I am currently rebuilding a 2000 750ss engine having had it sat around in bits for far too long. The front horizontal cylinder appears to have a bush with a restriction of about 1.5mm. The issue is I cant find the vertical cylinder bush. I do have another engine in bits but that bush appears to have no 1.5 restriction in the bore but drilled straight through at about 4 mm. To confuse myself further I took a look at the stein dense parts diagram for a number of the 2 valve engines and they all quoted the same part number for both heads. Also not possible to determine if from the picture it’s a borrow bore or lager bore. I’m totally confused now. Can anyone shine some light on my ignorance? Thanks in advance Keith I enclose a picture of the parts diagram MM
Hey Keith, having worked on a few 2V engines I feel I can help. I believe this is a restrictor, to allow the oil to soak a bit in the head before travelling back down. I would imagine this is to help take some heat away. I could be wrong though, I will get the oil feed diagram... I know exactly the two different bores you speak of, as I've been into a 600, 650, 900's and 944 engines. My own 900 had the smaller hole restrictor. You should have 2 of these, both cylinders should be the same. I have spare if you are struggling.
Yeah, I'm slightly wrong, I think it's to restrict flow into the head, so the BARREL fills first to take the heat away. Unless this diagram suggests otherwise. I have the external lines too
you have it ^ - it's a "tunable" oilflow restrictor. As great as some engine designers are, there are some areas that will always need a trial and error approach, plus the same head casting could be common for engines with different states of tune, oil pumps etc so a manufacturer retains the ability to minutely change oil feed/return depending on demand to an area.
As well as controlling the flow I'm thinking it would also control pressure, like putting your finger over the end of a hose. And as Chris mentioned the flow is helped by having a sort of one way valve so it is staged to the head. Much like the valves in the leg veins. I don't what the oil pumps are like on the 750 but certainly for the bevels they are low pressure gear pumps.
Likewise my M900 had them in both cylinders but, I assumed this is because the older 900 engines have the oil cooled cylinders and so, restricting the flow would allow them to fill properly before passing oil to the cams? Did the 750 have the oil cooled cylinders? (return pipes to the crankcase on the LH side) I didn't think they did, in which case the cylinder only acts as a pass-through to the head so maybe restrictor not required? but a hole definitely needed. I fitted direct oil feed lines to the cams on my M900, coming straight off the pump and so plugged the cylinder to head oil feed holes with grubscrews, so that all the oil in the cylinder jacket is used for cooling only, and then used solid dowels in the holes as locators (though the restrictor would have worked fine).
