If the McMaster-Carr #217 o-rings prove to be too loose fitting, then their next size down (#216) might do it.
The problem I would have with either of those sizes is that 1/8" thickness is too narrow - there will no crush at all between the inner face of the relief in the head/cam cover and the bolt-in rear cap. Nearly 1mm clearance instead. I think the ring needs to be compressed a little by the rear cap, as is the case with the old original square seals.
Again, may well be over-thinking… could always make up the difference with RTV, but I’m sure it should be possible to get something closer. The ones I’ve made are a really nice fit, the problem with those being that I have no knowledge of what they’re made of other than ‘rubber’.
Hi Roger yes i think your making it too complicated. …you could spend time and search for a perfect fit or search for new old stock…there is no oil pressure there so a nearly fitting o ring and rtv just like almost every restoration shop would do in lieu of them haveing the correct part…Steve
Just to tidy up the over-complication:
The BS217-N90 won’t really do it - see photos. The ID is too large and they are way too thin. Back to the home-mades, or the larger O-rings…
In order from left, original seal, home made 4mm thick unspecified rubber and the BS217.
Unfortunately FPE’s 216 only seems to be available in the same thickness.
Mold your own?
The #217 square o-rings arrived from McMaster-Carr, and they appear to be a sloppy fit in two ways:
- The inside diameter, at 1 3/16 inch, is slightly too large making the fit around the tach drive and end cap a little loose. The #216 o-ring at 1 1/8 inch inside diameter would probably be perfect. The photo below shows the slightly loose fit of the #217 square o-rings.
- The more problematic dimension, however, is the 1/8 inch X 1/8 inch size of the o-ring material. This allows only a loose fit within the cutouts of the cylinder head and cam covers. There’s not enough o-ring material for the three mating surfaces to ‘squish’ together and make a good seal without adding gobs of sealant. Look closely at the photos below to see the gaps between the o-rings and cylinder head.
The original o-rings supplied by the factory were roughly 4 mm X 4 mm (or 5/32 inch X 5/32 inch). This larger dimension fills the cutouts in the cylinder head and cam covers minimizing the amount of sealant needed.
Unfortunately, I don’t think anyone makes an oil resistant 5/32 X 5/32 inch square o-ring in a 1 1/8 inch inside diameter.