flexplate mayhem. Frustrated & questions... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

plumbcut · 04-18-2004, 08:30 PM

Here's the scenario- sbc 400 with original GM crank... I have had flexplate problems with this engine for years. No flexplates that Ive ever had will fit over the center of the crank, except for a pioneer SFI approved.. The non-SFI approved pioneers will not fit, nor will any other brands Ive tried.. (tried a few, definately not every brand, and the first one that was ever on there, I Think was GM)..

Okay, its weird problem but not really the end of the world. I figure the crank wasnt machined to exacting standards & tolerances, from the factory. Otherwise, all these replacement flexplates would have fit. The pioneer SFI flexplate is really nice, and I dont mind the extra $$ for the peice of mind. The other problem that I have, and it seems to be a little more serious, is too much run-out at the ring gear (front & back, not out of round) .... I dont know anybody thats ever dealt with this, but a lot of people have said something about shims between the crank & flexplate. Anyone have anything they can add? It sounds fairly simple. I have a dial indicator I can use to check for runout & stuff, but I havent found anybody that stocks the shims. Im going to check a machine shop tomorrow & maybe summit or jegs.

Does anybody have the spec for the diameter of that hole on a 70's sbc400 crank flexplate? I will get a micrometer and measure it and Ill bet dollars to donut holes that its out of spec by a few thou.

Im real hesitant about taking a grinder to the inside diameter of the flexplate to make room for the crank to slide thru. But if it comes down to it, I will clearance it.

anybody have anything to say about this? Like I said, monday I will get a new flexplate and check runout. Oh- my reason for replacing the flexplate in the first place is that the starter chewed it up and its missing teeth now

... Wish me luck, Ill keep you posted.

pc

onetoncrewcab · 04-18-2004, 08:45 PM

You didn't make any mention of it, but are you using the externally balanced flexplate? The 400 uses a different flex plate than the 350 (it is internally balanced). I believe there is also a difference between the harmonic balancers on the 350/400.

That was the only thing I could think of.
MJ

plumbcut · 04-18-2004, 08:55 PM

thanks for the reply.. Yes Im using the balanced flexplate... same with the harmonic balancer up front

pc

04-18-2004, 08:30 PM	#1
plumbcut california fill Join Date: Apr 2004 Location: Sunny So-Cal Posts: 694	flexplate mayhem. Frustrated & questions... Here's the scenario- sbc 400 with original GM crank... I have had flexplate problems with this engine for years. No flexplates that Ive ever had will fit over the center of the crank, except for a pioneer SFI approved.. The non-SFI approved pioneers will not fit, nor will any other brands Ive tried.. (tried a few, definately not every brand, and the first one that was ever on there, I Think was GM).. Okay, its weird problem but not really the end of the world. I figure the crank wasnt machined to exacting standards & tolerances, from the factory. Otherwise, all these replacement flexplates would have fit. The pioneer SFI flexplate is really nice, and I dont mind the extra $$ for the peice of mind. The other problem that I have, and it seems to be a little more serious, is too much run-out at the ring gear (front & back, not out of round) .... I dont know anybody thats ever dealt with this, but a lot of people have said something about shims between the crank & flexplate. Anyone have anything they can add? It sounds fairly simple. I have a dial indicator I can use to check for runout & stuff, but I havent found anybody that stocks the shims. Im going to check a machine shop tomorrow & maybe summit or jegs. Does anybody have the spec for the diameter of that hole on a 70's sbc400 crank flexplate? I will get a micrometer and measure it and Ill bet dollars to donut holes that its out of spec by a few thou. Im real hesitant about taking a grinder to the inside diameter of the flexplate to make room for the crank to slide thru. But if it comes down to it, I will clearance it. anybody have anything to say about this? Like I said, monday I will get a new flexplate and check runout. Oh- my reason for replacing the flexplate in the first place is that the starter chewed it up and its missing teeth now ... Wish me luck, Ill keep you posted. pc Last edited by plumbcut; 04-18-2004 at 08:32 PM.

04-18-2004, 08:45 PM	#2
onetoncrewcab One of my kids. Cute eh? Join Date: Oct 2001 Location: Mitchell, SD Posts: 1,286	You didn't make any mention of it, but are you using the externally balanced flexplate? The 400 uses a different flex plate than the 350 (it is internally balanced). I believe there is also a difference between the harmonic balancers on the 350/400. That was the only thing I could think of. MJ __________________ If a kid asks where rain comes from, I think a cute thing to tell him is “God is crying.” And if he asks why God is crying, another cute thing to tell him is “Probably because of something you did. John 3:16 2006 Chevy HHR 2000 Dodge Durango 1985 Jeep CJ-7 1970 Chevy Pickup

04-18-2004, 08:55 PM	#3
plumbcut california fill Join Date: Apr 2004 Location: Sunny So-Cal Posts: 694	thanks for the reply.. Yes Im using the balanced flexplate... same with the harmonic balancer up front pc



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