410 cracked flexplate
 

So, let's start from the beginning my truck had its 390fe engine built to a 410 using a 428 crank and new pistons at. 040. It was balanced assembled and stuck into the truck. A few years later I buy it and I get a horrible ear piercing turn over noise which I'm totally assuming is a cracked flex plate(this only happens when it's cranked). Now for my questions how will I replace this? I have the balancing sheet and the engine receipts. The hard part in my eyes is Rebalancing it. Would I absolutely have to pull the engine. And would I just need to find a 428flex plate with a certain weight?



This engines in a 74 highboy with a C6 if anyone cares

Don't assume it's the flex plate being cracked. It could be rubbing the separator plate because of crankshaft end-play, or it could even be the starter.

That being said, probably the only way to do it would be to take the old flexplate, balance a crankshaft to that flex plate, and then using a new flex plate, balance THAT to the spare crankshaft. Being that it's a 428 crank, this method is probably not feasible.

Check the paperwork - is your "410" crank still externally balanced? If not, the flywheel might be neutrally balanced.

I had this happen with an engine I built, a 289 for a specific vehicle I wanted to build. Was balanced, and some weight came off the flywheel to make it perfect. When plans changed, and I wanted to put it in place of a 1982 302 in a Ford LTD, it needed the flexplate for that AOD transmission. Only flexplates around were for externally-balaneced 302s. So I asked my machinist if it was possible, using a spare 289 crank I had, to balance the crank to the 289 flexplate, then balance the 302 flexplate to the 289 crank. He loved the idea, and took care of it. Worked out great. (He had to remove a whole lot of the weight on the 302 flexplate, but it worked).

Alright let's start off with the possibility that it isn't a flex plate. How can I rule out possibilities?

Visual inspection - not sure how a C6 is setup, is there a bellhousing cover on the bottom?

There's an inspection plate yes, I've taken a look before, no signs of metal shavings or anything like that. Teeth look fine. I can check again what am I looking for?

Any scraping of the seperator plate, or look up around the crank flange and see if you can see any cracks. Maybe turn the engine over a few times - anything obvious on the flexplate might be on top where you can't see it the first go-round.

Unbolt the converter while you are down there and slip it back the little it can go. Might give you a better look of the center hub and if it is cracked/damaged. Using a big screwdriver and gently pry against flexplate and block plate and see if it makes a cracking/creaking sound. Roll the engine around and test multiple spots. If it's cracked, it should make noise.

428's take their own flywheel/flexplate due to being externally balanced. Did the shop rebalance it for internal balance? That would eliminate the need for a 428 flexplate.

Have you pulled the starter to see if it is engaging improperly? That might be the screaming sound you hear. Something to check.
With the lower cover removed, can you see the starter nose? Maybe have someone crank it while you look/see. Might be able to narrow down the noise.

Simple and cheap first. If only when cranking the first thing I’d pull would be the starter. Have you checked that all 3 bolts are intact and tight? Then I’d pull the tranny and visually check everything. Look for obvious signs on the converter and check the flex plate. Yanking the engine would be my last resort.

If it's broke just find another 428 flex plate. The weight will be the same, you don't have to rebalance.

That depends on if the original balancer modified the balance of the flex plate or not.

A stock 428 flywheel would have the "pie" shaped weight built into the back making it easy to denote. Not sure what a flexplate has? Big weight welded on? I'm sure someone has the answer.