446 Engine

[Vivala446]

Wow… I’m overwhelmed and grateful for all the knowledge here. Let me see… I didn’t check the size of the pistons but the rods had the same numbers stamped on them as the old ones so i assumed (perhaps wrongly) that everything else was the same too. I didn’t touch the valves or pull the crank. Just took off the heads and oil pan and changed the pistons and rods. When the pistons are extend level with the head.

[Vivala446]

Wow, I’m overwhelmed and grateful for all the knowledge here! Let’s see… the new (used but new to me) rods had the same number stamped in them as the old ones so I assumed everything else was the same too. I didn’t mess with the valves or crank. Just pulled the heads and oil pan and replaced the pistons and rods. It ran on one cylinder before. Now, nothing.

[Spike188]

If it ran on one cylinder before the rod replacement, that hints that there was good compression on #2 cylinder.

Looking back at your first post for clues, where did you get the rings from that you used on the replacement pistons. You changed both pistons, did they come with rings?

Being a scavenger myself, and a onetime car jockey, I know salvaged piston can work. A dealer brought us a 305 GMC that had overheated and scored one cylinder. We did and in-frame bore using a hone with diesel as cutting fluid. A 0.030 oversize used piston was purchased and we hone the cylinder until the scoring disappeared and the piston fit. We even reused the salvage yard rings. That pickup engine outlasted the gentleman that bought it from the dealer.

You must have some mechanical knowledge or you would have attempt this. Since we don't know your mechanical background or what tools or experience you have, I will not make any assumptions.

Spike

[Vivala446]

I bought the replacement pistons off eBay. They came complete with the rods and rings already assembled.

As far as my mechanical knowledge, I’m self taught over the last 20 years or so. If something needs done I study up on it until I feel
comfortable enough to do it, then plan the job so it goes as smooth as possible. Doesn’t always work out, but I find when things are successful you end up knowing your machine better than anyone else.

[Eugen]

in my opinion you must determine if the pistons you got from ebay are the same size as the pistons you pulled out. I think I can save you some work. Those you pulled out must be stamped; if they say STD, you're lucky in a way, because no way you could fit an oversize piston to replace an standard size piston, if the bore was not worn out like crazy. If the stamp on the pistons you pulled out says 020 or 030, or even 010, it's possible that the pistons you got from ebay are STD. I would still expect some compression to show, not zero. Zero psi compression is at the opposite spectrum of Dave's 140 psi compression. Don't shoot @DavidBarkey

[Vivala446]

Unfortunately, I already tossed the old ones. I don’t recall anything being stamped into them and I know there isn’t on the eBay ones

[Eugen]

Unfortunately, I already tossed the old ones. I don’t recall anything being stamped into them and I know there isn’t on the eBay ones

It's a bit of a tough situation you're in, because you probably don't have the tools to measure the parts you have.

Usually ebay sellers do mention the size of the pistons. Have another look at the listing.

I'm still thinking there's gotta be something simple that has been overlooked though. Maybe one of the other guys see it, as I tend to over complicate myself thinking about things.

There will be a lot of ??? signs about it all unless you can go by the book. Are the heads flat, were the head gaskets new, all the head bolts torqued to spec and in the right order, and so on.

I doubt you get zero compression from orienting the rings wrong on the piston, but if you haven't paid attention to the ring gaps when you reassembled the engine, most likely that's also wrong.

At the very least you'll need a 3-4" micrometer and telescoping gauges to measure what you got there. The rods you got might not be a good fit for the original crankshaft. I totally understand you taking a risk, and when it works, you're flying. But it didn't, so I think you need to be careful now and go by the book, there's no need to throw the baby with the bath water. To measure the crankshaft you'll need a smaller micrometer, and I don't know which, as I don't have the Onan manual handy. Probably 1-2".

me here.

[Eugen]

Let me add this. Onan engines are not the cheapest to rebuild, but they'll last many years again, once rebuilt. In general, the tools to rebuild an engine are not cheap. Might be better to have a shop do it, if you don't think you'll do it again and again.

Also, in my opinion, don't give up. I got my first Case tractor because I did not have patience and I wanted one, it was not the best but it was good to begin with. In the next 3 years a lot of really good deals on Case tractors came up and I got some of them.

We'll help you as far as you want to go with this engine. If you were closer it'd be much easier too!

[DavidBarkey]

@Vivala446 After all I have read I feel one of two things is going on . Either the rings are stuck in the grooves / wrong piston size.
Or the head gasket is on wrong .
How to tell where the compression is going is to do a leak down test .
Take both head tins off , remove both spark plugs, remove breathe assembly and air hose to fan shroud , look down large hole in shroud with flash light , rotate crank shaft to top dead centre marked on flywheel (fan) , pull dip stick out ,open choke and throttle, using compressed air and blower with rubber tip, blow air in each spark plug hole and take note of where the air comes out . Rotate 1 turn and repeat .
The cylinder that is on compression will leak air out of the fault and the other will leak out what ever valve is open .
Example
- ring problem will come out dipstick tube.
- head gasket will come out where head and block meet .most likely the top
- valve problem will come out carb or exh on both sides on test 1 and test 2 .

note engine can be turned over by engaging the clutch and turning the pto pulley
Good luck and let us know what you find .

Dave

[propane1]

I think that some times when a piston has no marking it means it’s standard. Again I’m guessing.
When I rebuilt my TEA20 Ferguson tractor engine, I bought used sleeves and pistons from a fella in Ontario, who deals in old tractor parts. I also got new piston rings, gaskets and bearings too. I had to reuse one ring off the old pistons to fit in the bottom oil ring groove of the new used pistons. The new rings would not fit this lower ring groove. These engines had an oil ring at the very bottom of the piston below the wrist pin.
Any way, the old 51 Ferguson works great. Engine was done about four years ago.


Noel