Engine blowby

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Jwbarker
Posts: 6
Joined: Wed Dec 09, 2020 11:41 am
Zip Code: 99344

Engine blowby

Postby Jwbarker » Wed Dec 09, 2020 11:56 am

I have a 1950 cub that has quite a bit of blowby and collects a little oil around two spark plugs was thinginking about pulling the pistons and checking them out and maybe replacing the rings would this remedy the issue if the sleeves and pistons are in good condition I think part of the problem is the last owner eplaced the head and didn’t check anything else so the new head is putting more pressure on the rings causing the blowby thanks for any and all input

Cubfriend
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Location: Grand Island, Nebraska

Re: Engine blowby

Postby Cubfriend » Wed Dec 09, 2020 2:44 pm

As a rule Cub blocks are not sleeved like a Farmall A would be. The cylinders are bored to size and honed to fit the pistons. If there was damage to a cylinder it is possible to put a repair sleeve in the block. Usually cheaper to find another block though.
Frank

Jwbarker
Posts: 6
Joined: Wed Dec 09, 2020 11:41 am
Zip Code: 99344

Re: Engine blowby

Postby Jwbarker » Wed Dec 09, 2020 3:11 pm

Thanks I will tear it down and see what is wrong

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Glen
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Location: Wa.

Re: Engine blowby

Postby Glen » Thu Dec 10, 2020 1:45 am

Hi,
Cubs originally have no cylinder sleeves, the cylinders are made in the block.

There are many posts on here about people using an oil additive in the engine oil, and getting good results reducing oil burning, and blowby.
Seafoam is one of the products they use. It takes time for it to work.

The additives help free stuck piston rings. It is easier than taking the engine apart.
One of the experts on here said to use the additive for 2 oil change periods. The 1950 Cub operator's manual says to change the oil every 120 hours of use.
They have said make the engine do some work, and warm it up.

A new head wouldn't cause any more compression than the old head, if the combustion chambers are the same shape.

I would give the engine a compression test, and see what it has, you can tell if the engine is still good.
The Cub service manual says a Cub engine has 120 lbs of compression, that is probably for a new engine.
Less is ok, they still work. 90, 100, 110 lbs is still ok.
Remove all 4 spark plugs at the same time, and use the starter for the compression test.
Open the throttle fully when doing the test, so the engine can get air.
If you write down the results, and you will have a record.

If it has good compression, but uses oil, the oil rings could be stuck. The additive may help it, unless the rings are wornout.

Something to remember is, pistons and cylinders don't stay round when they wear, they get sort of an egg shape.
It discusses this in the Cub service manual. So if you put new round rings in an engine that has egg shaped worn cylinders, the rings may not seal right.
The egg shape probably can't be seen, you have to measure the pistons and cylinders to find it.
Reboring is the fix for that problem, then the cylinders are round again. :)

Jwbarker
Posts: 6
Joined: Wed Dec 09, 2020 11:41 am
Zip Code: 99344

Re: Engine blowby

Postby Jwbarker » Thu Dec 10, 2020 3:04 pm

thank you for the information I was looking through the pdf manuals tryin to figure out what one to use


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