How tight should the motor be after a rebuild?

Leonok · Jun 27, 2008

How tight should and motor be after a rebuild. I am in the process of rebuilding an F-20 with new rings and resurfaced rod bearings. The rod bearings all have 3 thousands clearance. The motor will turn fairly easily with a pry bar until I tighten the last rod bearing—it doesn’t matter which one is last. After tightening the last rod bearing the motor tightens up quite a bit. I can still rotate the crank one full revolution but it takes a lot more effort. Is this normal or too tight? Thanks

Ron in Nebr · Jun 27, 2008

Did you put in new pistons/sleeves? Or just put new rings on the old pistons? Sometimes new rings on old pistons will make a motor tight if the ring grooves haven't been completely cleaned. The old carbon in the grooves won't let the rings compress fully. Just an idea...

old · Jun 27, 2008

As Ron says and also. Did you use a good lube on ALL the parts when you put it together?? If you didn't then thats part of the problem. But yes when you install new bearings etc they can/will be tight till they have been run and things brake in a bit
Hobby farm

Anonymous-0 · Jun 27, 2008

without the pistons you should be able to turn easily, put the rings in and have to use a wrench or short bar about 30 to 40 foot pounds

Bob · Jun 27, 2008

If the rod bearings REALLY have three thousandths of on inch oil clearance, tightening them all down should have NO effect.

F-Dean · Jun 27, 2008

I use .005 on my F-20s. Be sure to lube everything when you put it together for the last time. I use automatic transmission fluid when making the last assembly prior to putting the cotter pins in. Your F-20 will probably turn less than 1,000 rpm so it is not as critical at a higher rpm engine.

Anonymous-0 · Jun 27, 2008

Use Plastigage. They sell it at most auto parts stores. Put a piece on the crank journal, tighten it down and then take it off and measure the width of the crushed plastigage against the measurements on the package. It will give you accurate clearances.
I just rebuilt an F-20 and using had to add a couple shims to get the clearance right. I set the clearance at about .005.

Anonymous-0 · Jun 27, 2008

In addition to what's already been mentioned, I like to get a torque wrech on the crank and measure the ft.lbs. added with each rod-piston assembly installed, then if you have one jump way up above the rest you know it has a problem.

NDS · Jun 27, 2008

Back in the day I have seen them put up so tight you would have to pull start them. Not good I know but they would loosen up and go out and make several crops probaly more hours than any F 20 will ever run from now on today.

steelfronts · Jun 27, 2008

In my experience with F20s if the crank isn't ground you will find the journals to be oblong from the thousands of hours of being pounded on from the top an bottom. Your new rod bearings may be tight on the side.Chec the roundness of your crankshaft if it is in fact round and you do have .003 clearance you're fine. The after the fact test is after running it the first time shut it off an see if it will turn over hot.

El Toro · Jun 28, 2008

You should measure or have someone measure your crankshaft to see if its egg-shaped. Could be your problem. Hal

How tight should the motor be after a rebuild?

Leonok

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Ron in Nebr

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