The International Cessna 170 Association

Below are a couple pictures of my primer plunger. The plunger portion works great, but when I go to rotate the plunger to lock the primer the plastic knob just spins on the shaft. I end up having to wiggle it while trying to rotate it to get it locked.

Does anyone know how the plastic knob is attached to the shaft? Or how can I remove the plastic knob without damaging it?

I did find this thread with the same issue where they used some CA to glue it. I may have to do the same thing, but if anyone knows how the knob comes off I would like to know. viewtopic.php?p=122940

Thanks,
Doug

Doug,
Here’s a couple of pictures of the primer plunger disassembled. Click on photos to enlarge them.The primer I bought online apparently suffered the same fate yours did. Someone tried gluing and then drilling a hole for a roll pin to keep the knob from spinning. Note that the knob is secured in tension by serrations on the brass fitting. This is similar how the other panel knobs are secured to their bowden cables. What is supposed to keep the knob from spinning on these serrations is a pair of crimps in the serrations as can be seen in the second photo. To separate the brass fitting from the aluminum plunger gently pull out the small pin that locks the plunger in the closed position and then unscrew the brass fitting. The knob can be removed from the brass part by heating the brass with a propane torch while pulling on the knob. I haven’t decided on the best fix for this. Maybe use a strong Loctite together with inserts as described here.

The link you provided is an excellent article, Gary. Thanks for posting it!

Another possibility might be good ol’ JB Weld.

Gary,

Those pictures are gold. They explain the situation perfectly. Thanks for taking the time to share. I'll ponder on my options now that I have more info.

Doug

Glad to help. Let us know how you repair this Doug. To prevent the knob from spinning, it seems it would have been a better design if the knob was pressed on a shaft with a square cross section.

So I decided to punt the final fix on this issue and just bought some thin CA glue and put a few drops down between the knob and the shaft. It has been working well for the last dozen flights. It is super nice being able to easily rotate the primer to lock it again.

If the glue doesn't last I'll have to figure out what my next step will be.

Doug

dstates wrote: ↑Wed Apr 17, 2024 1:09 pm So I decided to punt the final fix on this issue and just bought some thin CA glue and put a few drops down between the knob and the shaft. It has been working well for the last dozen flights. It is super nice being able to easily rotate the primer to lock it again.

If the glue doesn't last I'll have to figure out what my next step will be.

Doug

I visited the WentWorth Salvage Booth at AirVenture/Oshkosh…and left with what appeared to be a NOS primer assy…. paid $5. (to go into the SkyHawk trainer in order to get rid of the goofy-primer-knob “fix” some previous owner had concocted.)