When to machine a crank

I've been hearing about guys just slipping new bearings into motors. What would make a fellow decide just to slide bearings in and not do a complete overhaul? Where is that dividing line? I have a Farmall 300 that need a new head gasket and thinking of just sliding in new bearings. Just picking your guys's brains on all this.

I've always heard that if your fingernail catches on the surface, it needs polished. This is probably old model T thinking.

Make sure to check that the rod journal is not out of round.

Beagle

You have to measure it to know what to do with it
not just throw parts at it . Need to measure diameter
and also whether its egg shaped to see if it need to
be machined undersized

The old school thinking was: If it was not knocking before the new bearings, it certainly will not knock after new bearings. or This engine made it xxxhours/miles without knocking by replacing the bearings I should get another xxxhours/miles. Both have a grain of truth but every engine is different so results will vary. As already mentioned, when you pull a cap, either main or rod, inspect both surfaces. If there are noticeable grooves on the journal and the old bearings are worn a couple layers down, get the crank turned or polished. If you remove the crank, measure for out of round journals. If nothing obvious appears wrong roll in new bearings. In all other cases new bearings are a band aid at best.

Does it have a working oil pressure gauge?

Idle oil pressure when up to operating temp is a good indicator of the health of the bottom end.

Every manufacturer is different, but if the pressure drops to near nothing, the bearings, and possibly the oil pump, are getting worn.

A leaking rear main seal can also be an indicator of worn main bearings. Worn bearings cause the crank to bounce around and beat the seal out, along with passing too much oil between the bearing and journal, flooding the seal area.

Don't be tempted to put undersize bearings on a standard crank! Cranks wear more on the outside of the rod throws because that's where the maximum load is applied. Even if it measures small there, 90* from there it will still be near standard specs. Too tight is worse than too loose!

first are u mechanicaly Inclined ? mechanic's know when and to and how. cause that has me wondering by u asking this question as your
sliding in new brg's. yourself. may i ask... do you know how this is done? as your saying guys just slipping bearings into motors...
[engines] . you look how brgs are wearing, measure your clearances with plastiguage on tear down. look and measure the crank. it is not
nessesary to be regrinding the crank if all is good and can be repaired with a set of brg's. plus the amount of hrs. on the engine, and oil
changes are in play also. the more often you change oil the less engine wear you will have. plus using the correct oil for the weather and
not pulling the guts out of it like steady lugging. and is this engine using oil? need to know that also. thats another thing that is never
said here. never ever hear someone say my tractor is burning a gallon of oil a day so i better give it an overhaul. and if you plan is to
do brg's at least do the rings also. as i always say when someone says they just overhauled the engine i get scared. just nicking the
crankshaft with a rod bolt is something you will never know till the next tear down. and all the other stuff not measured or checked. pull
a piston out replace the rings and stuff it back in , and good . ya right. lots more to it than that. everything inside the engine is
telling a story so read it. and when someone gives free info listen closely and observe that info, not get all pizzy pants and tell a
ticketed mechanic off like he has no clue, then they do the opposite. there is a few here like that . my squawking for the day.

If it were mine, and had fairly good oil pressure, I would just take care of the leaking gasket, and be done with it. Stan

Yip

Oscar Leroy.

If you are just putting on a new head
gasket for what ever reason (like
leaking?), and not doing anything down
below, I wouldn't worry about it. I
wouldn't tear the bottom end apart if all I
was doing was a head gasket. No need in
going down there, unless you suspect a
problem.

Now if you were putting on a rebuilt head,
and or new rings, or doing somewhat of a
partial overhaul, it'd be a good idea to
check things out. Or if you had the bottom
end apart and was there anyways, it'd be a
good time and idea for new bearings.

As others have stated, better know what
your doing when replacing rod and main
bearings. Alot more skill level required in
changing bearings, than say, changing the
oil or a head gasket. It takes more know
how. Not really so much in doing it by the
book right, but knowing what'll work, and
what won't. Knowing what your looking at.
New bearings isn't going to do any good if
it has a scored up crank. An experienced
mechanic that has seen some bad ones, and
some good ones, can tell alot by just
looking. And tell more by measuring. And
the measuring thing is an art of its own.
Alot of preciseness to doing that. No room
for sloppiness when it comes to the
measuring. Or really for doing overhauls in
general. Sloppy work will lead to big
problems.

I think most good repair manuals will give the recommended out of round and tapered measurements
allowed ,if any.

Suggest posting how to do a head gasket. reason is if youre just guessing on bearing install ,then I thinking youre just guessing on head
gaskets repair.

Redos are costly ,ask for more help !

I have an 8N that has had 6 rebuilds.
I was a teen when I rebuilt it the first time.
Just put rings in it then.
Put rings and bearings 4 times.
Then sleeve kit with new pistons the last time.
Just slipped in new bearings the last 5 times.
I am going to do rings and bearings again soon.
It has great oil pressure.
It has been in the family since new and it still gets used all the time.
Same age as me.
1952 model.

Ya know the old adage, if it ain't broke don't fix it? It's a pretty reliable guideline.
It also matches up fairly well with, fix it til it's broke. gm

I would grind at any more wear than .002 thou . If a bearing is too loose all the oil pressure will dump at that wide gap.

If it is not overly using oil. not very low oil pressure amd
if not making noise then don't pull the bottom apart. Doing a
tractor now because it had a knock and was told it was piston
and rod bearings by a mechanic. Tore it down and found the
knock was somebody had put a bit longer bolt in to hold
distributor on. Went for the over haul BEFORE found what was
making the knock. new bearings but NO crank grind and this
was at the garage. So unless it has one of the 3 tuings DO
NOT pull the bottom apart!!!!

X 2 on rustred......

Bob.

Mine did get an overhaul but it was because of when it was pulled apart to get to the top end of rod bearings found the thin steel sleves were cracked vertical but your tractor is too new to have ever had the steel sleves as your tractor has the thicker cast sleves. I had a Ferguson TO30 that I did have to have crank ground, Good oil pressure cold, none warm, no noise. Garage pulled pan and main bearing cap and one look at crank is Had to go 40 under all it took to know it had to be ground. Dont think rod jurnels were touched. But that was back in the 60's.

I'm with Dr. Sportster on this about 1-3 would need ground. And if things are working good with good oil pressure then just do the gasket and run it.