I recently purchased a CRF50 dirt bike and was going to do an oil change on it. I was talking with a friend and they recommended I take the following steps when I do my first oil change to help "clear things out":

  1. Drain oil
  2. Remove spark plug
  3. Fill Crank case with gasoline through oil filler opening
  4. Cycle the kickstarter several times
  5. Drain the gasoline
  6. Fill with oil

I don't know a lot about cars/motorcycles but I've never heard of anyone doing this and it sounds a little sketchy to me.

Would doing this potentially damage the engine? Does doing this actually help clean the engine?

  • Is this a 2-stroke or 4-stroke engine? Commented Apr 23, 2020 at 19:07
  • 1
    I would not do it, fire hazard.. Its not necessary, if the oil is thick and sludgy, then use naptha to thin it , then drain.
    – Moab
    Commented Apr 23, 2020 at 19:15
  • @Pᴀᴜʟsᴛᴇʀ2 - it's a 4 stroke
    – Broham
    Commented Apr 23, 2020 at 20:08
  • 4
    Oh heck no. You'll never get ALL the gas out, and it will dilute the new oil and make it MUCH more flammable. Commented Apr 24, 2020 at 6:13
  • 2
    @barbecue - point of order - he's doing this on a motorcycle so technically he'd be blowing his ass off. Commented Apr 24, 2020 at 22:03

3 Answers 3


Gasoline flushes the oil out of the very spots where you need lubrication, so the next time you start it, the bearing surfaces are "dry" until the oil pressure builds up enough to relubricate them.

This technique is mechanic folklore and should not ever have been done. If you think the current oil is excessively dirty, there are engine "detergent" products that you can add to the old oil and run for a while to help free up sludge and dirt and get it down into the oil sump.

Not only is it a fire hazard (as noted above in one of the comments) it's extremely hard on your engine. Don't do it!!

  • 3
    Note that if sludge and deposits are really of concern, then I'd add the the "detergent" to the new oil and let it run for a while, then change the oil again.
    – Makyen
    Commented Apr 24, 2020 at 3:13
  • 1
    Another concern is what do you do with the gasoline when it is drained. Oil recycler don't want waste oil diluted with that much fuel.
    – mikes
    Commented Apr 24, 2020 at 9:35
  • 4
    "mechanic folklore " "never set a car battery on a concrete floor or it will discharge it, put a wood board under it" was one of my favorites.
    – Moab
    Commented Apr 24, 2020 at 11:57
  • To be fair, leaving a battery on a plain concrete floor poses some risk of water or other liquid pooling; placing it on a board at least removes that risk. The reasoning is likely back-justification because obviously nobody would ever put batteries into a puddle... @Moab
    – Nij
    Commented Apr 26, 2020 at 1:26

There are solvent additives for cars; You add a quart to the crankcase shortly ( several miles ?) before an oil change. Filling the crank case with gas sounds excessive and I think you are pretty close to an explosion/fire , depending on how much oxygen is in the crankcase. As an old person , I can understand the intention. When you dropped a pan or pulled rocker covers of a used American car from the 50s, 60s, 70s : you could expect to use a putty knife to scrape out 1/4" thick sludge.


A Honda CRF50 holds a total of 0.8 litre of oil (28 fluid oz.), according to page 3-12 of this manual. https://coodie.com/crf50/Honda%20CRF50F%20Service%20Repair%20Manual%202004-2015%20-%20Chapter%203%20-%20Maintenance.pdf

I would drain out about 5-6 oz of oil, (about 20%), and add 5-6 oz of Seafoam Motor Treatment.

Take the bike out for about a 10 minute ride to warm the engine (and oil) up. Then drain and replace the oil.

Seafoam Motor Treatment can also be added to your fuel to help clean out the carb.

Do some research on Seafoam Engine Treatment. A lot of old car, truck and bike owners swear by it.


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