smiling_simon wrote:
justminis wrote:
These are both 1275 cranks. The brass plug in the big end journal indicates this is a cross drilled crank and the plug is used to plug off that oil drilling.
Can you explain this a bit further? I had my crank cross-drilled on the mains and was told by someone to watch out for loss of oil pressure? Mines an A+ crank.
Many small journal 1275 cranks (including Cooper S) had the big ends drilled as normal then cross drilled, and the original outer hole plugged with these brass plugs.
The idea was not to get more oil up there, but to put the oil into the bearing at a lesser radius from the crank axis. This reduces oil throwoff due to centrifugal force.
With A+ engines the designers thoughtfully(!!??) put a plain bearing into all 3 main bearing caps, with no oil groove. This was ostensibly to give greater load bearing capacity for turbo engines. Unfortunately, as the mains only have 1 drilling to the big ends, the big ends all get starved for 1/2 a revolution (from 9 o'clock to 3 o'clock, where the load is greatest..
There are 2 ways to fix this:
1. Use A series bearings (cheaper, too). You just need to provide an offset tang slot for the bearing shells. or,
2. Cross drill the mains so there are 2 oil pickup points to the bigends.
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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
