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Old 04-20-2013, 09:14 PM   #1
lavid2002
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Default How do you check main + rod clearances?

Hey everyone,

So I am in the process of doing research for an engine build. I have asked several people how they check main + rod bearing clearances and I keep getting different answers. Everyone talkes about plasti-gauge like it is the duct tape of engine building.

Can someone please elaborate on this for me? These are the only ways I know of

-Plasti gauge (everyone seems to look down on this IDK why. Because you can only get one measurement instead of micing all around and getting several measurements?)

-Using micrometers (Take several measurements in just a few seconds of the main bearing journals and the connecting rods. Take measurements of the crank and compare and use the right bearing size?)

I asked a guy who was building a shortblock and he was telling me that a lot of people have issues with these blocks because the size of the mains changes with the torque applied to them. Apparently one guy on the forums did some tests with align honing, ARP studs and still couldn't get the specs he wanted.


I know I am making this out to sound like it's some mystical thing. Long story short, how do YOU check your clearances in your shortblock?
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Last edited by lavid2002; 04-20-2013 at 09:29 PM.
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Old 04-20-2013, 09:46 PM   #2
the suicidal eggroll
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Plastigage for me.

Micrometers and bore gauges are finicky and you can easily get a variance between subsequent measurements that's equal to or greater than your target clearance. It takes a lot of practice and a lot of discipline to use a micrometer and bore gauge properly, not to mention a very expensive set of micrometers and bore gauges in order to reliably measure your true bearing clearance.

Meanwhile, plastigage is cheap, relatively accurate, and requires no calibration or training.
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Old 04-21-2013, 12:24 AM   #3
aboothman
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Lets just sY that I am currently putting together an engine with a ground crank. At 90 from the oil holes some of the main crank journals measure 2.35170 while at the edge of the same 90 point they measure 2.35200

Meanwhile they get LARGER near the oil holes...to the tune of 2.36190-220

Would this be a deal breaker with plastigage? Well I suppose that depends where you measure from. Not going to get into a battle as there are plenty of engine running "fine" that were assembled while properly using it. I just prefer to do things right and dont mind the time and money to do so.

FYI my mics and bore gages are hardly "finicky". They are consistent and reliable. I guess it is the same as plastigage: It depends whether you are using it/them right or wrong.

Last edited by aboothman; 04-21-2013 at 12:31 AM.
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Old 04-21-2013, 03:24 PM   #4
lavid2002
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So it seems like the primaty benefit of micrometers is that you can check several spots at once.

Obvious question...

Why not use both? Check with a mic to see if the journals are cylindrical (Only really focusing on deviation), and use plasti gauge for the actual clearance measurement?

I am not building a 500 WHP car, and I am keeping the factory redline.
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Old 04-21-2013, 03:42 PM   #5
aboothman
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Well there is more to it than that. Plastigage is realistically accurate to about .001"

Any Mic or gauge worth using in one of these engines is typically accurate to .0001" It stands to reason that an instrument that is 10x more accurate could be considered "finicky", but only in the fact that it is 10x more accurate. If you use consistent methodology every time you will get consistent results. They are not very complicated instruments.

As for your question, if you are going to bother checking the mic with the crank main journals it is only another step or 2 to check the case with main bearings installed. Either way you have to bolt the block halves together. IMO it is actually easier to check the mains with a bore gauge than to bolt it together with the crank installed and deal with plastigage.

Of course I prefer to keep the crank sealed away unless I am measuring it or assembling the block.

And again, you get to measure near the parting line and 90 degrees from the parting line. You get much more precise measurements that come in handy when you need to swap bearing halves around.

As for rods it is actually easier to check them with a bore gauge IMO. You do not have to deal with the crank at all, and with the crank already mic'ed, it should only take 15-20 minutes to torque up the rods with bearings and get an accurate measurement. It takes as long to figure out the proper configuration for the bore gauge and to calibrate it to the mic If you are already setup, then it is a cakewalk.

There are places where it becomes fairly difficult to measure with either, such as the piston pin clearance, or the cam journal to cam clearance. My bore gauge does not go under 1.0" so I am limited in my options there...
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Old 04-21-2013, 07:51 PM   #6
theotherguy
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Quote:
Originally Posted by lavid2002 View Post
So it seems like the primaty benefit of micrometers is that you can check several spots at once.

Obvious question...

Why not use both? Check with a mic to see if the journals are cylindrical (Only really focusing on deviation), and use plasti gauge for the actual clearance measurement?

I am not building a 500 WHP car, and I am keeping the factory redline.
That is how I do it. I've built over a dozen engines over the last 30 years but that is not very many. It takes me awhile to get repeated results with a mic and I can check it against the plastigauge. Mains are easier for me as is taper on the journals with plastigauge. It is what the FSM calls for.
If I wanted a 500 whp engine, I wouldn't build it myself, Dom and MPS surely would but I can squeek by on a stock style build which is basically checking that the parts that I have are within factory specs.
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Old 04-21-2013, 08:08 PM   #7
lavid2002
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Interesting. I am going to do some more research on this process. If I find a crankshaft or bearing set that is out of spec what are my options? Keep swapping bearing halves until I get what I want?

