Dangers of overfilling engine

was asked this the other day.

so what are the dangers of over filling (not by alot) an engine?

of the top of my head things i can thing of are:

increased pressure leading to blown gaskets
increased engine temp as more oil means more heat
increased chance of getting oil into the cooling system

anything else you want to add?

Locked my gearbox, was stuck in 3rd, blew a bit into the airbox - that was overfilled by about 500ml


That was once it heated up to a certain point, was an interesting ride home with only one gear available

btw how did you get almost an extra pint of oil in there? no dipstick or sightglass?

in extreme cases

valve stem seals

crank shaft seals

ive seen car engines that have been filled almost to the top and run, ripping the pistons off the crankshaft, stripping cam belts and generally screwing the engine.

Dipstick = me!

I kept lobbing it in and turning it over to check get the oil circulating a bit, then checking it…guess I wasn’t waiting long enough before checking (the oil’s pumped into the engine and rad then) and has to trickle down into the oil pan before you get an accurate reading.

i did it to mine and it leaked by busting the cam chain tensioner gasket. easy and cheap fix so I was lucky :pinch:

The cars I’ve tinkered with (i.e. old) had a breather
from the crankcase to the airbox. And I remember overfilling
a Talbot and getting huge clouds of smoke as soon as
the revs were held high, i.e. on the motorway :w00t:

I’d expect most engines would just burn off extra oil
in a similar manner and hopefully before the extra
pressure did any damage.

The gear change gets notchy on the SV when there’s
more than about 100ml too much or too little.

Diesel engines are great fun if you over fill them enough for the oil to get from crankcase breather to the air cleaner. The road behind disappears in a cloud of smoke and you accelerate at full throttle waiting for either the whole thing to go bang or run out of overfilled oil.

Or of course for the brakes to burn out, as this was on the M25:w00t:

Lot of rubbish being talked ! Old wife tales !
Bike crankcases are vented ! Overpressure is nearly impossible ! Oil pressure is created by a pump not head of oil
Megga overfill like 33% might just cause a little froth to get in the airbox !

As for gearbox locking sorry something else caused it !

I had customers bikes in with 2/3 times too much oil in them all that suffered was the air filters and the plugs when oil was drawn up the breather system into the air box and from there into the cylinders. very smoky but no permanent damage except air filter and plugs

Oh and by the way lots/most sprint /drag racers over fill by 20% or more to allow for slop under hard acceleration !

Diesels are different overfill a diesel engine sump is indeed a cause of run away engines !

interesting post Dave…

Hmm - I did an oil change, first ride after it the gearbox locked when it got hot, I dropped the oil and put it back minus 500ml, ran it for 4000 miles more with no problems and gear shifts as smooth as a baby’s bottom. I’m not a religious man, so ruling out ‘acts of god’, the oil level is the only explanation…

If you over fill get a bendy straw. (The type the american use in milkshakes) Put it in the oil filler hole and suck the required amount out.


I suspect the " locked Gearbox" was a stuck clutch too much oil can give problems with clutch not disengaging but no way would excess oil affect the gearbox itself

Oh yes I am a professional mechanic been working in the bike trade since 1977 so seen most things and heard some whoppers told by dealers ! and owners !

I recently drained 8/9 litres out of a BMW r1100 that holds 3.75 now problems at all ! and we had an ER5 in with more than 10 litres in it I gave up measuring ! again just rode in, but in that case complaining it was smoky ! and the clutch sticky !

It happens, not as often as low oil but way less problems and easer / cheaper to fix

I suspect not, as I can clutchless shift but in this case I was standing on the gear shifter and it wouldn’t change.