Bearing tolerances

[e30m20TurbO7]

Is .002 the correct spec for the main bearing tolerances?
on the other side of the measuring paper it's says .051
It's about the same size at .002 am I okay if they are al .051 /.002
91CC8F095AC5-4658-00000302B542D5FF_zps7f2a19c2.jpg[/IMG] also main cap 4

Main cap 3^
main cap 3


main cap 2
main cap 2

[jlevie]

My Bentley is out in the shop, so I can't look up the correct clearance, but it is a lot less than .002". The general rule of thumb is .0001" per inch of journal diameter.

Plasti-gauge is good for a sanity check, but not good enough for picking the bearings. Main bearings come in three "colors" to accommodate manufacturing variations and minor wear. You measure each journal to a tenth of a thousandth and then, using the table in the Bentley (or TIS) to pick the color for that diameter. Rod bearings come in two "colors" and the same procedure is used. If you are lucky (the most common case) one color of main and one color of rod bearings will work. But I've see cases where all three colors of mains and both sets colors are rods are required.

Not getting the clearance right will lead to early bearing failure. Too tight is intuitively obvious, but too loose is just as bad as the hydraulic forces that maintain the lubricating layer will be disrupted. When the clearance is correct, the bearing and journal will never touch while the engine is running.

[redlightpete]

The Bentley manual calls out 0.03 to 0.07 mm for 'plastigauge' measurements of main bearing clearance. That's for 'red and blue' classification. The other colors are tighter for whatever reason. I think the general rule is 0.001" per inch of journal diameter, not 0.0001".

[tinkerputzer]

.002" is good. The .051 on the other side of the paper is mm. I don't have the specs In front of me but iirc main journal specs go up to .0028" The main journals normally mic at around 2.3xx" which means if you follow the .001"/" of journal you want .0023" of clearance.

Pretty sure jlevie meant .001" / " of journal not .0001". When he said it is less than .002" he is thinking of rod clearances which should be around .0017"-.0018".

[efficient]

Did you use standard size bearings or oversized?

[jlevie]

The Bentley manual calls out 0.03 to 0.07 mm for 'plastigauge' measurements of main bearing clearance. That's for 'red and blue' classification. The other colors are tighter for whatever reason. I think the general rule is 0.001" per inch of journal diameter, not 0.0001".

The main journals of an M20 engine are 60mm (2.36") in diameter and (as noted the radial) clearance is .03-.07mm, which is 0.00012-0.00022". That is pretty darn close to 0.0001 per inch of journal diameter. 0.001 per inch is ten times too much.

Did you use standard size bearings or oversized?

You use the bearings that are appropriate for the journal diameters, see post #2 above.

[whodwho]

The main journals of an M20 engine are 60mm (2.36") in diameter and (as noted the radial) clearance is .03-.07mm, which is 0.00012-0.00022". That is pretty darn close to 0.0001 per inch of journal diameter. 0.001 per inch is ten times too much.

What are you using to convert mm to in?

0.03-0.07mm is 0.0011811-0.00275591"

http://www.google.com/search?hl=en&q=.07+mm+in+inches&oq=.07+mm+in+inches&gs_l=serp.3..19.47212.47307.0.47487.2.2.0.0.0.0.57.109.2.2.0.les%3B..0.0...1c.1.4.serp.dGUUDPiT67M

[e30m20TurbO7]

Piston 1

Piston 2

Piston 3

Piston 4

Piston 5

Piston 6

Main cap 1

Main cap 2

Main cap 3

Main cap 4. Too tight ??

Main cap 5

Main cap 6


[IMG]http://i65.photobucket.com/albums/h210/adamslayerxyz/6FB79346

[tinkerputzer]

The main journals of an M20 engine are 60mm (2.36") in diameter and (as noted the radial) clearance is .03-.07mm, which is 0.00012-0.00022". That is pretty darn close to 0.0001 per inch of journal diameter. 0.001 per inch is ten times too much.

You've got one too many zeros. The rule is one thousandth (.001) of an inch per inch of journal diameter. Not one ten thousandth (.0001).

[!kid]

Number 4 is to tight. when I build engines (nissan VQ) at work I do .0018-.002 oil clearance on the mains and rods.