Slotted Around the bearing Top End Caps on Merc 44s, not on the Bottom??

I was machining down a bunch of top and bottom end caps for Merc 44 this evening and it dawned on me that 2 top end caps have slots that ring the bearing though still separating the bearing with a casting wall, this being that these 2 end caps are not the solid castings I am used to that other than bearing and seal(s) are solid. None of my bottom end caps have these slots as part of the casting surrounding the bearing. Never had a top cap failure either so, what is Merc's idea behind those die cast in slots? Is that supposed to be some air/fuel cooling idea? They just trying to save on aluminum by saving some in this kind of diecasting? Trying to save on overall weight of the engine? All of these or none of them?? Last time these caps were used were on Quincy Flathead 4s at the time with different owners from the caster oil coating varnished on. I would not normally use them because the ring of slots increase the crankcase CCs which racing engine's in principle do want an excess of. Was there a "heating" problem with Quincy Fatheads top cap bearing system causing them to use a slotted type end cap to shed heat??? I turned off 8 top cap collars and 8 bottom cap spouts and thise slotted ones were the only 2 there of their kind I got here. I came up with no duplicates out of the other pile of tops and bottoms too from some Merc 400s I salvaged parts from, so where would those have come from anyway?? From another model of Merc???

Don't read too much into the two design versions... the 'slotted' caps are just an earlier version, used on the Mk 30's and Mk 55's. They interchange with the later (I'm guessing 1958 or so... I know '59's had them) 'solid' caps, so either works.

I prefer the 'solid' cap for the cc volume reasons you stated, plus they seem to be better at not stretching on ID, as there is more material, so the bearing is retained better. I've never had a 'solid' cap drop the bearing. They stay pressed in, bottomed in the bore. Of course, you wouldn't use any cap with obvious damage, such as a damaged bore ID with a ridge of displaced aluminum from someone previously pushing in a bearing unevenly.

There are two versions (or more) of the 'solid' cap... early ones are a match dimensionally with the earlier slotted version, later yet versions (Mk 500) are about .100" shorter; that is they don't project as far into the block. That was to provide clearance for the later cranks with the thicker top counterweights (about .100" thicker, what a coincidence). Obviously, you won't interchange those.

Also, some have an oiler hole between the the seal and bearing for an external oil line. Use it or not. I prefer to use that feature, even on race motors.

Hope this helps.

Jerry Wienandt