30 min epoxy on a 54mm motor retainer

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dhkaiser

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At this time I don't intend to fly any 54mm motors, I plan on a 38mm adapter. How bad did I screw up? Will it be ok on 38mm motors?
 
At this time I don't intend to fly any 54mm motors, I plan on a 38mm adapter. How bad did I screw up? Will it be ok on 38mm motors?

Retainers have flown on epoxy before, although JB weld is the manufacturer's recommended due to it's heat tolerance.
 
I would not recommend long burn motors though, as Nytrunner says JB Weld is recommended. Actually having used regular epoxy a heat gun might allow you to soften the epoxy enough to get the retainer off, then clean up the tube and retainer and re-attach with JB weld.
 
Have used 30 min epoxy on my 75/5120 amraam level two flight motor mount with no problem!
 
Epoxy is fine. It's all I use, up to 98mm. Unless you have ejection pressure, there's really nothing forcing the motor out.
 
I've been curious about whether JB Weld is actually that much better than regular epoxy for a while. We have a lot of lore about that, but there are reasons for doubt.

Data is scarce. There is a seemingly genuine data sheet here: https://www.aeroelectric.com/Mfgr_Data/JB_Weld/JB-Weld-Technical-Datasheet.pdf . It repeats the main website's claim of 500/600F continuous/intermittent max temp, but doesn't give Tg (basically the softening point). Instead they quote an ASTM D648 deflection temperature of 102.5F. That is practically room temperature and is not a good proxy for a Tg rating; showing that but not a Tg could be an obfuscation.

The JB Weld tensile strength is given as 3980 psi, which is poor at about 40% of the value for Aeropoxy etc.

On the MSDS, (available at https://www.jb-weld.co.uk/images/technical-data-sheets/Steel_Reinforced_Epoxy_Twin_Tubes.pdf, the decomposition temperature is listed as ">220 C (>428 F)" which is lower than the claimed max continuous operating temperature. To me, the inconsistencies and lack of a Tg spec raise a red flag about the marketing claims. Genuine, documented 500F Tg epoxy like Cotronics is extremely expensive and I think there is zero chance that JB Weld is actually in the same league.

Anyway, back directly to topic, I think you could use just about any reasonable epoxy for retainers. They are only exposed to significant heat for 10-20 seconds before ejection, and the ejection event is a single (possibly double) pressure impulse. And it starts to soften at Tg; if it got brittle there would be more cause for concern. For 75mm and up I recommend the flange type retainers.
 
With a little heat (from a heat gun) the epoxy will soften quickly, making the retainer easy to remove. Heat softens most common epoxies, quickly. If you have never tried it, it might surprise you just how easily. A little scraping action followed by some sandpaper will clean the end of the tube, and an appropriate tool will clean the i.d. of the retainer ring. Scraping it back down to metal, and then a little sanding on the i.d. will have it ready for some jb weld.
 
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