What causes cracked coils?

I've had problems with my ignition only when it rains. I hardly ever get caught in the rain, so I haven't really worried about it. Well, since I'm planning a cross state trip next weekend I thought it would be a good idea to look at my system a little closer to see what gets wet.
When it rains, I lose both outer cylinders, so tracing the wires, I figured it had to be my left hand coil.
I discovered a crack extending radially around the front end of the coil, the same end the primary wires go into it. I also looked at the R/H coil and it too is cracked, but not all the way through into the windings.
For my trip I just cleaned up the coils and smeared some silicone sealer around the cracks as a temporary measure. I can't test it until it rains, but I'm reasonably sure that this is my problem.
So does anybody know why both of my coils would crack in the same exact place? Age? Heat? Vibration?
I can't tell if they are the original coils or not, but the mounting screws do show signs of screwdriver abuse.

id say heat and vibration are main causes.

Thanks Plummen, that's kinda what I figured. But it's just kind of weird how they both are cracking in the same place, and I was wondering if maybe it was because of the way they mount to the frame. I was also wondering if maybe any of the aftermarket coils were cracking in the same place. Maybe they should be shock mounted with bonding jumpers (flexible grounds) installed? But if the aftermarket coils aren't cracking, I won't bother. After all, my coils may be 30 years old and it's about time they died anyway.

i always thought that was a stupid place to mount the coils with all the heat coming off of motor constantly baking them,i like them mounted under seat myself.

Alot of times you can simulate the rain with a spray bottle, that way you can isolate where the water goes for more accurate repair

here was mine on the 650. i second the age and vibration. heat does not help either. i just got a used set of accel coils off ebay and have been real happy

I like that idea of putting them under the seat. But when I buy coils, they're gonna be brand new. And if I lose the air silencer, and maybe go to pods also losing the air box, I'll have enough room in there to mount a camper shell I think. At least a little storage box.
Thanks guys, mama's got her dancin shoes on. Seeya.

One of my carb test bikes has a set of cracked Dyna coils... these coils are old; real old. They are cracked and the cracks allow them to short when it is damp or rainy. I fixed the problem permanently with JB Weld. The silicon won't hold up. Nor will liquid rubber (have tried both). The JB Weld is a very durable epoxy and bonds well to the plastic that covers the coils.

I think the cracks are a combo of age, vibration and heat. Heat is likely the real killer as it turns any type of plastic brittle over time and then the vibration probably works its damage.

Thanks George. I was out of JB weld the day before, so I just used what I had. But I picked some up today, and I'll be doing it over tomorrow.

riverroad wrote:

I like that idea of putting them under the seat. But when I buy coils, they're gonna be brand new. And if I lose the air silencer, and maybe go to pods also losing the air box, I'll have enough room in there to mount a camper shell I think. At least a little storage box.
Thanks guys, mama's got her dancin shoes on. Seeya.

Internal coil heat and age are generally the worst ememy of a coil. These coils will live probably 30 yrs in the stock location. Leaving the ignition switch on one time can cook them pretty good. If you put the coils back, say, "under the seat". Then you would have to run extra long plugwires back up to the plugs?? Hardly worth the effort in my opinion.

i just think about all that heat rising off top of motor having no where to but up when you are sitting there,even when you are moving theres a lot of heat consentrated in that area.theres lots of air movement back there behind those side covers,just ask anybody whose lost a sidecover.the plug wires are no big deal make a trip to advance auto parts and they will hook ya right up!

Try to invision plug wires 4'+ long routed from under the seat? This is a no-brainer. Stock mounting location is just fine! Don't create a problem where one doesn't exist.