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What is platiguage?

blakebmxr3

NAXJA Forum User
Location
Robins AFB GA
I have an 89 xj with a 4.0 engine with 180,000 miles on it and there is a loud rattling noise that is constant at idle and acceleration, hot and cold. My oil pressure starts out(cold) at barely 36psi, then drops significantly as it warms up and eventually has next to no oil pressure at idle. My plans are to replace the rear main seal, oil pan gasket, oil pump/pickup, switch to 20-50 oil and an upgraded oil filter. I want to replace the main crank bearings and rod bearings, but I've been told that changing the bearings could ultimately destroy my engine. So im not sure what to do. Anyways, I've been doing a lot of reading on these forums about bearings and heard a lot about "plastiguage" and i dont know what it is. How do i determine what size/type of bearings to replace the old ones?
 
A friend of mine just got done doing the exact thing you are talking about.

Low oil pressure and a rod knock prompted him to tear into the bottom end of the motor.

I convinced him to replace rod and main bearings with standard size bearings (not undersize ones for a ground crank). There are also bearings made that are .001" small, -but I advise against doing that.

Replace all the bearings, and change out the oil pump.

Don't worry about plastigauge unless you are changing to an undersize bearing, -which you wouldn't do unless you have the crank ground undersize.

Plastigauge is small cylindrically shaped wax that you lay across the surface of the bearing. You then torque the bearing cap to specified torque to crush the bearing. Then you remove the cap. The plastigauge is made to squish out flat. The width of the squished plastigauge is measured using a special scale included with the platigauge set (to determine bearing clearance).

My friend who just did the bearing/oil pump overhaul in his driveway resurrected his 4.0 from a knocking rattling POS to a nice smooth running engine that has in excess of 50 PSI cold and 35-40 PSI hot.

You will be amazed how much oil pressure you'll gain by restoring the clearances in the bottom end of the motor. These oil pumps are also pretty much "done" after 150k anyway, -so that helps out bigtime also.

Work slow and methodical, -don't get in a hurry. Use fuel line hose to slip over the rod bolts to protect the crank journals, -and do one at a time. Keep things clean, -and use assembly grease on the bearings, -but keep the backsides of the bearing inserts clean and dry.

Fill the new oil pump with power punch or STP so that it primes quickly when you light it off.

Don't keep running that motor with the rod knocking though, -or else you may need to get that rod resized.
 
Ok thanks, that helps a lot, the whole "plastiguage" part of the bearings threw me off when i was reading it from other posts.

So as long as the crank itself is'nt scored then then i should just get what? Just the factory size bearings? (this is my first attempt at the bottom half of the motor)

And i've heard a lot about high volume oil pumps, should i go there? Or just stick with the factory one?
 
Do you have a set of mikes? You should, if you plan on doing any major engine work (0-3" at least...)

Plastigage is handy, but I've never liked it. If I suspect rod bearings or main bearings, I'll drop the pan, expose the crank (taking it out, if I'm doing mains) and measure the damn thing directly. Even if you don't note an "undersize code" staped into the block, you may still find that you need -.001" bearings due to normal wear. The .001" undersize would allow you to maintain oil clearance (ideally .0015", but .001"-.003" is acceptable, as I recall...) with a slightly worn crankshaft.

If there is any scoring, plan on replacing the crankshaft entirely - you'll end up chewing up your bearings. Some slight striations (visible) are fine - but if you see anything like that, use your thumbnail to check the potential scoring by feel. If you feel anything but smooth metal, replace the crank. No sense in risking failure...
 
I just did the rod bearings in mine, they were $5 each at autozone. At 220K miles the mains looked fine and the clearances checked good, the rod bearings were worn. I plastigauged everything as I was putting it back together but in the end I did just what ratman suggested and put the same size bearings in that I took out (they'rte marked on the back of the bearing, mine were STD. Plastigauge is not perfect, but it does tell you of something is way too loose or way too tight.
 
Thanks again!
what about the high volume oil pump? should i go there?
and if I do need to replace the crank, what would a new one cost me? Because i found rebuilt 4.0's (long blocks) for $1100 online, is it wise to try the bearings first to regain oil pressure, or is my engine more dead than I think? And what about timing? Should i go there?
 
Low oil pressure is typically due to excessive rod/main clearance - replacing the bearings should clear that up (as long as the crank is okeh.)

I think the last crank kit I got ran about $175 + core deposit (which I lost half of - the crank I replace was SHOT due to a failed oil pump!) which wasn't too bad.

I'd not consider a HV oil pump really necessary, the OEM ones are fine. I can't recall any significant oiling issues with the AMC I6 engine series.

Timing? Why, are you having trouble?
 
I'd use the .001" under bearings. What you're doing is exactly what they're made for. They've been around for at least 50 years, and the people at Federal Mogul aren't stupid; they wouldn't still be making them if they were just a gimmick.
 
