I always thought all B18 and B20 flywheels were identical.
However, after looking through GCPs on-line catalogs, it appears that B18s were equipped with two different part numbers.

Does anyone know what the difference is between flywheel part #418258 vs. #419634 other than "early vs. late"?
Are there physical differences? Are they not interchangeable?

The #419634 number is listed as a B18 or B20 flywheel.
I have a #418258 and know someone who wants to install it on a late (1973) B20.

Thanks.

Thanks to DerekUK for this:

http://i.imgur.com/5iKQtwA.jpg
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-Matt I ♥ my ♂

Late B20s have an eight bolt flywheel instead of a six bolt flywheel, but you probably knew that.
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john

Thanks and yes, I was aware of the 8-bolt pattern.
However, the two part numbers that I referenced are both 6-bolt flywheels.

I should clarify that the interested party specifically asked for a 6-bolt flywheel for a B20. I don't know what he'll use it on - maybe not the aforementioned '73.

The "metric" 8-bolt B20 started in 1974 along with K-jet FI. Maybe Planetman knows about the two flywheels--this was news to me. -- Dave

The early B18 flywheels didn't have locating pins for the pressure plate.

And they didn't have the central relief for new style of clutch discs that came with diaphragm style pressure plates. The relief is to allow room for the outer row of rivets that hold the clutch disc hub together from riding on the flywheel after about half way into the life of the clutch disc.

And the early flywheel uses a different ring gear. The ring gear for the later flywheel was used on all of the 8 bolt 4 cylinder motors with manual transmissions at least through the B23 OHC and they might also fit the B230 flywheels.




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Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90502
hiperformanceautoservice.com or oldvolvosonly.com

I guess the plot thickens...
Here's a photo.
You can see it has locating pins. I don't know what the "central relief" diameter is supposed to be.
I could not find a part # on the flywheel itself, but did see one on the ring gear (418358). Based on the parts manual I saw on gcp's site, I am assuming the the flywheel is a 418258. That flywheel is the one listed as using that particular ring gear.

Perhaps derek uk is correct and this flywheel was converted some time in the past.

Just realized the photo didn't post...

The early B18 flywheels had a smaller diameter central recess than the later ones. This extra diameter is via small step machined around the recess. This change may have coincided with the use of diaphragm clutches rather than the arm and spring type in about 64-65. I think the central part of the drive plate has a fatter central casting. The later flywheels also have 3 location pins for alignment adjacent to the fixing bolts. Early ones can have the centre machined down and also have pins fitted but re balancing is then advised.
If I knew how to post a pic I would, but will send if you post a usable email address.

Send pic(s) my way and I'll get it/them posted here for all to see.. I, too, am now curious.

forcefieldmatt-at-gmail
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-Matt I ♥ my ♂