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External vs. Internal Balanced Flexplate Guide

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Working in customer service, sometimes I get calls from customers having questions on their parts, or how something works, and though I try to do my best to explain it to them, sometimes it still doesn’t make sense, and I need assistance from our tech department to explain a little bit better.

When choosing a new flywheel or flexplate or harmonic balancer for your engine, you will need to know if either will require external balance parts. If, say, building an SBC 400 that requires an external balance flexplate and you use an internal balance flexplate you will get a serious vibration when the engine is running. This is a common occurrence that I see a lot working in customer service and can save you some money and time in returning parts. Or save even more money by not damaging your new or fresh rebuilt engine.

You’ve spent a lot of hard-earned money for that perfect engine for your vehicle, so spend a little time and get the right parts to outfit that engine to make it purr. The following is a comprehensive list of internal and external balance engines. And interesting enough, there are combinations of the two. Internal balance, for the harmonic balancer, external balance for the flywheel flex plate such as 87-95 SBC including LT1 and LT4 Opti-Spark engines. Pictured is a 1986-1997 Chevy 153 Tooth Flexplate with the external balance weight highlighted. You may also need a Harmonic Balancer.

If you are unsure of what year engine you are working on, simply do an internet search of the casting numbers on the block. These casting numbers can be tricky to locate or see if the engine is installed in the vehicle. For instance, on Chevrolet engines the casting numbers are usually located on the back of the block behind the intake manifold and cast in the top of the mounting boss for the transmission, like the one pictured here. However, Chevy also would place a stamped number on the front of the block passenger’s side by the water pump just under the cylinder head, as shown here.

Both sets of numbers are usually covered in grease and grime and sometimes painted over as well. Clean the area up with a wire brush to expose the numbers. You can also use a mirror to see the back of the block when the engine is in the vehicle. And a little extra light never hurt either.

Searching casting numbers also works well for identifying cylinder heads. The internet these days provides a vast data base of knowledge, if you are unable to navigate the web I’m sure there are friends or family can help. Once you know what engine you are working on locate it on the following list to confirm what balance it requires for accessories.

Here is a list of engines that is helpful to determine if it is internal, external or both. Take some time to research to save some time and money and get the correct parts the first time!

Small Block Chevrolet

Gen1-86 and prior – (267/283/302/305/327/350) Internal Balance

400 SBC-External Balance

87-95 (Includes LT1/LT4 Opti-spark) Internal Front/External Rear

92-2000- Internal Front/External Rear

LS Engines-Internally Balanced

LT Engines- (2016+ model year) Internal Balance

Big Block Chevrolet

396-427 Internal Balanced

454-502-External Balance

572-Internal Balance


L29/L21-External Balance


L18-External Balance

8.0L Vortec- Internal



Oldsmobile Small Block

260-307-330-350-403-External**strong text

Oldsmobile Big Block


Gen 1 (to 1966)


Gen 2 (1966-1970)


Gen 3 (1907-1991)304-360-390-401 External (Though they share the same crank flange, 304, 360 and 401 do not interchange.)

Chrysler Small Block

318 A (POLY)- Internal

273-318-340 (340 to 1971 Steel Crank) LA-Internal

340 (1972 up w/Cast Crank) External


Chrysler MAGNUM Engine

318 (5.2) Internal

360 (5.9) External

5.7L, 6.1L, 6.2L, 6.4L Hemi- Internal

Chrysler Big Block

B Engines 1958-1971 Internal (Forged Crank), 1972-1978 External (Cast Crank)


400 External

RB Engines 1958-1971 Internal (Forged Crank), 1972-1978 External (Cast Crank)


426 Wedge-Internal

440 (1966-1971) Forged Crank-Internal

440 (1972-1978) Cast Crank-External

440 Six Pack-External (Forged Crank w/ Heavy Rods)

425 HEMI-Interal

Ford Small Block

289-302-Ext 28oz

302 HO-Ext 50oz

351W-Ext 28oz

351C- Ext 28oz

351M/400-Ext 28oz

Ford Big Block


Ford Y Block

239-256-Internal (1954 Cars Only-Cleveland Blocks)


239-256-External (Trucks only-Dearborn Blocks)

Ford FE





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