Flathead Blow By Simple Solution

It’s a look! It screams 2020!

Lately I have been had to wear a mask while out alone on WECATCHEM. The blow by smell is so bad you can taste it. Forget that the Boatress complains that her cloths smell like engine!

We will all just sit back here. Right girls?

After googling around and talking to folks, Boat Buzz, forums. They all say the same stuff. Rings and yada yada. For the record if you google a headache, it’s says cancer. Why does everything have to go to some extreme. Seems like we live in that time for sure. Here is a short clip to see the smoke.


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Now, I did some think’n. 6th grade engineering think’n. And thought. mmmmm, I haven’t changed this oil in a while. I wonder? I mean it smells like used oil. So. Then I got to think’n some more. It’s possible that the oil has broken down and lost some of its viscosity.  Hold on.. I need to take a break. That was a deep word I had to drag out of my brain.

Getting dark

The BEST tool ever.

Anyway. I changed the oil. Some drama huh? And guess what? Really it’s in the headline of the story! You need to guess? It fixed it. BIG TIME! No smell, no oil all over the deck near the breather. Just rumble rumble. Funny thing. Machines of a certain age, do not adapt to modern standards. They smell, they putter, they need a kick in the ass every now and then, and coil change very often. New spark plugs help as well. They really do.

Ahhhh thats beter

So if you are about to rebuild your engine because of Oil Blow by, change the oil first and try It.


17 replies
  1. tparsons56
    tparsons56 says:

    I hope you are correct in your hypothesis [I know – big word]. We had more of an exhaust smell then burning oil smell in the cockpit this summer with the M engine. I checked the torque on all the manifold bolts and got a 48″ pipe wrench to tighten down the tailpiece/exhaust pipe nut but to no avail.

    I changed the oil [Brad Penn 30W] before layup this fall so hopefully next summer things are better.

    A quick question: When extracting the old oil I use a Jabsco oil change pump [we used to have a 43′ motoryacht w/twin diesels that held 5 gallons of oil each – not doing that with a hand pump]. I pumped the old oil out through the dipstick hole but I noticed you removed the trans plate – is this the more common practice?

    • Bilge Rat
      Bilge Rat says:

      There is always about 2 pints of oil in the transmission that can’t be sucked out using the dipstick. The block and transmission castings have an edge that allows oil to flow between the two through some holes, but the angle of the engine traps oil at the rear of the transmission.

      Since this is at least a yearly task, I used stainless steel bolts and washers on the transmission cover to allow removal without wrecking the paint. Some say you can remove one bolt and snake the suction line through the hole if you know how to bend it just right.

  2. Old Salt
    Old Salt says:

    For blow by I’ve been using Shell Rotella 30 T1 Straight Grade for years in my model K engine. It seems to help with the blow by…

  3. Bilge Rat
    Bilge Rat says:

    Later Chris Craft engines had a closed breather and PCV valve in the line to help prevent the dreaded blow-by smell from ruining the wood boat experience. However, if blow-by was really bad, I’m sure it would be coming out of the oil fill cap under pressure.

    This pic is of my MCL with the breather highlighted.

  4. re c
    re c says:

    The problem is that you are combustion blow by past the rings.,
    Exhaust fumes are coming out of your crankcase ventilation tube. Rings cannot completely seal off exhaust fumes. Install a PCV valve like C-C did on later models and like was required on all cars in starting in the 60’s or 70’s. This isn’t a perfect solution but it is a substantial improvement.

  5. tparsons56
    tparsons56 says:

    A lot of good information here – Thanks everyone. You do learn something new every day – I just wish I had known this before I filled up the crankcase with new oil!

  6. Dick Dow
    Dick Dow says:

    Best perfume in the world – a lot better than that flowery stuff! At least that is what you can tell Suzy… 😉 I thought “Wecatchem” is in line for a new engine? Regardless, changing the oil makes a significant difference (for a while) when the motor has some hours on it.

  7. David Hughes
    David Hughes says:

    What type of oil are you using? Modern synthetic oils don’t do well in older engines with large clearances in the mechanical components as compared to new engines built with CNC computer controlled machines. I use Marvel Mystery Oil in my hemi’s…an old habit I picked up during my British motorcycle days.

  8. Mr. Asshat
    Mr. Asshat says:

    Hey David, some of my communist friends and I would like to know if you know exactly what that Mystery Oil actually does when mixed with the standard oil you happen to be using? Does it mess with the viscosity or ? Have you used it with both standard and synthetics? I have always found these type of products a bit hocus pocus and only mask a problem that should be addressed. I could look it up but you said you’ve been using for years. Just curious as to your experience.

  9. Waldo
    Waldo says:

    I have used Rotella 30 wt non-detergent with 1 Quart Lucas oil treatment in CC 131, in 1955 Continental. Runs great, no blow by and no oil smell. Had some smell with 30 wt Penzoil. Old mechanic told me the Rotella is what they use in Detroit Diesel 2 stroke. They were the original stink bomb. I have been very satisfied with the blend.

  10. Troy in ANE
    Troy in ANE says:

    Dictionary dot com
    Blow-by: leakage of the air-fuel mixture or of combustion gases between a piston and the cylinder wall into the crankcase of an automobile.

    One of the problems with Forums is that every member gives an answer based on their own experience which can be relevant or not.

    Another problem is that questions are often miss stated. If you asked “What is causing my Blow-by?” then by definition you most likely have a ring to cylinder issue. If on the other hand you ask “Why do I have smoke coming out my breather tube?” you may very well get very different answers. Obviously since changing your oil seems to have fixed the problem it is not really “Blow-by”.

    Just my thoughts on the matter.

    Oh the images I could post for this subject!

  11. G. Gordon Gray
    G. Gordon Gray says:

    Since we all must wear a mask, where did you get yours ??
    I’ve been looking for an appropriate woodyboater mask.
    No Luck

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