icaneat50eggs
icaneat50eggs Dork
9/8/18 8:46 a.m.

The 95 Miata I picked up cheap had the engine pulled and started on a rebuild.  The previous owner had torn it down and had it bored and new bearings and pistons installed.  He put back in the original rods.  That’s as far as he got so I’m going to put in better rods now before I put it back together.  My question is can I reuse the just installed rod bearings or do I need to get new ones. 

Suprf1y
Suprf1y UltimaDork
9/8/18 8:57 a.m.

You can reuse them, but check the clearances.

jimbbski
jimbbski Dork
9/8/18 11:37 a.m.

Yeah,  you need to install the rod bearings in the rods and then bolt the two half's together.  Then measure the I.D. of the rod and compare that to the crank.

If you don't have the correct tools to do this take it to a shop that does. Of course you can just install them with some plastic-gage and check them that way but it's not as accurate.

Knurled.
Knurled. MegaDork
9/8/18 11:42 a.m.
jimbbski said:

Yeah,  you need to install the rod bearings in the rods and then bolt the two half's together.  Then measure the I.D. of the rod and compare that to the crank.

If you don't have the correct tools to do this take it to a shop that does. Of course you can just install them with some plastic-gage and check them that way but it's not as accurate.

Plastigage tends to be more accurate, not less.  It doesn't take any more finesse than making sure the surfaces are dry and that you don't rotate the crank.

 

It's real easy to get incorrect measurements with calipers/micrometers, especially when you are looking for accuracy to the ten-thousandth.

Vigo
Vigo UltimaDork
9/8/18 1:57 p.m.

^2nd that, plastigauge will be more accurate in this case. Now, if the big end of the rod itself was not perfectly sized or shaped, then plastigauge is not going to help you diagnose that issue. But it's also pretty unlikely. 

Suprf1y
Suprf1y UltimaDork
9/8/18 3:07 p.m.

Plastigage is fine, but let's not be silly, it will never be as accurate as the proper tools.

jimbbski
jimbbski Dork
9/8/18 4:08 p.m.
Knurled. said:
jimbbski said:

Yeah,  you need to install the rod bearings in the rods and then bolt the two half's together.  Then measure the I.D. of the rod and compare that to the crank.

If you don't have the correct tools to do this take it to a shop that does. Of course you can just install them with some plastic-gage and check them that way but it's not as accurate.

Plastigage tends to be more accurate, not less.  It doesn't take any more finesse than making sure the surfaces are dry and that you don't rotate the crank.

 

It's real easy to get incorrect measurements with calipers/micrometers, especially when you are looking for accuracy to the ten-thousandth.

IF plastigage was more accurate why does every machine shop use micrometers, calipers, etc. to measure things?

Knurled.
Knurled. MegaDork
9/8/18 5:21 p.m.

In reply to jimbbski :

It is assumed that machinists know how to properly use them, and that they are using regularly-calibrated decent quality ball-end bore gauges and such, not the $30 digital calipers from Harbor Freight.

 

This is why I said "tends to be more accurate".  For the money, and for the lack of technique required to use it, you can't beat it.  You can easily be off by more than the clearance just by improper technique and cheap tools, this is unacceptable when you are shooting for, say, .0020" clearance with .0025 being excessive.

 

And some machinists DO use Plastigage on assembly, to verify everything, in addition to the machine shop tools.

mikedd969
mikedd969 New Reader
9/8/18 7:21 p.m.

Good micrometers and bore gauges are not cheap, and they do require skill and experience to use properly.  It's not truly difficult, but it is easy to get wrong.  I inherited mine from my grandfather or I probably would not have them.  Good instruments usually come with something to check the calibration/accuracy, make sure you do that before you make a critical measurement.    

Plastigage is great for measuring bearing tolerances. I used it for years, still do. Simple, easy and cheap, just follow the directions and take your time. Haste has ruined more engine builds than just about anything else. The only difference is, now that I have a set of quality micrometers and bore gauges, I use both.....just for peace of mind....

For the OP.  Have your old rod journals measured by a machine shop and order your new ones accordingly, you should be fine.  If you can't, then a new set of quality bearings probably won't break the bank.

 

 

buzzboy
buzzboy Reader
9/8/18 7:57 p.m.

I like plastigage because it can show you things like my F up here

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