rocker tins, cups, curious holes

[poorbiker]

Bought a complete set of lower tins at a local swap meet a few months back. OEM, in good shape, greasy was all.
Got around to cleaning them for an up coming build. Once cleaned I found in each cup located at top and toward rocker box
on the high side of each cup a very small hole, one in each tin.
was this some bodies failed attempt at helping to drain the oil or is this some little trick that I am not privey too...

[Jerry Wieland]

The holes in one of the cups could be OEM but not all 4. There is somewhat of a controversy about whether or not all knuckleheads had them. The perceived reason is to let air in to replace the oil being evacuated by the suction of the crankcase. Usually the hole if in the intake tins if I remember right.

Jerry

[poorbiker]

I wonder if someone thought "if one is good maybe three more are better" lol.
What potential issues would there be in leaving them? I could see some problems
but would like more input, thnx Jerry for the info.........

[ricmoran]

Jerry, you perception is correct. The 1/16" holes were in the intake tin lower body only. They are there to prevent top end over oiling and were used when the top end went from open spring covers to sealed. Without them the vacuum to suck the oil out of the top end can be compromised. You'll know if it is if you get oil coming out of the exhaust tins onto the headers.

[poorbiker]

Ricmoran,

Thnx for the reply, were the holes in both of the intake cups and none in the exhaust, correct.

[Chris Haynes]

Ricmoran,

Thnx for the reply, were the holes in both of the intake cups and none in the exhaust, correct.

Correct...

[poorbiker]

Okay, thnx.
I already brazed up the ones in the exhaust, will leave the ones in the intakes. I knew about the early 36s being open to air but not the vent holes on the later ones once they put covers on.
Can I ask why they decided the same reason for the holes were not nessesary on the exhaust cups. thnx again.

[poorbiker]

Also, do any of you know where I might find/buy a .30 over breather valve. I have Jims tool ( reamer) that will do knuckles (I been when needed ) using it on Pans. But it dont look like Jims sells a .30 over valve for knuckles.....

[Matt McManus]

S&S makes one for a knucklehead

[poorbiker]

Thnx. Appreciate the info

[RichO]

I know the early knucks had a hole in both bottom intake tins but I have a '40 with it's original heads and tins and it only has a hole in the rear bottom intake tin. We've (a few in my circle of friends, I have a few so don't laugh) who think they( the factory) found that one worked o.k. and so they eliminated the front intake one.

[ricmoran]

Okay, thnx. Can I ask why they decided the same reason for the holes were not necessary on the exhaust cups. thnx again.

Can't be certain about why these were seen only on intake and not exhaust. I think perhaps this has to do with the breather timing, where perhaps the breather is closed on the intake stroke (or could be vice-versa, closed on exhaust stroke....not certain) that a vacuum is produced that when a small relief hole is produced allows enough vacuum bleed off that allows the vacuum to draw the oil down from both exhaust and intake tins at the same time.

As I wrote above I am not 100% certain of this of where the timing of the breather comes into play with the vacuum and oil evacuation from the tins, but vacuum is created within the engine on one of the strokes and that vacuum is what aids in oil removal from the tins. With the exhaust tins being lower than the intake spring tins, the exhaust tins would have more oil accumulation than the intake tins. No evacuation and the oil accumulates to a point where the engine goes from vacuum to pressure causing what I believe the accumulated oil to blow out between the cover, lower tin and gasket.

[poorbiker]

That makes sense, about breather timing, maybe vent holes in the exhaust tins just becomes overkill where vaccum is concerned.
Speaking about breathers I still cant find an aftermarket std or .30 over for a Knuck, S&S does sell one but it is for the Pans, needing shims.
The smaller tit end breather, really dont want to use shims. Cases are geat but breather hole needs either repaired or reamed and on Knuckles
.30 over is close on the bore. Anyway thnx Ricoman for the info

[Jerry Wieland]

Personally I prefer the panhead style of breather with the shims. I use them in knuckleheads all the time. Just make sure they have the return oiling hole like knuckleheads use. I really doubt if anybody is making the knucklehead style in that big of an oversize. Jerry

[poorbiker]

Hey Jerry,
S&S makes a .30 over along with the std. Both for the Pans. I may not have a choice but to use the later style. Actually the one S&S makes is for the Knuckle but has the small tit still requiring the use of shims