In some cases polish the crank?
In extreme cases align hone?

It doesn't seem like micing the crank and rods and mains would be difficult. I watched a video of a machinist online and he just "felt" the piston pins.
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Old 01-16-2014, 04:24 PM   #8
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Sharing my little experience

I used plastigauge to measure my clearance on my first build that lasted 4 years/40000km before I got a permanent oil pressure drop after overheating the oil (around 5 psi). Guessing the build would have lasted longer without that lapping day. But still there was some marking on the crank and the bearing on the rod. Nothing major on the mains.

From what I remember, plastigauge showed that I was suppose to be around 0.001 all over the place for the mains and the rod.

For my new build I'll be using mic (~30$) and bore gauge (~100$) and did a test run on my old block. It's 4 years after, possible that things moved a bit, but the result are crazy.

Rods (not so far from what I got)
Rod 1: 0.0008
Rod 2: 0.0011
Rod 3: 0.0012
Rod 4: 0.0012

Mains (all over the place):
1: 0.00045
2: 0.0023
3: 0.0013
4: 0.0033
5: 0.0010

From the little test I did It's easier to adjust the bearing with +0.001 and -0.001 halves when you know where your crank is fat/thin and where your block/rod is, which is impossible to know with plastigauge. I've check a few articles on how to do it and did a few tests and it's impressive how accurate and constant you can get. Having an old crank/block to test is a good idea. Measuring the same thing over and over again and getting always the same result with no more than 0.0001 variation.

My theory is that plastigauge for the main is very hard, as it is hard not to get the weight of the crank on the plastigauge while torquing the block together. For the rods, once you find a technique to torque them with not rotation, it's ok.

I've seen many debates about this and I was skeptical as well. But after testing it myselft I have to agree with mic/bore gauge way of doing it. It's also easier than I though.
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Old 01-16-2014, 07:03 PM   #9
fastblueufo
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Quote:
Originally Posted by aboothman View Post
Well there is more to it than that. Plastigage is realistically accurate to about .001"

Any Mic or gauge worth using in one of these engines is typically accurate to .0001" It stands to reason that an instrument that is 10x more accurate could be considered "finicky", but only in the fact that it is 10x more accurate. If you use consistent methodology every time you will get consistent results. They are not very complicated instruments.

As for your question, if you are going to bother checking the mic with the crank main journals it is only another step or 2 to check the case with main bearings installed. Either way you have to bolt the block halves together. IMO it is actually easier to check the mains with a bore gauge than to bolt it together with the crank installed and deal with plastigage.

Of course I prefer to keep the crank sealed away unless I am measuring it or assembling the block.

And again, you get to measure near the parting line and 90 degrees from the parting line. You get much more precise measurements that come in handy when you need to swap bearing halves around.

As for rods it is actually easier to check them with a bore gauge IMO. You do not have to deal with the crank at all, and with the crank already mic'ed, it should only take 15-20 minutes to torque up the rods with bearings and get an accurate measurement. It takes as long to figure out the proper configuration for the bore gauge and to calibrate it to the mic If you are already setup, then it is a cakewalk.

There are places where it becomes fairly difficult to measure with either, such as the piston pin clearance, or the cam journal to cam clearance. My bore gauge does not go under 1.0" so I am limited in my options there...
Plastigauge is not accurate to .001". I break out plastigauge and test to my mics and bore gauge periodically and alot has to do with how accurate it is. Temp at test, how old the plastigauge is, if an oily film is on metal being tested etc etc.

When measuring subaru mains you need a clearance between .001 and .0015 max. If you want to do it right you will make #3 main .0005"-.001 more clearance than the other mains. #2 and #4 main can have the least clearance of all of them because they dont feed any rods.

When measuring to such tight specs why risk a good build with a cheap, inaccurate measuring device. Do it right with mics and bore gauge.
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Old 01-16-2014, 11:32 PM   #10
Serkan
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Quote:
Originally Posted by the suicidal eggroll View Post

Micrometers and bore gauges are finicky

They are if you don't know how to use it. And it is not hard to learn.
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Old 01-17-2014, 09:08 PM   #11
aboothman
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If you read what I wrote then you would realize that we agree. Accuracy of .001" (at best) is not what I would even consider accurate when you are dealing with clearance of .001-.002" After factoring in +-.001" that leaves you .000"-.003"

Quote:
Originally Posted by fastblueufo View Post
Plastigauge is not accurate to .001". I break out plastigauge and test to my mics and bore gauge periodically and alot has to do with how accurate it is. Temp at test, how old the plastigauge is, if an oily film is on metal being tested etc etc.

When measuring subaru mains you need a clearance between .001 and .0015 max. If you want to do it right you will make #3 main .0005"-.001 more clearance than the other mains. #2 and #4 main can have the least clearance of all of them because they dont feed any rods.

When measuring to such tight specs why risk a good build with a cheap, inaccurate measuring device. Do it right with mics and bore gauge.
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