Main Bearing Shells

[Alan Pugh]

Hi everyone,

I have stripped my engine down and found something quite confusing. I am the third owner of this car and when I bought it off the second owner it had done 175200 km (109000 miles). When the second owner bought it, it had done 42500 km and was 4 years old. To his knowledge the engine was original and had not been reconditioned when he bought it. I have confirmed through records that it is indeed the engine that the car left the factory with.

When pulling the engine down all parts are Nissan and the general condition of the engine backs up what I have found out as far as being unreconditioned.

Now the confusing part. The crankshaft main journals all measure up as standard along with the big end journals. Out of the seven sets of main bearing shells 6 pairs are .014 undersize and show above normal wear and 1 pair is standard showing normal wear. The undersize bearings have not damaged the crank in any way. All big end bearings are standard.

Is this a mistake in the factory or has anyone else come across this before in their engine.

Everything else in the engine is standard and shows the normal wear I would have expected, except for the timing chain and components which was more worn than I was expecting.

I am now in the process of cleaning all components and am going to fit a standard set of mains when putting the engine back together.

I would be interested to know if anyone has a theory as to why or how this could have happened.

Alan.

[HS30-H]

The crankshaft main journals all measure up as standard along with the big end journals. Out of the seven sets of main bearing shells 6 pairs are .014 undersize and show above normal wear and 1 pair is standard showing normal wear. The undersize bearings have not damaged the crank in any way. All big end bearings are standard.

Hi Alan,

When you say "undersize", are you getting the information from the stamping / engraving on the backs of the main bearing shells?

The reason I ask is that I'm wondering if the 'undersize' is that of the main bearing bore rather than the main journal size of the crank.......

I've seen a few engines with oversized main bearing bores ( presumably line-bored oversize at the Factory ) but with stock sized crank main journals, and a few engines with oversized bearings in big ends ( again, presumably because they were machined oversize at the Factory, and a 'fatter' bearing was used to correct it ).

Could it be that you are misunderstanding exactly what part is 'undersized'? What are the bores for the main bearings in the block measuring up at without the bearings in them?

Worth a shot?

[Phred]

HS30-H is on the correct path. I would be very surprised to find this engine had been able to run for 100,000 plus miles with with bearing clearance of .014. Even a competition/high volume oil pump would not be able to supply enough pressure/volume, and indicated oil pressure would have been VERY low. To find out whats up, torque all the main caps in place, and measure the inside dimensions of the bearing bores. Stock specs are: 2.3866/2.3873

Then do the same thing with the bearings in place. Measure the crank main journal O.D. and then subtract that measurement from the Bearing I.D., and that will be the actual bearing clearance.

Phred

[Alan Pugh]

Hi Alan & Phred,

Ok, i've put my glasses on (my wife yells at me all the time for not wearing them) and have come up with the correct info.

Alan, yes I was going off the stamping on the back of the shell halves, they are as follows:

#1 0.014

#2 0.014

#3 STD

#4 0.012

#5 0.014

#6 STD

#7 0.014

Each shell half also has it's own part number, the second half being E3000 for the STD shells and E3010 for the others.

There is also damage to the two STD bearrings that would only happen with a clearance that was too large. This leads me to believe that the two STD shells are the mistake and the rest are the correct undersize.

Phred, I measured the main bearing bore diameter in the block and it comes out at 2.3096 - 2.3097 and not the 2.3866/2.3873 that you mention.

To me now, it is looking like i've got a crank with mains that are at the smaller size of the standard tollerance, even though they are still just in the standard size range. Five of the seven bearings have been running with approx 0.003 clearance and the other two with quite a bit more.

It pays to wear your glasses doesn't it.

Alan.

[Phred]

Alan,

Sorry to have mis-quoted the bearing bore size. The one I gave you is for an L-28, which is the last engine I built, and had that number on my desk. If needed, I can look up the L-24 Bearing bore specs at work and post them later. I do have the L-24 Std. journal specs. 2.1631/2.1636

Phred