Balancing assy without flexplate optional?

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68gtxman

68gtxman

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I checked with my machinist regarding the balancing of my 340 rotating assembly and he says I should give him my harmonic balancer but that the flexplate is optional, though recommended. The harmonic balancer is a new SFI approved unit.

Well, I had planned on using my old 273 flexplate with the new 340-6 engine. I also planned on leaving in the 273 engine until the new one was ready to go in, but if I do this, I won't have the flexplate to give to the balancing shop.

Should I buy a new flexplate? Can I safely reuse the old flexplate and flexplate bolts? Should I balance without the flexplate?

What do you gentlemen recommend?
 
If it's neutral/internally balanced, you don't have to have a neutral flexplate on it. Theoretically, the flexplate and bolts should balance out at "0" so adding it "shouldn't" change the balance. But I'm nit-picky and would like to have everything assembled just to be sure. It wouldn't be a bad idea to use the old one, but I don't like the OEM versions that have the "windows" in the ears. Seen too many break in even mild performance applications.

If it's a cast crank 340 that's externally balanced, you must have the correct, weighted flexplate. (Don't really know how they do it if the weight is on the converter.)

What year 273 are you replacing? The earlier ones had a small converter snout and it will be loose in the crankshaft if you reuse the converter! I believe the small snout converter/trans combo also uses different converter splines. Even if you have a later 273/trans, if you are using an aftermarket converter, the converter/flexplate bolt size and spacing will need to be double-checked.
 
The 273 is a 1967. I was also planning to use the original torque converter as well. The 340 is a 1969 and has a forged crank.
 
Is this a performance build?

If so, I would go with the SFI plate and order up a converter built for the combo. If performance is what you want, then IMO it would be another 6- 700 well spent.

If it's just a stock build with just cruising in mind, then no.
 
Does the 69 340 crank have the same converter register that the 67 273 converter has?
 
Just stock for cruising and shows. My wife will be driving it and she is scared to death of fast cars. Don't tell her that a 340-6 is quick!:burnout:
 
.........67...........should be a small crank register...........
 
The "register" in the crank flexplate flange is the raised part, similar to the raised area on an axle that the brake drum fits on. But the crank version is measured internally while axle version is measure externally. The exterior diameter where the flexplate fits over, like a brake drum, may have stayed the same. Not sure of that either - been a while since I found out the hard way.

I believe the small crank register is closer to 1.5" while the "normal" larger, later one is closer to 2". But I don't have the exact numbers handy.

"Congerter"??? :-k: Now you're really confusing things! lol Just kidding. :hello2:
 
OK. It sounds like I should balance the new crank assembly without the flexplate. Then when it's time to make the switch, verify that the old flexplate and torque converter matches up with the new crankshaft flange. If not, then it,s time for a different setup.

Let me know if I don't have this right. Many thanks!
 
your small snout pre 68 convertor will not center properly in the larger "post 67" crank register if you dont use a "T-flite patty" .25 thick adapter ring or a machined ring with the proper ID/OD. It WILL crack the stock flex plate at the mounting ears over time. solid B/M style will probably wallow the holes but will hold. Slip .25 ring over old 1.55 to get you a 1.80 "large snout"

"....the pre-'68 904 (has) the 1.55" torque converter pilot diameter; all the 727s and the '68-up 904 have 1.8" pilot diameter...." same goes for Auto cranks. Manual cranks were machined for pilot bearing
 
pishta said:
your small snout pre 68 convertor will not center properly in the larger "post 67" crank register if you dont use a "T-flite patty" .25 thick adapter ring or a machined ring with the proper ID/OD. It WILL crack the stock flex plate at the mounting ears over time. solid B/M style will probably wallow the holes but will hold. Slip .25 ring over old 1.55 to get you a 1.80 "large snout"

"....the pre-'68 904 (has) the 1.55" torque converter pilot diameter; all the 727s and the '68-up 904 have 1.8" pilot diameter...." same goes for Auto cranks. Manual cranks were machined for pilot bearing
Click to expand...

Thanks Pishta! OK. Can anyone tell me where I can get this adapter? Anyone not using one they may have on their spare parts shelf?
 
Well, thank you all for your advice! I ordered a new SFI flexplate, so it would be stronger than the old one, plus it includes new ARP bolts. Plus the snout adapter from Patty. This should do the trick!
 
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