flex plate issues AGAIN !!

[uk-barracuda]

my flex plate cracked a couple of months ago, so i replaced it, and managed to find another flex plate near by, but it was used, but i still put it on, after around 100 miles it cracked again,

but the 2nd hand flex plate i put on, wasnt 100% flat, it was slightly bent, and when i bolted it up, it straigtend it, but came to take it of today and when i took 1 bolt out it just bent back again,

i have thoughts that the owner before me, when he replaced the engine to a 318 he must of wrested it on the flex plate and it weakend it,

but why should it crack twice, could it be that the 2 old flex plates were weak, or is it my box, i drove the car today just to give it a little bit of a run, and it drove spot on didnt miss a beat, apart from the tapping from the flex plate

could it be that it is unbalanced some where,

anybody got any ideas,

 

[uk-barracuda]

EDIT :

car is a 1967 Barracuda,

904 box, original had a 273ci, now has a 318ci motor in it,

also i have noticed that when i put the 2nd flex plate on, i didnt notice any dowls, there was none there, does this mean anything

 

[oldkimmer]

...............If ur talking the dowel pins in the back of the block, u shure do need them.........kim.......

 

[uk-barracuda]

...............If ur talking the dowel pins in the back of the block, u shure do need them.........kim.......

cheers, but what do the dowel pins do

 

[1968FormulaS340]

The dowel pins align everything. Never use a bent flex-plate unless you enjoy changing flex-plates.

 

[stroked 340]

the dowel pins align everything. Never use a bent flex-plate unless you enjoy changing flex-plates.

x2.........

 

[uk-barracuda]

cheers, just got the whirling noise, when in reverse, did it all time, but only seems to do it in reverse now,

i have now put the new flexp plate on