As 5-90 said, got a mike!!!!! the .001 is for a standard crank that has been polished and requires the .001 under size bearing to get the oil clears right.

If you don't have a mike and or just going to plastic gauge them........put the standard ones back in(New) the bearing is most of the ware will be comes form so they should be just fine unless the crank is scared. then you will need to remove the crank and have it turn or by a crank kit!


Flash.
 
Installing .001" undersize bearings without taking the crank out would be a risky proposition in my opinion.

No, the people at federal mogul are obviously not stupid, -however, typically .001" under bearings are specifically made for cranks that have been removed and polished. The problem with crank journals is that they don't "wear" round, -ever, -so even trying to mic the journal while it's still in the motor is iffy at best. I am a machinist, so I know all about mics and bore gauges, -however, even being a machinist I'll say this, -plastigauge is inexpensive, -and it is accurate enough to let you know if you're going to have a problem, -most certainly good enough for keeping a guy whose putting a motor together out of trouble.

So, if ya wanna gamble on the .001" bearings, -do as several others have suggested, -but I personally wouldn't do it. It is all too easy to spin a bearing that is too tight, -then you're in worse shape than you're in right now.

High volume oil pump, -not necessary. Waste of power, -and takes valuable oil from your sump and puts it into the top end of the motor. When the motor is on a steep incline, keeping the oil pump pickup covered is a serious concern. High volume pumps typically suck down the oil level in the sump, -not good for wheelin'.

I don't want to sound like I'm disagreeing with everyone, -it just that I've most definitely been there and done that.

My .02
 
Re: What is plastiguage?

Ok, I'll be doing this at a shop so I'll have mikes and stuff....so its best i just pull the crank entirely out then? Woulnd't I have to pull it to do the rod bearings any way? I really dont want to gamble since this is my only vehicle(other one is being sold) so I'll probably stick with whatever size bearings i pull out. Any advice for the rod bearings???

The HV oil pump is now out, thanks for the input!
 
Re: What is plastiguage?

blakebmxr3 said:
Ok, I'll be doing this at a shop so I'll have mikes and stuff....so its best i just pull the crank entirely out then? Woulnd't I have to pull it to do the rod bearings any way? I really dont want to gamble since this is my only vehicle(other one is being sold) so I'll probably stick with whatever size bearings i pull out. Any advice for the rod bearings???

The HV oil pump is now out, thanks for the input!
If the crank is not scored (they usually aren't unless the oil wasn't changed regularly), -then just install a full set of standard rod and main bearings (and your new oil pump) and call it good. No mic's, -no plastigage, -just git 'er done.

No, the crank does not have to come out to do rod bearings.

If any journals are scored, -then re-evaluate your situation and decide if you're going to go any further with this motor. Lightly scored journals don't mean a crank is bad, -it just means that it needs to be polished, -by a crank shop. Then those .001" undersize bearings would come into play.....

If the crank looks ok, -this is an afternoon job in the driveway.

No offense, but a lot of people pick up a chinese made mic, and instantly they are a machinist who knows how to measure parts. Besides, anyone who tells you they can accurately measure a mainline bore, -or a rod bore using a telescoping gauge and a mic is on some serious dope. Dial bore gauges (set with an NIST traceable calibrated set-ring) are needed to accurately measure the bearings in the bores. So, having the best mic in the world will do nothing to accurately determine clearances. That is why plastigauge is so appealing for most folks who don't have access to the high dollar measuring stuff.

What I am trying to say is that an inexperienced person will get more accurate results using plastigauge.

I am not implying anything, I'm just trying to keep you "out of the weeds"


Rich.
 
Re: What is plastiguage?

ratman572 said:
If the crank is not scored (they usually aren't unless the oil wasn't changed regularly), -then just install a full set of standard rod and main bearings (and your new oil pump) and call it good. No mic's, -no plastigage, -just git 'er done.

No, the crank does not have to come out to do rod bearings.

If any journals are scored, -then re-evaluate your situation and decide if you're going to go any further with this motor. Lightly scored journals don't mean a crank is bad, -it just means that it needs to be polished, -by a crank shop. Then those .001" undersize bearings would come into play.....

If the crank looks ok, -this is an afternoon job in the driveway.

No offense, but a lot of people pick up a chinese made mic, and instantly they are a machinist who knows how to measure parts. Besides, anyone who tells you they can accurately measure a mainline bore, -or a rod bore using a telescoping gauge and a mic is on some serious dope. Dial bore gauges (set with an NIST traceable calibrated set-ring) are needed to accurately measure the bearings in the bores. So, having the best mic in the world will do nothing to accurately determine clearances. That is why plastigauge is so appealing for most folks who don't have access to the high dollar measuring stuff.

What I am trying to say is that an inexperienced person will get more accurate results using plastigauge.

I am not implying anything, I'm just trying to keep you "out of the weeds"
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I will 2nt that!;)



Flash.
 
