engine rebuilding?

JL Ray · Aug 31, 2018

Not being one who has rebuilt an engine before. IF an engine has been bored out let say .030. When putting it back together, do you replace the pistons with bigger ones? If not, do you buy a ring set that is bigger?
When turning a crank let say .030. You buy new .030 mover bearings. Then you Plasti-gage to check fit with crank. What if its out of tolerance, to loose or too tight? How does that get taken care of?

Patsdeere · Aug 31, 2018

Yes you would get bigger pistons and obviously rings.

In regards to bearings, you hope the machine shop know what they are doing and don't screw up. The only way to fix would be grind down to the next standard undersized bearing or weld up and turn down to the correct size. Neither is a really way. If machine shop screwed up that badly, don't know if I would give them a chance to fix it ...

IanC · Aug 31, 2018

You need to buy a service manual for your machine. That will give you the tolerances, and torque values. If this is a mission critical piece of equipment (tractor used to feed livestock, etc.) you should find a good mentor (I would suggest somebody who is an airframe & powerplant certified mechanic). If an engine is rebuilt correctly it should last as long or longer than the first, but often they do not because of the "It ain't right, but it'll work" attitude of the person assembling the engine. And yes the pistons will be oversize, and the correct way is to have your machine shop hone the cylinders to correct clearance to the pistons you HAVE, not just nominal specs.

Steve@Advance · Aug 31, 2018

Yes, you have to match the piston and rings to the bore size.

Typically, when an engine is rebored during an overhaul, the pistons are purchased before it is bored. They are measured then the cylinders are bored to the proper clearance for that particular piston set. If a piston needs to be replaced afterward, you can purchase a single over size piston, check the clearance with a micrometer and telescoping gauge, and it should be within tolerance provided the bore isn't worn. If too tight the cylinder can be honed. If too loose, you will have to go to the next oversize and and rebore the block.

The rings will also be sized to the bore spec. They will need to be checked though, the compression rings need .004" end gap per inch of cylinder bore. Oil rings are not as critical, generally .010" gap minimum.

The crank bearings are more critical. The crank will need to be miked for diameter and out of round conditions. If it has been ground undersize, that amount is deducted from the original size, and replacement bearings are ordered accordingly. Still, everything has to be check for final fit. Also the connecting rods need to be checked for proper bore. If any doubt, if the bearing spun, the caps have been switched, they will need to be reconditioned back to factory spec.

Most home shop mechanics don't have the proper equipment to take the measurements, and certainly don't have the necessary machinery to make corrections. Best to leave that up to a reputable machine shop.

Janicholson · Aug 31, 2018

An engine bored to a standard oversize must have oversized pistons installed (fitted to clearance for new) The ring set is also sized to the new bore/pistons, with end gap set to specification. Big end bearing inserts are made in under sizes because the journal gets smaller as it is refinished. Measurement of the engine should be done with professional level tools, and professional level tactics. Jim

jimg.allentown · Aug 31, 2018

There is a lot more to engine rebuilding than piston fit and bearing fit. These items are just the beginning and the foundation of an engine.
Engine bearings are normally either standard or undersized. Why? Because when a crank is reground, material is removed making the journal diameter smaller. That then requires smaller (undersize) bearings for a proper fit.
Other items come into play for the basic short block assembly. Crankshaft end play is also important. Connecting rod side clearance is also important. Piston skirt to bore clearance is also important. All of these things go into assembling a short block that will stand up to a day's work.
Next comes cylinder heads and the intake system and the oiling system. The pistons, crankshaft, and rods are just the beginning.

gene bender · Aug 31, 2018

You got it backwards when you grind he crank you need thicker bearings. And they are oversized.

Bob · Aug 31, 2018

(quoted from post at 13:58:21 08/31/18) You got it backwards when you grind he crank you need thicker bearings. And they are oversized.

C'mon, Gene, we've been through this before... when a crankshaft is reground, it's then UNDERSIZED, as are the required bearings.

You are gonna CONFUSE the "newbie" telling him they are "oversized".

jimg.allentown · Aug 31, 2018

Nope. The size that counts is the inside diameter of the bearing. After crank grinding, it is smaller. That means undersized. Ask any machine shop or parts supplier. They will tell you the same as I did.

gene bender · Aug 31, 2018

OK just measure a 30 bearing when the crank has been ground 30 and you will find the bearing will be 30 thicker than a standard I spent too many hrs making new ones cause we had to ground the shaft down so then new bushing would be 30 thicker to fit the undersize shaft

IanC · Aug 31, 2018

Caterpillar used to and still may supply main bearings that were undersize ID, and oversize OD for engines that had just had a boring bar through the crank centerline rather than the caps lowered and line bored.

teddy52food · Aug 31, 2018

Each bearing half would be .015 thicker to fit a .030 ground shaft.

