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Discussion Starter · #1 ·
hello everyone,
a lil history on the motor so i buy a dart 406 sbc short block online it was supposed to be a 2 piece rear main seal so i order a oil pan from same company. it gets here and pan doesnt fit i call they say guess we messed up its a 1 piece rms block and you need a oil pan for a 86up sbc 400 w/dipstick on pass side. fast forward 5 years and dyno shop over revs engine and blows out rear main seal due to a faulty valve cover breather(my own fault) so now im trying to order a new flex plate and converter and everywhere that i call is telling me theres no such thing as a sbc 400 or dart 406 with a 1 piece rms. yet im looking at my flex plate with the large weight that has both ends with the u shape cut out. any body have any thought on this? thanx
 

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hello everyone,
a lil history on the motor so i buy a dart 406 sbc short block online it was supposed to be a 2 piece rear main seal so i order a oil pan from same company. it gets here and pan doesnt fit i call they say guess we messed up its a 1 piece rms block and you need a oil pan for a 86up sbc 400 w/dipstick on pass side. fast forward 5 years and dyno shop over revs engine and blows out rear main seal due to a faulty valve cover breather(my own fault) so now im trying to order a new flex plate and converter and everywhere that i call is telling me theres no such thing as a sbc 400 or dart 406 with a 1 piece rms. yet im looking at my flex plate with the large weight that has both ends with the u shape cut out. any body have any thought on this? thanx
Post a picture. GM didn't make a 400 with a 1 piece rear main seal unless it was converted in some way. The 400 blocks were made untill 1980 and the 1 piece deal didn't show up until years later. Perhaps what you are saying is you have a DART aftermarket block? If so, the counter parts guys aren't likely to be of help. I suspect you are confused or given wrong info so if you will post plenty of good pics of everything you have we can help you sort it out and get you the right pan including the felxplate, the back of the crank, and the block part number.
 

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Your 3.750 crankshaft if externally balanced would need an external balanced flexplate that would match the rear main seal type of your crankshaft and if your crank is aftermarket and a one piece seal style then you would need a flexplate for an external balanced crankshaft with a one piece rear main seal. Below is a few options if you have a one piece rear main seal and your crank is external balanced.

Summit Racing SUM-G104SFI Summit Racing® SFI-Approved Flexplates | Summit Racing is a 168 tooth

Allstar Performance ALL26831 Allstar Performance Flexplates | Summit Racing is a 153 tooth.
 

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All flexplates for any size one-piece rear main seal SBC are external balanced....there are no internal balance 1-piece rear seal flexplates, since when GM went to the 1-piece seal style rear crank flange they lost the offset weight built into the crank flange shape...now it is full round instead of offset.
To counter this, all 1-piece seal flexplates (and flywheels) have an external balance weight. Even 350's and 305's have this weight, if they are 1-piece seal cranks.


All you need is a 1-piece seal flexplate or an 1986-2001 SBC with the correct diameter(number of teeth to match your current plate....12-3/4" with 153 teeth or 14" with 168 teeth.

Dart uses a 1-piece seal aftermarket 3.75" stroke crank, and their block is machined for the '86-up 1-piece seal and oil pan fit.

You can't look anything up for fitment to a 400, at least not for the bottom end of the block, ....since none of those were ever 1-piece seal from GM,..... last stock 400 was made in 1980 when everyhting was still 2-piece style.

If you have a 1-piece seal crank but the instructions call for "external balance", that typically means you need the harmonic balancer to also be offset weight...like a stock 400 balancer is.....since all the 1-piece seal flexplates and flywheels are already external balance.
 

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Ericnova72 a quick question just incase anyone else reads this someday. I know normally most of the aftermarket 3.750 stroker cranks for a one piece rear main seal or two piece seal is externally balanced but I know on some stroker kits with the 6.0 rod they can make them internal balanced so do they still use the extarnal weighted flexplate on the rear and a neutral balancer on the front then?


I know they have the stroker cranks of the 3.750 internal balanced but I have never looked up that information and wondered now just since this was asked I know they make them in one and two piece rear seal designs and just wondered if the stroker cranks that are the one piece style seal do they when are internal balanced do the same as the 86 and up have just the weighted flexplate on the back still while having the neutral balancer on the front?

I never came across that before and never thought about that part and know it would make it confusing to some folks and would make it hard for some folks who are using an aftermarket crank in there build to look up stuff if using factory specs.
 

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You was already typing and answered my questions before I could read it. That is what I figured but I just wanted to make sure on my knowledge and for anyone who reads this post in the future they will have that information as well. Two thumbs up for ya :)
 

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Discussion Starter · #9 ·
All flexplates for any size one-piece rear main seal SBC are external balanced....there are no internal balance 1-piece rear seal flexplates, since when GM went to the 1-piece seal style rear crank flange they lost the offset weight built into the crank flange shape...now it is full round instead of offset.
To counter this, all 1-piece seal flexplates (and flywheels) have an external balance weight. Even 350's and 305's have this weight, if they are 1-piece seal cranks.


All you need is a 1-piece seal flexplate or an 1986-2001 SBC with the correct diameter(number of teeth to match your current plate....12-3/4" with 153 teeth or 14" with 168 teeth.

Dart uses a 1-piece seal aftermarket 3.75" stroke crank, and their block is machined for the '86-up 1-piece seal and oil pan fit.

You can't look anything up for fitment to a 400, at least not for the bottom end of the block, ....since none of those were ever 1-piece seal from GM,..... last stock 400 was made in 1980 when everyhting was still 2-piece style.

If you have a 1-piece seal crank but the instructions call for "external balance", that typically means you need the harmonic balancer to also be offset weight...like a stock 400 balancer is.....since all the 1-piece seal flexplates and flywheels are already external balance.
hi eric thanx for the response. not sure about the harmonic balancer being offset or not. im running a crank hub instead but this is a pic of the 1 that they sent with the motor. its 6 3/4 " x 1 1/8"
 

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Discussion Starter · #10 ·
Over rev blowing out the rear main is the real mystery here.
hmmm. well im guessing thats what happened. 1 of the valve cover breathers was faulty. new out of the box and i didnt blow though it to check i just assumed it was good. it wasnt. they brought it up to 7000rpm and it started dripping oil. so i tow truck home and instead of just living with it(a drip or 2 a day) i pull the motor and get a lil ahead of myself and forget to unbolt flex plate from converter. end up bending the flex plate
 
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