226 rear main bearing cap
 

I am rebuilding a 226 and am about to install the main bearing caps. I read some where (perhaps the instructions for the rear seal from Best gaskets) that the block to cap mating surface for the rear main bearing cap should be sealed. Why? I understand why in a SBC engine since the rear seal is integral with the cap. On the 226 though, the back cap is fully inside the oil pan and the rear main seal. Has anyone felt the need to seal this cap and if so, why?

I would say "absolutely not"! How could you get the proper clearance?? Isn't the flat 6 like the flat 8's, with a separate seal holder?

Absolutely, the flathead six also has a separate seal holder which is why I question sealing the caps even though I don't think I have the channels cut into the block as shown in your photos. Sealing these would be useless in that case. I don't believe it would do any harm regarding clearances since you use an anaerobic sealant.* Perhaps the instructions I saw were generic since this procedure is necessary on certain engines including all older small block chevys.

*You could have clearance issues if you used a silicon based sealant though.

Even an anaerobic sealant would add a couple thou to clearances. I've never heard of any sealants except on the pan rail where it joins the rope seals. What oil flow are they trying to stop??

I was a Chevrolet dealer mechanic for many years before defecting to Ford. On Chevy motors the back of the rear bearing cap protrudes past the pan and holds the rear main seal. If the cap is not sealed oil will leak past the cap and eventually onto the floor fooling many people to think the rear main was leaking. The anaerobic sealant should not add any thickness as it is designed to only fill porous areas while the rest is squeezed out. Silicon typically will not do this but leaves a thickness across the area being sealed.
That being said, I am leaving the cap unsealed as on these engines it is not needed.

I found where I read about sealing the bearing caps. I was the instructions that Best Gasket Co. has for the graphite rope seals. Specifically it says "Coat all cap to block mating surfaces with sealant. DO Not apply sealant on rope ends."

The caveat is that the instructions say "general instructions - not application specific". I am now sure it does not apply to our flatheads.

Check this article. Rear Main Seal Repair | Hemmings Motor News

Even there, they aren't coating the actual bearing portion of the block or cap. They're using black Permatex goo.

This is what I was talking about which is referenced in the Hemmings article;
"Larry likes to place some sealant on the block where the cap meets the seal." In our vehicles the main bearing cap does not meet the seal so sealing is unnecessary I believe. I NEVER advocated coating the actual bearing portion of the block or cap.