Main Bearing Clearance Check

earossi · 06:21 PM

As I continue the rebuild of my 3.6 motor, I am at the stage of doing bearing clearances on the crankshaft. I am initially relying on Plastigage to give me the clearances. This is the first split of the case, as far as I am aware. When I pulled the crankshaft, it and the bearings (both mains and rod) looked perfect. It's one of those revelations that makes you wonder "why" you made the decision to split the case. So, I sent the crank out for dye penetrant inspection and to have the crank polished. I did not measure the crank before sending it to the machine shop.

When I installed the rods and rod bearings for clearance checks, they came out just perfect with clearances at mid point of the range.

So, the next step was to do a bearing check on the mains. I installed the new bearings in the case (which I had spent about 6 hours cleaning to this point!). I installed the crank, dropped in the Plastigage, installed the other case half and torqued up all the through bolts and case perimeter studs to the specs, which for this motor are called out as 37 ft-lb for the through bolts and 17 ft-lb on the case perimeter nuts. The through bolts were torqued to a cross pattern, while the permitter nuts were just done in series.

When I pulled the upper case half, I was surprised to find the clearances right at the maximum spec. So, I have to determine why that was the case. Every one of the mains was at the extreme max, so the case appears to have set down evenly.

Tomorrow, I will pull the crank out of the motor so that I can mike the mains. And, we'll go from there.

There has been discussion on this board about poor quality bearings. I miked the thickness of the new mains as well as the ones that came out of the motor, and they are identical (using a caliper which is the best that I have to do that).

So, are there other checks I should make?

AlfonsoR · 05-09-2013, 06:39 PM

If you are within specs, even at the max, and the old bearings look good then you should not change a thing.

One additional check you can do is to install the old bearings and see how that comes out on plastigage, then of course, compare the two results. A lot of people tend to think that new, as in new bearings, is better. That is not necessarily the case. If you have a bearing system that had been functioning well over 20+ years and it is still within specs, sometimes, that is better than introducing a new component (new bearings) to the mix. Anyway, that's my opinion. Some people will not agree.

Hope things work out for you.

HawgRyder · HawgRyder's Garage

Magnafluxing and dye penetrant test are great...and expensive.
I always "ring" a crank before I use it or send it for check.
If the crank rings like a bell...it's probably not cracked...and if it's straight...it's ready.
Cheap check...and old habits die slowly.
BTW ... I have arrived at machine shops to pick up "checked " cranks....and have "rung" them only to hear a very sour sound....cracked!!
One shop tried to sell me 4 in a row...I don't go there any more.
Bob

KTL · 05-10-2013, 06:15 AM

Ernie,

If you know of someone who has a bore gauge, you could use that method as an alternative/2nd opinion to the Plastigauge. You measure the bore with the bearings installed (that includes #8 nose bearing, since that one aligns the front of the case) and the case fully torqued with both thru bolts and perimeter nuts.

Or, you could assemble the case and bring it over to AutoMachine and they could run their gauge in there & give you some numbers?

earossi · 06:36 AM

Quote:

Originally Posted by KTL

Ernie,

If you know of someone who has a bore gauge, you could use that method as an alternative/2nd opinion to the Plastigauge. You measure the bore with the bearings installed (that includes #8 nose bearing, since that one aligns the front of the case) and the case fully torqued with both thru bolts and perimeter nuts.

Or, you could assemble the case and bring it over to AutoMachine and they could run their gauge in there & give you some numbers?

Hi Kevin,

I have a bore gauge, but find it a bit cumbersome to use to get accurate and repeatable measurements, so I don't have a lot of confidence in my ability to use it; but, if I don't resolve my issue, I'll try the bore gauge with my "limited" skills, and then, if necessary, make the trek to AutoMachine.

earossi · 05-10-2013, 07:13 AM

OK. I pulled the crank out of the case this morning and just finished miking the mains, and they are all well within tolerance. I checked the tolerance spec in both the shop manual and the Porsche spec booklet for my car, so I feel that the specs are probably correct. I also took 3 to 4 readings on every journal to gain confidence in the results. Factory specs call for the journals to be 59.971 to 59.990 mm. My minimum readings were found on two of the journals and measured 59.975 mm, so well within spec.

