446 Engine

[propane1]

So both pistons were replaced ? Right or wrong ?

Noel

[Eugen]

Good. The BF-MS pistons are both 1/8 smaller than the original B43!

That's a big gap!

You have to pull them out and get the right size.

[Vivala446]

That’s correct. Both were replaced.

I held the old piston up to the one currently in the cylinder and it looked to be the same size. No local auto parts store has calipers on a loaner program.

Am I to understand both BF-MS and B43M have their own individual STD sizes?

[Timj]

That’s correct. Both were replaced.

I held the old piston up to the one currently in the cylinder and it looked to be the same size. No local auto parts store has calipers on a loaner program.

Am I to understand both BF-MS and B43M have their own individual STD sizes?

Yes, they are both standard for that particular engine as it was built. So the standard in MS is an eighth inch smaller.

[Vivala446]

Ok. I’m going to really show my ignorance here (but I learned long ago a truly smart man admits when he doesn’t know something) it’s still possible that I just have the wrong size pistons for the cylinder?

[Eugen]

Don't worry, we have many beginners here, including me! We all learn.


Yes, 100% you got the wrong size pistons. The pistons you got are from a different type of Onan engine. The pistons you got from eBay are 1/8 of an inch smaller diameter than the pistons that are the correct size for your engine.

You have the B43M engine model. The pistons you got are from the BF-MS engine model.

As the engine gets older, the bore of the cylinder gets worn out. All engine start with STD size pistons, and the diameter of the cylinder bore for STD pistons for your engine is 3.25 inches. The manual states that the maximum clearance between the piston and the cylinder wall is 0.0053 inches. Compare this, 0.0053 inches to 0.125 inches (1/8) which is what you have now, probably even bigger because of wear.

So, no doubt, you pull those pistons out, they are the wrong size.

[Vivala446]

Alright. Thank you all so much. I can’t thank everyone enough!

[Spike188]

This is a standard piston that is beyond end of life. Notice it is smaller than the bore to the point of exposing the top ring.

This is the same engine with a .020 oversized piston after the cylinder was bored to match the piston

Here is an example of how to check ring gap.

Spike

[Vivala446]

I went out to the garage and measured the pistons as best I could with a tape measure (for lack of anything else) and the ones I ordered to appear to be a hair smaller than the original one

[Vivala446]

Would it be best to get brand new rings to use on the correct pistons?