# Flywheel Identification 1970 400 or Other



## 1971LemansWisc (Apr 18, 2021)

All - Looking for some help to identify (confirm) my Factory Pontiac Flywheel.

I was told it is from a 1970 Pontiac 400 - I plan to use it on a 1971 Pontiac 400. (After a resurfacing and clean up.) 

It is drilled for a 10 1/2" Clutch Disc. 

The only Part Number I can find is on the Ring Gear and it is F 5 8621677 (No Casting Number on the Back)

Thank you in advance!


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## 1971LemansWisc (Apr 18, 2021)

Still interested if someone has a GM Part Number for this Flywheel.

Wallace Racing Website says 1964-1979 Pontiac Flywheels are interchangeable so I should be good here. 

Found a good thread (April 2019) on this site recommending I make sure the resurfacing doesn't impact the Balancing - it shouldn't ... but still work the check. 

This leads next to the whole topic ... internal external neutral Balancing as it applies to the Harmonic Balancer, Crankshaft and Flywheel ... more reading to do here.


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## PontiacJim (Dec 29, 2012)

1971LemansWisc said:


> Still interested if someone has a GM Part Number for this Flywheel.
> 
> Wallace Racing Website says 1964-1979 Pontiac Flywheels are interchangeable so I should be good here.
> 
> ...


The flywheel is cast iron. Looks like the ring gear is chewed up. Also appears, looking at what I can see in the photo, that it has been turned down in the past and there is little "meat" left on the face. You want to make sure that the flywheel comes in in spec as far as thickness - just like turning down brake drums or disc brakes. If the casting gets too thin, it can crack/break and explode. The fact that it is cast iron AND almost 50 years old, I would not use it. Most purchase an aftermarket steel flywheel which is much safer AND new.

An _internally balanced_ engine is a *neutral balanced* engine using no external components to correct an imbalance. 

*Most all* factory Pontiacs where "internally balanced" BUT to a required specification and is generally referred to as internally balanced. I have read that the unbalance was 6 or 7 in-oz.. The harmonic balancer is neutral balanced. All factory flywheels/flexplates were then manufactured/weighted to offset this slight unbalance - so the engine is also "exteranally balanced" in the sense that the flywheel/flexplate is used to bring the slight unbalance of the internal assembly to a "neutral balance" - or no vibrations. So with most all Pontiac engines being balanced this way, most all Pontiace flywheels/flexplates had and can be used on any engine without rebalancing - but there are exceptions due to engine type/changes.

If you look at your photos, the one showing the backside of the flywheel has the three holes grouped together on the outer ring/edge. The flywheel is drilled to remove material, thus removing weight, and balancing the assembly.

When you buy a replacement steel flywheel, it is "neutral balanced." You would want to take it and your Pontiac flywheel to a machine shop and have them match the balance on the steel flywheel to the stock Pontiac flywheel - IF all you were doing is changing the flywheel to an existing factory/running engine.

If you were rebuilding the engine, things like new pistons/pin and aftermarket rods, this will change the balance of the engine and the engine will need to be rebalanced.


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## 1971LemansWisc (Apr 18, 2021)

Pontiac Jim -

Thank you for your recommendation and the explanation.

Poking around a little on the Internet I see multiple Manufactures of compatible Flywheels i.e., Ram, Luk, McLeod.

These are some common specifications:

Fits GM 10.5, 11" (My Clutch is 10.5”)
2.75″ register bore
35 pounds (Range) 
Internal balance
166 tooth ring gear
Meets SFI 1.1 specifications
Made in the USA (Try to buy here whenever possible.)

I would ask if Forum Members had a Flywheel preference …

My old cast Flywheel will make a nice decorative paperweight.

The internal-external-neutral Balancing topic makes more sense now.


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## PontiacJim (Dec 29, 2012)

1971LemansWisc said:


> Pontiac Jim -
> 
> Thank you for your recommendation and the explanation.
> 
> ...



The steel flywheel is the best/safest way to go.

Most will have a dual bolt pattern for either the stock 10.5" (technically a 10.4" clutch) or some Pontiacs did have the 11" size. So no big deal there, but the 11" is the better choice to use if you plan on purchasing a new set-up. 11" disc has more surface area and more surface area means more grip. But, nothing wrong with the 10.5". Most aftermarket set-ups will provide a HP rating for their "kit". Pedal pressures can also vary and some will list this as well. The diaphragm style pressure plate typically gives you an easier pedal feel.

I used the Hays steel flywheel - just my choice.

166 teeth is the ring gear and what you want.

SFI = The SFI Foundation, Inc. is a non-profit organization established in 1978. They issue and administer standards for quality and performance of racing equipment. The SFI program establishes recognized levels of performance and quality for a given product. This results in a safer racing environment for everyone.

So something said to me SFI requirments generally means it is applicable for racing. Do you need it? I am sure the cast iron flywheel was not SFI certified. LOL So no, you don't necessarily need to and often those SFI certified products cost more.

2.75" register - that is the center hole diameter as it fits over the crankshaft. The later 1976-1981 Pontiac engines used a 2.6" hole(register).

Flywheels can be listed having a specific weight. 30 lbs seems about average. Some can be as heavy as 40 lbs or you can get a lightweight aluminum flywheel at 15 lbs. You want a heavy flywheel for a heavy car and can use an aluminum for a very lightweight car. The flywheel build inertia as it spins and when you let out the clutch, that spinning weight prevents the engine from stalling out. If you used a 15 lb flywheel on a 4,000 lb GTO, you would be stalling the engine out real fast unless you slipped the clutch a whole bunch to get the car rolling - if you could if you had 2.73 gears. So light clutch, light car, and gearing like 4.56, 4.88, or 5.10 so it will move. The engine will rev faster and that is the benefit, but it has to be used in a light car. Heavier flywheels will rev slower, but it is much harder to stall out the engine when you let out the clutch.

What you want to check is to make sure you use the correct flywheel bolts for a flywheel. The bolts for an automatic flexplate have a taller head on them. The taller head will strike the clutch disc springs, so you want the bolts which have the shorter heads.

If you purchase a clutch "kit", check that the throwout bearing is correct for your car. Sometimes you get the wrong length as they come in 3 different length with the "long" and "short" being the most commonly used. This has been covered here on the forums and if you use the Search Community search engine above you should find it.

Finally, most aftermarket flywheels will say "neutral balance." Do as I said earlier and take the new flywheel and the factory flywheel to a local machine shop and ask them to balance the new flywheel to match the factory unit. It should not be very expensive and you won't have any vibrations in the engine. The bolt holes on the flywheel have a specific pattern, so make sure the balancer person matches these up when he removes any metal from the new flywheel.


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## 1971LemansWisc (Apr 18, 2021)

Thank you!


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