  Replacing the washers and cotter pins removed the excess slack and the latch is working now.

For those with door closure problems, the upper latch assembly is a bit of a B**** to get out.  I strongly suggest removing the clevis pin on the cable turnbuckle first, even though it is tight.  It is in a tight space but can be reached.  I used a magnet to hold the pin while I removed the cotter pin and washer, then retreived the clevis pin from above using the magnet.  (Don't worry if you drop it, it is easy to retreive with a magnet retreiver even with the latch still in place.)  Once this link is removed, the mechanism is easy to remove.  Just remove two AN bolts under the door seal at the top and the sheet metal screws holding the bottom latch plate to the door frame skin.

Assembly is in reverse order.  The turnbuckle on the cable end may need 1/2 turn one way or the other, but it should not take more than that to get everything working correctly.  Shortening the cable with the turnbuckle will tighten the door seal, turn the turnbuckle out and the latch moves further aft when open due to its eccentric pin.  If the cable linkage is too long the latch mechanism will not go overcenter, and the door will not lock.  If the linkage is too short, the door will lock, but with insufficient travel on the lower door pin and outside door handle.

RFB