You don't necessarily need to pull the crank to do the rod bearings, you should to do the mains (you can do the mains with the crank in place, but it's difficult.)

The reason I keep saying to measure the crankpins directly is simple - if the "undersize codes" are obscured (I'd have to look up where they're stamped, in fact!) and you just get the standard size bearings with -.010" pins, you're sunk and getting a new crank from the off. I can't think of anything that will wreck a crankshaft faster.

And (I didn't mention this here, but I do cover it in detail elsewhere,) you don't just measure a crankpin in one place and call it quits. You've got to check for taper, out-of-round, and barrel-facing, so you end up measuring each crankpin at least four times (twice at right angles to each other, and again at each "side" of the journal. Ideally, it's more like six - three along one diameter, shift to right angles, and three again.)

Plastigage is convenient for most people - I just don't happen to like it. Then again, I buy decent mikes (Japanese, German, or US-made - not RPT NOT Chinese!) and practise using them, and I check them periodically against the "good" set of gage blocks at school to make sure they haven't wandered.

But - and this is something I consider critical - you want to at least make sure you don't have an over/undersize situation in front of you and get "standard" parts. Engines come from the factory sometimes with rod bearings -.010", main bearings -.010", and/or cylinder bores +.030" - so you should at least check.

I'll have to go over my notes one of these days and do a writeup on "oversize codes" - but any FSM will also explain it. Do you have one? Chilton's might cover it as well - but I'm not so sure about Haynes.
 
5-90 said:
The reason I keep saying to measure the crankpins directly is simple - if the "undersize codes" are obscured (I'd have to look up where they're stamped, in fact!) and you just get the standard size bearings with -.010" pins, you're sunk and getting a new crank from the off. I can't think of anything that will wreck a crankshaft faster.
I agree, -but these are few and far between. And aren't the bearing inserts usually marked on the backside of the shell (ie: .010, .020, etc.?)

5-90 said:
And (I didn't mention this here, but I do cover it in detail elsewhere,) you don't just measure a crankpin in one place and call it quits. You've got to check for taper, out-of-round, and barrel-facing, so you end up measuring each crankpin at least four times (twice at right angles to each other, and again at each "side" of the journal. Ideally, it's more like six - three along one diameter, shift to right angles, and three again.)
I agree, that is typical of checking any bearing journal, in an engine, pump, etc.

5-90 said:
Plastigage is convenient for most people - I just don't happen to like it. Then again, I buy decent mikes (Japanese, German, or US-made - not RPT NOT Chinese!) and practise using them, and I check them periodically against the "good" set of gage blocks at school to make sure they haven't wandered.
First of all, I was not implying that you use chinese mic's. The point I was trying to make was that most guys don't want to spend $150 for a 2-3 Starret, -so they opt for the less expensive stuff and "assume" it is accurate across it's range. For the layperson, plastigage is the ticket.

5-90 said:
But - and this is something I consider critical - you want to at least make sure you don't have an over/undersize situation in front of you and get "standard" parts. Engines come from the factory sometimes with rod bearings -.010", main bearings -.010", and/or cylinder bores +.030" - so you should at least check.
I agree with this, but if all you're trying to do is measure .010", -you can do that with a plastic dial caliper.....


I am not trying to start a flame war or pi$$ anyone off. I am merely trying to keep someone from spending needless money, -and keep them out of trouble (or reinventing the wheel when it comes to slipping in a set of bearings in the driveway). I also do not claim to have all the answers. I have also re-bearinged many engines using plastigauge succesfully, -even though I have tools that will allow me to measure to 50 millionths of an inch. In my 16 years of being a machinist, I have also seen people measure things with the best tools at their disposal, and do it inaccurately. Most people would be hard pressed to accurately measure anything (if it's not something they do all the time) -and I'm not implying that you are one of those people either.

Yes, I am a new to the forum with my 5 posts and all, -but I have been around the block a few times on this stuff and will try to help someone from making a mistake whenever I can, -especially if it's a mistake that I have made and learned a hard lesson as a result.

Peace.
 
Thanks again for the bearing info! I think what I'll do is just see if the crank is good or bad, if its bad, hell I'll just put a new one in! I just want to avoid a pricy engine swap if its not necessary.

Do any of you recommend any particular brand of bearings, should i just get them from auto zone?

And I'll look at the chilton's manual i have, plus i have access to ALLDATA as well.
 
"Usually" they're marked, but I've seen plenty of cases where the marking gets washed off of the back of the bearing shell (it's only ink, after all. They can't rightly stamp a bearing once it's been poured...)

Fortunately, I can count the number of mismarks I've seen on bearing in the last 30 years on the fingers of one nose, so they manage to get that right. Far more common is to remove a bearing, wipe the oil off the back, and find that the oil has taken the ink with it...
 
blakebmxr3 said:
what brand bearings do y'all recommend???
They are all made my several major manufacturers, and repackaged by practically everybody.

I say go with what you can get the best deal on and who is easy to deal with.
 
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