Transmission adapters, breaking flex plates, and trying to learn from EcotecMiata's mistakes

Hey everyone, I have a build thread going over yonder, but I thought it would be better to post this question in the main forum to hopefully reach a wider audience.

Background: I am planning on using this adapter to bolt a T5 or TKX with a SBC bell housing onto an Ecotec, and benefit from the larger clutch from the V8.

With the adapter kit, I'd be bolting the following parts together in a big sandwich:

Ecotec crankshaft <- Ecotec flex plate <- adapter <- SBC flywheel
(I am not using the triangular plate they show in the picture of the kit, that's for driving a torque converter.)

Miata guys have been using a similar adapter from EcotecMiata to bolt Miata transmissions onto Ecotecs for a while now, and some of them have been having problems breaking flex plates. The endurance road race guys, at least anecdotally, have found that over 50 hours of track time, the flex plate tends to let go. The sample size is small, of course, and I have no idea what 50 hours of track time equates to on the street (perhaps years, perhaps forever) but it's a point of failure I'd like to try to eliminate if I can.

My theory for why some people were breaking flex plates with the EcotecMiata adapter is that because the flex plate no longer has a torque converter bolted to it for support, it's getting bent somehow; maybe it bends a little sometimes when the starter engages it, or maybe some guys were installing flex plates that were already bent without knowing it. In any case, once the flex plate is bent, it can fatigue itself to death at high RPM. Example flex plate carnage pictures from an Ecotec Miata Facebook group:

EcotecMiata recently released a billet chromoly flywheel that replaces their old adapter solution, and thus eliminates this problem. I asked Speed Gems, who make the Ecotec-to-SBC adapter, whether they could come up with something similar, and their answer was basically that with enough money, anything is possible. That's about what I expected, and before going that route, I'd like to try to make the adapter solution work.

It seems like the simplest fix would be to support the flex plate by just bolting it to the back of the flywheel, like it would normally be bolted to a torque converter. That would mean drilling the flywheel in 3 places and machining 3 stand-offs to go between the flywheel and the flex plate. With the bell housing adapter bolted onto the block, there would be no way to tighten bolts from the block side of the flex plate like you would with a torque converter, except maybe through the starter hole, which sounds like a pain. So it seems like the bolts would have to go in from the clutch disc side of the flywheel, with countersunk holes, and I may end up welding nuts, or just the whole stand-offs, onto the flex plate.

I would then take the flex plate + adapter + flywheel assembly to a machine shop to be balanced together. Since it would be getting balanced after the fact, I don't think the location of the holes in the flywheel would even need to be extremely precise; probably something I could do myself with transfer punches and a drill press. I have a buddy with a bench top lathe who could help me with the spacers.

So, does this make sense? Any feedback or constructive criticism on this whole scheme would be greatly appreciated. Thanks!

I think you're probably on to something with the analysis of why the flex plates are failing.  I suspect that they get bent from the starter drive.

The whole arrangement of having a second plate just to support the ring gear is a bit, um, crude.  The cleanest solution is probably the custom flywheel but, done correctly, that'll be expensive.

Is there a possibility of mounting a Chevy starter and using the ring gear on the flywheel?

You're idea of attaching the flex plate to the back of the flywheel is probably the most cost effective and easiest to implement.  You might be able to use off the shelf Male/Female standoffs threaded into the back of the flywheel and then install bolts through the flex plate into the standoffs through the starter holes.  You could also use male/male standoffs with nuts.  I'm not crazy about going in from the flywheel side unless you can do it outside of the clutch surface area.  It'll probably work fine but I think it's better to avoid that if you can.

Whatever approach you use there should be a flat surface on the back side of the flywheel for the face of the spacer/standoff to seat against.  You may have to have someone with a vertical mill do that for you.

This is my Oldsmobile to ford Tremec solution I have been running for 2 years. Flywheel and flex plate touch so there's some support and my flexplate is beefier.

I'll be changing it soon because it's all very heavy.

In your situation I ask how thick is your adapter between flex plate and flywheel? Can you add 3 or 4 arms to the adapter in order to support the flex plate? Then you could bolt it together before installing the flywheel. I would expect 1/8" steel or aluminum to be plenty of extra support to do the job.

In reply to APEowner :

Thanks. Yeah, I took a quick look at the McMaster catalog and there are some standoffs that look like they would be pretty close to working. Come to think of it, I must have gotten to the Ecotec torque converter bolts through the starter hole when I pulled the engine at the junkyard. I was really tired by that point in the day and my memories are a little fuzzy...

Based on a 2-minute Google search, reverse-mount SBC bell housings seem to exist, but I'm only seeing them from Quarter Master, designed for their small-diameter clutches. I'd like to use a 10.5" or 11" stock-style SBC clutch if possible. I doubt I'll have room in the tunnel for the starter to mount on the bell housing anyway, but I'll keep looking.

I'll be using an SFI aluminum or chromoly SBC flywheel; not going to trust a $50 cast one when I'm already in this deep. The SFI flywheels tend to be machined flat on the back, so depending on how things line up I may get lucky and have a flat surface to work with, otherwise you're right, I'd have to get it spot faced.

Would it also be a good idea to get the flywheel resurfaced after drilling, just to take care of any distortion? That seems like it might be overkill.

So the 55-56 chevy used a bellhousing mounted starter.  I have one of those bells, but also there is a midplate available that sandwiches between engine and trans that accepts a starter in the standard low passenger side location for swapping in a newer transmission without starter provisions.  I envision this plate plus a hub adapter to fit the sbc flywheel to the ecotec crank in the correct location probably being your cheapest route.  
 

https://www.summitracing.com/parts/dck-10100?seid=srese1&gclid=cj0kcqiatql-brc0arisaf4k3wfac2f7cbfwdka_oxkosngihb5s1y99jvtikyaecgw16th-vk_igbsaao9bealw_wcb

Dumb question incoming!

What's the point of running the flexplate and a flywheel? Is it just to interface with the starter?

In reply to thatsnowinnebago (Forum Supporter) :

Sounds like in this kit the original starter engages the original flexplate and then the sbc flywheel is physically larger and is just a flywheel and the ring gear is dead weight

my idea probably has the least rotating mass

I am shocked that this is a failure, but it sounds like the flex plate is failing due to crank harmonics, due to there being no load on it.  Plus the torque converter probably forces it to hold its shape more.

I'd look for a way to bung a starter in to drive the Chevy flywheel, and eliminate the Ecotec flexplate entirely.

I don't have it in my hands yet, but I believe the crank adapter is around an inch thick, so the flex plate and flywheel will be far from touching like asphalt_gundam's setup.

The early SBC starter solution is interesting. There isn't a lot going on around the lower right side of an Ecotec block, so it might fit. Looks like my options for that style starter are either to stack the plate that Patrick linked on top of the Speed Gems bell housing adapter, or use a cast iron 365 bell. Given those choices, I'd take the plate--what's one more? Either way I'd be hacking a chunk out of the Speed Gems bell housing adapter to make it work. Actually, Speed Gems could probably just add a provision for a SBC starter to their kit, either the early style or the later style that bolts in from the side, but then that pesky cost of having something engineered and produced for me vs. just buying parts off the shelf comes up again...

I still like the idea of bracing the flex plate to the flywheel the best, though. The steel friction surfaces on aluminum flywheels have lots of flat head machine screws holding them on from the clutch side; surely I could get away with 3. Actually, if I use an aluminum flywheel, I might be able to hide the screw heads going into the flex plate stand-offs under the replaceable friction surface. Not sure whether it would line up, but that would be slick.