I went over the mating flanges of the case halves looking for anything to explain the poor Plastigage readings. I didn't find anything. I found a couple of specks of old sealant that was squeeze out at the parting lines and I only found it by sliding a razor blade over the flanges (that's how small the several specs were!). And, since these "specks" were outboard of the mating flange faces, I don't believe they explain the differences.

There are only two things that need to be checked. First, I did not have the nose bearing installed when I set the crank into the case. Kevin (KTL) suggested that the nose bearing needs to be installed since it helps position the case. Worth a try.

The second thing to do is to assemble the case halves (with or without bearings??) and run a bore gauge through several of the bores to see if the case might have been machined oversized. I made a feeble attempt to prove this out by making sure that the new bearing shells where firmly seated and then I checked, visually, to see if they were even with or proud of the split line on the case.......and they were flush.

I'm going to reassemble the case with the nose bearing and see what I get with Plastigauge. If the readings are still out of tolerance, I'll run a test with a bore gauge. I'll report back as I move through this. Thanks for all the good comments.

KTL · 05-10-2013, 07:50 AM

If the STD bearing fit in the case clean & tight, your case bores should not be oversized. You'll want to install the bearings in the case if you do the bore gauge measurement. Your bearing ID measurements vs. your mic reading for each main journal will give you your clearance specs. I say do it. You might be surprised how different the plastigauge is vs. the bore gauge.

The key to the bore gauge usage is the rocking of the gauge. Do the rocking slowly and check each bore in a number of places. As the dial indicator waves back & forth at you, you note the lowest reading you see on the gauge. Does take some practice/technique to get repeatable measurements & it's a bit tedious. But I think it's a better indication of your clearances.

I think precision measurement should be used where possible and a bore gauge + mic works here. Certainly the squish approach with something like clay is a necessary method in instances like piston & head clearance checks where you can't get an instrument in there. For these bearing clearances, I think mics win out over Plastigauge.

AlfonsoR · 05-11-2013, 09:35 PM

You may want to check the clearance with the old bearings. The new bearings might have slightly less crush and give you slightly larger readings. The place to measure the plastigage is 90 deg from the split line. If you go significantly above or below that, the readings will probably change because when the bearing is properly installed, it will have a slight oval shape, not round. Shell bearings are designed with an oil relief (larger clearance) at the split line.

irobertson · irobertson's Garage

I measured mine using both the bore gauge method and plastigauge. The bore gauge/mic calculations gave me 0.0030 on average. The plastigauge came out at 0.002.

earossi · 05-12-2013, 12:27 PM

Quote:

Originally Posted by AlfonsoR

You may want to check the clearance with the old bearings. The new bearings might have slightly less crush and give you slightly larger readings. The place to measure the plastigage is 90 deg from the split line. If you go bsignificantly above or below that, the readings will probably change because when the bearing is properly installed, it will have a slight oval shape, not round. Shell bearings are designed with an oil relief (larger clearance) at the split line.

Alfonso,

Good idea since I did save my old bearings and since they, visually, look perfect. But, I did not keep them separated by pair. So, can I just randomly select two bearing shells and install them for a cross check? In other words, for a matched set, will I get different crushes? I would guess that I would given that there is a tolerance on each of the bearing saddles.

AlfonsoR · 05-13-2013, 09:15 PM

You're right, they need to be a matched set. Sorry, no help there.

You may want to do as "irobertson" and KTL (Kevin) suggested, measure with a bore gage or bore mic.

earossi · 05-14-2013, 02:30 PM

I spent today bolting up the case with bearings installed. I then spent some time becoming comfortable with my bore gauge. I set the bore gauge and zeroed it out using the mic I used to measure the crankshaft journal. After a while, I was at a point where I could get repeatable readings using the gauge. Using the bore gauge on the first bearing, bearing clearances varied between 0.0015" to 0.0025". Took about a half dozen readings on the first bearing and averaged them. Doing so, it appears that my bearing clearances average 0.002" or 0.051 mm which is within normal range.

I then attempted to take readings on the second and third bearings, which is the furthest my bore gauge could reach into the case. These bearings were much more difficult to get repeatable readings on; but, in general I got similar readings to the first bearing.

