sorry its kinda long post but I put everything I knew to help figure it out. k,i'm puzzled,half brain dead right now anyway but still don't know what to do. I have an 87 camaro with an auto tranny. The motor that was in it was a 305. I had a motor built,this built motor is a 69-80 model block(got that from the engine codes). The guy that built the motor put this odd shaped piece on the black. Its a circle shape with about a 4 by 4 inch lobe hanging off the side. I think thats for balancing because i'm supposed to use a non balancing flywheel. I took my 153 tooth(counted them to make sure) flexplate from my 305 to use on my new engine,the hole in the center of the flexplate had to small of a diameter to fit over the crank. I went to a parts store and picked up a 168 tooth,I didn't remember to check and see if it would fit over the crank(although it looked like it would). But when I put the 168 tooth up to my converter the holes to mount the flex plate to the converter didn't line up. Plus the flexplate seemed to big to even fit in the bell housing,real tight fit. I don't know if I'm right but I think the 168 plates are 14 inch right? Or do they make 168's in various overall diameters? The 153 is like 12 and 3/4 or something. So can a 14in 168 plate be used in our cars? Last thing that was odd to me is when I looked through some manuals I saw the odd lobe thing. It said its purpose was to convert my 350 to accept 400sbc flexplates....? I just don't know what to do,I was told you sometimes have to change your bell housing to make a larger flexplate fit,which is more than I wanted to deal with.... I just need to know what I need to do to get this done... Thanks in advance for any advice or input. If I need to put anymore info just ask.

buy the correct flexplate?

if you tell the parts store what car you have, they should be able to get the proper flexplate.

you should need a flexplate for a 2pc rear main seal motor, so engines made up to 85. You were most likely given one for a 1 pc rear, which has a smaller hole.

I know the flywheel size is 153 tooth, so the flywheel size should be the same.

you have e a 87 car, which would require a 1 pc rear main flexplate, but you have an engine that predates that, so you need a 2 pc rear main seal flexplate. Ask for one from an 85 camaro

the weight on the flexplate is for balancing. generally they are used to put a 400 crank in a 350 to make a 383. the 400 was externally balanced

ok, understand some better know,lol. When he built the motor he told me to use a 168. If I use a 153 will it not balance right and damage the engine or run just fine? I would be nice if I could use a 153 tho because of my starter. I just wonder why I was told to use a 168,what reason...?

So could that mean he used a 400 crank in my motor? Its just a stock 350 rebuild,not bored to a 383.

But yea,now I know how to tell them for the most part what I need. Thanks for the advice so far. The guy behind the counter was clueless....heh,actually said he was just there for customer service and doesn't know a whole lot.

Kinda would seem like I have no choice but to use a 153 because of the bolt pattern on the converter plus the size of the 168 compared to the bell housing....

Flexplates are available in in 153 and 168 tooth ;both int and ext balance. There are two convertor bolt patterns; 10.75" and 11.5"and not all flexplates have both .

I believe your stock convertor has the 10.75" convertor pattern (need to confirm ) so as stated above you need a 153 tooth pre '86 2pce seal flexplate with 10.75" convertor pattern.
Most aftermarket ones have both convertor patterns like this one
http://www.summitracing.com/parts/TCI-399573/.


I would be asking engine builder why he used the ext balance plate on your engine.Not needed for normal 350 setup.

k,so it doesn't seem to mater what number of teeth I use. That was what I was most worried about. They will prolly have one in shop I can measure. Sounds like maybe the motor will go in tomorrow,hopefully have time,lol. Thanks for all the help.

Teeth will be determined by your starter type.
Offset bolt pattern starter for 168 straight / bolt pattern for 153

I removed a 305 from a 1986 Z28 305 parts car and sold it, and it had a weird balance weight on the flexplate. That took me totally by suprise! I didn't know that 305's had any external balance.

It sounds like you may have gotten a 400 crank in your engine, and that would be a two- piece rear main type crank with the different, larger bolt circle on the crankshaft flange, as already mentioned. The oblong plates are usually added onto the flexplates when you don't have the matching 400 flexplate to go with the externally balanced 400 crank, I think that's opposite of what you may have been thinking in your earlier post.
If it is a 400 crank, you should also have a large, externally balanced harmonic balancer on the front of the crank as well. This harmonic balancer would have an area on the back edge of it's outside diameter that is undercut for about 2/3 ds of it's diameter.

Or maybe the engine builder was trying to imitate the 305's flex plate external balance weight? Can you ask him why he did it?

The 1 pce seal crank engine are ext balance at rear only because the round crank flange to suit the new 1 pce seal did not allow casting any balance weights there ; as is the case with the earlier int balance engines. 1 pce seal engines are still neutral at front.
As you note , only the 400 was ext balance both ends with a weighted balancer.