David G · Aug 31, 2018

JL,

Do not let everyone scare you away, people just want to help.

Pistons and cylinders are standard or oversized, crankshafts and bearings are undersized.

Please post more questions.

used red MN · Sep 1, 2018

Yep agreed, rod or main crankshaft bearing journals are finish ground to a certain diameter spec. For EXAMPLE say your engine has a specified rod bearing journal of 2.000 inches it wears down below the allowed tolerance and needs resized. So a machine called a crankshaft grinder is used to very precisely grind the journal down 0.010? undersize. So now the bearing insert halves needed are 0.005 inches thicker to fit the smaller diameter journal. So they would now fit the new small diameter rod journal that is 1.090 inches. That ...undersize... diameter is how normal machinists nomenclature or naming would refer to them. I agree with finding someone to mentor you through this process there are a lot of steps that need performed properly during and engine rebuild to ensure longevity.

IowaTJ · Sep 1, 2018

2.000 minus 0.010 is 1.990, not 1.090!

gene bender · Sep 1, 2018

correct now how do you call that undersize sure glad they don't work on my engines

jimg.allentown · Sep 1, 2018

If you use oversize bearings, your engine will most certainly knock - and loudly!
In order to standardize nomenclature, certain standard means of describing parts have been implemented for as far back as engines have been built.
The size of the bearing refers to the INSIDE DIAMETER of that bearing. NOT the thickness of that bearing. Crankshafts that have been reground to a standard undersize require undersized bearings. That is why there is a standard in describing them.
There have been uses of oversized bearings in applications where the inside of the connecting rod has been machined to a larger diameter.

Supposing you were rebuilding an engine, and you needed replacement bearings, the parts man would ask you if you want standard or undersized bearings. Don't take my word for it. See for yourself. Better yet, pick up an old Motor's repair manual and read it for yourself.

We can argue about this for as long as you like, but you will find that it will be a small minority that call bearings oversize and the majority will call them as they are - UNDERsize.

used red MN · Sep 1, 2018

TJ you are correct my mistake. So much for trying to make my post as accurate and detailed as possible so it would be taken as valid. I?ll chalk that one up to it being too late when I posted. Good thing I?m not a machinist. Lol.

dr sportster · Sep 1, 2018

If the crank grinder has done a good job there won't be any clearance issues. There are many good books from schools out there very inexpensive . When you bore pistons are oversized .When you hone if conditions are right you can re-ring the same pistons.

showcrop · Sep 1, 2018

jl ray, some of these guys are making this way more complicated than it really is. What actually happens is you ask around about who runs a good automotive machine shop. Then you give them a call and tell them what you are doing and they will give you an idea what it will cost. Then you drop your short block and head to if you wish off with them. You will go over with them what services you want and don't want. They will give you an idea of when it will be ready and THEY WILL ASK YOU "DO YOU WANT US TO GET YOU THE NEW PISTONS, BEARINGS, RINGS" you have a little more discussion and say "YES". This insures that the bearings match the crank as it ends up being ground, and that your pistons and rings will be just right for the bores. There is no way of knowing exactly what the final bore will be or how much the crank needs to be ground until it is done. THIS IS WHAT THESE GUYS DO, not me, not you.

sotxbill · Sep 1, 2018

(quoted from post at 08:18:02 09/01/18) correct now how do you call that undersize sure glad they don't work on my engines

Everyone calls it different. I call them oversized..... You can call them anything you want as long as they make up the metal removed.....

Andy Martin · Sep 1, 2018

It must be a regional issue. We always called the thicker shell "oversized" because it was thicker to get a smaller bore to fit a turned crank.

But I really think neither of you guys are a stupid as you come across. You understand what the other guy is saying. You're just trying to force your terminology on people who use different terminology.

But if you order from Summit Racing you'd better ask for undersize.

bumblebee · Sep 1, 2018

I am no expert but I have rebuilt some engines. After reading all these comments I wouldn't dare rebuild an engine cause I am really confused!!!!!!!!!!!!!

Bob · Sep 1, 2018

(quoted from post at 16:18:11 09/01/18) It must be a regional issue. We always called the thicker shell "oversized" because it was thicker to get a smaller bore to fit a turned crank.

But I really think neither of you guys are a stupid as you come across. You understand what the other guy is saying. You're just trying to force your terminology on people who use different terminology.

But if you order from Summit Racing you'd better ask for undersize.

Regional slang and colloquialisms fascinate me, (Coke, pop, soda, etc.), but when you need to order bearings for a crankshaft that's been ground, by industry convention, they are called UNDERSIZED.

NOT sure why few of you guys can't accept the term the rest of the mechanical world uses???

engine rebuilding?

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