For grins, I did as Alfonso suggested and installed one of the "old" bearings that had 100k miles on them for the test I did above. Understanding that the bearing halves were not a matched set, I just did it to see what I would find. Though inconclusive, it appears that the "old" bearings are at the same clearance as my new Glycols or 0.002".

So, I appears that my Plastigauge check was probably done at close to the extreme of the material and may not have been as accurate as needed.

AlfonsoR · 05-14-2013, 07:09 PM

Excellent work!! Glad it worked out for you. By the way what is the measuring length you would recommend for the bore gage? I need to buy one for my rebuild. I'm assuming if your gage had been a little longer, it might have been easier to measure?

earossi · 05-14-2013, 07:23 PM

Quote:

Originally Posted by AlfonsoR

Excellent work!! Glad it worked out for you. By the way what is the measuring length you would recommend for the bore gage? I need to buy one for my rebuild. I'm assuming if your gage had been a little longer, it might have been easier to measure?

Alfonso,

I really don't know much about bore gauges, so do not rely on what I say to guide you. But, my bore gauge is about 10-12 inches long, which allows you to reach the first three bearings in the 911 case. I would guess that you would need one that is approximately twice that length to reach all of the bearing saddles. My guess is that you can purchase an extension that will allow you that extended reach. I chose mine, originally, to measure my cylinders for wear and out-of-roundness.

KTL · 05-15-2013, 06:44 AM

Ernie is correct. The typical bore gauge kit us DIY'ers would buy is not long enough to reach the mains closer to the pulley side of the crank. Pros like Costa Mesa's John Edwards have bore gauges that are about three times as long as the one I have. Gotta be mucho expensivo!!!

Porsche 911 Machine Work & Racing - YouTube

irobertson · irobertson's Garage

Here is a shot of my gauge and case. It is long enough to get half the bearings going in one end, and the other half from the other end.
One thing I should mention. I did use it to measure the rod big ends without bearings installed, and in doing so, damaged that tip (tiny flat spot) by riding it back and forth on the cross hatch from the machine work. I had to carefully round it off and polish it again to avoid scratching my bearing surfaces. It was the same tip for the mains as it was for the empty rod ends.

KTL · 05-16-2013, 07:29 PM

I have the same size gauge. Works going in from the flywheel end. Notsomuch on the pulley end. You can't rely on the main measurements from the pulley end when #8 nose bearing is not in place. That bearing locates/aligns the front of the case.

earossi · 05-26-2013, 07:42 PM

Still not happy with my struggled useage of the bore gauge, I went out and purchased a set of Starrett ID mikes off of Ebay to double check the bore gauge readings. Happy to say that though not a perfect duplication of bore gauge readings, the ID mikes were close enough to convince myself that my skills in using the bore gauge are at least passable.

Thanks to all for the advice along the way.

Walt Fricke · 05-26-2013, 10:04 PM

Has anyone used a bore gauge to see how much having the #8 in place or omitted affects measurements?

I can see the #8 as possibly keeping the two sides of the case from being up or down, as it is a tight fit, although only pinned on one side. Wouldn't affect side to side measurements?

And if the case were shuffle pinned (or, as I kind of prefer, shuffle bushed), those would keep things where they were supposed to be. Or, installing the IS shaft with new bearings might do to keep the up/down part in register, if it were inclined to wander?

So if a standard bore gauge length got you to the 3d main from the pulley end, you could get them all?

earossi · 05-27-2013, 05:38 AM

Quote:

Originally Posted by irobertson

I measured mine using both the bore gauge method and plastigauge. The bore gauge/mic calculations gave me 0.0030 on average. The plastigauge came out at 0.002.

Interesting. That is a huge difference (Plastigauge versus bore gage). But, you are in the same dilemma I am in. At 0.0030 inches with your bore gauge, you are out of spec according to the shop manual, by about 0.004 mm, which is not very much out of spec....but it does exceed the factory spec of 0.010 to 0.072 mm.

Comments from others would be appreciated on this. If you are building a race motor, the larger clearance would probably be OK which would reduce internal friction.....but, race motors are built to last hours not a hundred thousand miles.