Board index Flathead Power-Technical Questions, Answers, and Suggestions Big Twin Flatties Engine Flush

Engine Flush

Post Mon Apr 06, 2009 6:39 pm

Posts: 83
Do any of you guys use engine flush before a oil change in a UL Model ? I mean one of these kerosene based engine flushes, Amsoil Engine Flush etc. I've always used on my previous cars and never been a problem, wondering if this can be done to my old sidevalve. The last owner used dino oil and no filter, so I expect to find a good amount of crud/sludge...

Post Mon Apr 06, 2009 8:14 pm

Posts: 2688
Location: Los Angeles, CA
Sometimes it takes a stick to get crud out of an oil bag. The early 1936,1937 model tanks have a screen in them the will plug up easily with crud or moisture. There was a service bulletin about cutting the screen to help oil flow.
If you do use an engine flush only use it at at an idle. Don't ride the bike with it in it.

Post Mon Apr 06, 2009 10:33 pm

Posts: 49
Chris is absolutely right about the flush procedure; don't ride the bike! I will go even further and say don't even start the engine.

If you read any of the excellent documents that were published specifically for the military flatheads (the Army "TM" manuals) they will all tell you that flushing the oil tank was a static procedure. They did not use a kerosene based proprietary flushing product, they used pure KEROSENE, and they did it without running the engine. As Chris mentioned, it required some kind of mechanical means to help dislodge the sludge and get it into solution so you could drain it out of the oil tank, a stick, agitation, whatever.

Run the engine until it reaches operating temperature with the old oil in it. If the scavenger pump is doing its job it will pull most of the dirty oil out of the sump and return it to the tank. Drain the tank completely. Add your solvent of choice (kerosene or proprietary engine flush) and slosh it around as best you can, use a stick to stir it if you like, and drain it completely. Refill with clean oil and repeat at normal change intervals. If your cases have a drain plug (some don't) you can remove the plug and drain the sump after the initial warm up and before, or while, you do the tank flush.

The best advice is to change the oil frequently and when you ride the bike you should ride it long enough and far enough to thoroughly warm the oil so that it circulates and gets hot enough to flash all the moisture out of the system.


Post Tue Apr 07, 2009 5:09 pm

Posts: 83
Thanks Chris and Mike!

Return to Big Twin Flatties