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Flathead engine oils

Moderators: Curt!, Pa



Posts: 20

Joined: Tue Nov 13, 2012 7:53 am

Location: East Central Illinois

Post Fri Apr 05, 2013 6:50 pm

Re: Flathead engine oils

Just my $0.02 worth.......I've always used Valvoline Racing 50 wt. in my 45........arc1953


Posts: 90

Joined: Tue Dec 25, 2007 4:04 pm

Location: Norway

Post Sun Apr 07, 2013 3:08 am

Re: Flathead engine oils

knucklebolt wrote:Heavier oils, and synthetic oils especially, I believe, have much greater shear strength than light oils. So, just for the sake of debate, you might speed up oil delivery a bit with a light oil, but you lose some shear strength. More quantity, less quality. ??

Having said that, I suppose it could be true that roller bearings don't require the shear strength that a plain bearing engine would.

I tend to lean towards heavier oils, I'm just not convinced that lighter oils flow that much faster, or that it makes that much difference with a roller bearing engine. I think our imaginations tend to get carried away thinking that heavy oil flows like gear oil or maple surup. With plain bearings, I think that's probably more true, or more important, to get oil to the rod and main bearings faster. Synthetic 20w50 seems like a good compromise for me...but the last time I had a 45 was back in the straight weight days, and I'm just now putting a big twin flathead together...so I've not tried it yet. But that's what I'll start out with.

I think you might be missing my point. Heavy oil do not give lesser speed on the oil. And it do not contribute to slow down oil delivery unless the oil is so thick that the pump struggles to suck the oil form the tank and cavitates. However on flatheads the relief valve sends oil to the cam chest. Too heavy oil causes the relief valve to open early and sending more oil to cam chest and less oil to the rods and pistons. This is not the case om sporsters, because they do not have this valve.


Posts: 1588

Joined: Tue Nov 27, 2007 2:29 am

Post Sun Apr 07, 2013 10:14 am

Re: Flathead engine oils

personally, as a general comment I use the lightest oil I can get away with. On old engines, especially roller bearing ones with low to very low system pressures and posibly, part of the circulation controlled by internal pressure differentials, this is often a matter of what will stay in the engine, and not leak everywhere and spray out of the breather, if fitted, or the cam cover or rocker boxes if not!! Silkolene SAE 40 is good http://www.opieoils.co.uk/c-1489-classi ... -oils.aspx and the Castrol Classic range needs no introduction http://www.castrol.com/castrol/sectiong ... 99#7094382

I tend to use straight 50 for machines working hard and 20-50 for general year-round use or road use on machines doing limited mileage, mainly because it's easier to start the bike and my knees aren't what they were
Shoot, a man could have a good weekend in Dallas with all that stuff...

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