283 Too Much Main Bearing Clearance

[Domenic T.]

Re: 283 Too Much Main Bearing Clearance

Mark,
The oil passage is only in the block and not on the cap. The foil go's on one side only. I suppose if one wanted it on both sides they could Trim the hole. We used to peel the foil from chewing gum wrap and put it over a dime and rub the foil to mold the dime then trim it and set it down for someone to find. That is about the thickness of the shim foil and it is rubbed smooth and trimmed.
Also, I think Duke has the answer, align bore. Common thing to do. I believe they did that when I had engines blue printed years ago.

Dom

[Thomas H.]

Re: 283 Too Much Main Bearing Clearance

Good discussion going on, thank you for all the input. I'm learning a thing or two.....

I have assembled several engines over the years (but I am no expert) with no issues and I attribute that to the fact that I check every little detail which is how this appeared. Never had a problem with excessive clearance before.

A couple of comments:

When I got this block, it was a std bore block and had the caps with it. I can't prove or disprove that the caps that are on the block now are the "born with" caps or not. For the sake of what we are discussing lets go with they are not the originals.

After checking and double checking my micrometer readings and bore gauge settings I started to believe the readings I was getting. For the heck of it I checked the #1 bearing with plasitgage. The reading I got from that was 0.002" which is right where it should be. Took everything back out and redid my micrometer / dial bore readings and came up with 0.0035" - repeatable. So I don't have too much faith in the plastigage reading (never really did either.....).

I started thinking, why the .002 with plastigage? Is it because the crank is installed and I had all of the journals torqued down possibly causing the crank to bend a bit? Maybe pointing to some misalignment in the main journals - ie non original caps........?

Tonight I will check the parting lines, go through each journal again and take measurements and then do the plastigage on each journal which may show some deviation.

As much as I hate to have an issue with this which will cause another trip to the machine shop, I'm having fun doing it.

Tom

PS: the machine shop I deal with is very competent and been around for years so no worries there.

[Domenic T.]

Re: 283 Too Much Main Bearing Clearance

Tom,
The plastigauge is more accurate than one may think. That's what we used for years. Also hopefully your crank isn't bending when tightened. After you get all the good plastigauge readings, oil the crank, tighten the caps and give it a spin by hand. If it had the slightest bend it would not spin.
I have the instruments you used when measuring and find that I get readings that are close but a few tenths .0002 off when combining 2 instruments. The every day pro's are most likely better than I at that but I find it hard.
Good luck with it and run your plastigauge numbers past a few and see what they say.

Dom

[Duke W.]

Re: 283 Too Much Main Bearing Clearance

Mark,
I think Duke has the answer, align bore. Common thing to do. I believe they did that when I had engines blue printed years ago.

Dom

As a rule, I don't recommend align boring if the caps are believed to be original. During the "forensic teardaow" I recommend, when the block is stripped except for the crank, rotate the crank slowly around several times and check for any spots that have excess drag. Then give the crank a good spin, and it should rotate like its on roller bearings. If the above two tests are passed, the bore alignment is likely okay.

As a final test with the crank out, reinstall/torque the main caps, then use a machinists bar and .0015" feeler gage to check main bore alignment by placing the bar across the tops and bottoms of the bores and other angles by rotating the engine stand. Place the .0015" feeler between the bar and each cap. Pull the gage out and note the amount of force required. If all are about equal, the bores are in alignment and no further work is necessary.

The problem is that most machine shops will align bore, apriori, without taking measurements, so one must discuss this with the shop and be certain that they understand and comply the customers rules of engagement. If not, find a shop that will.

Measuring main bore ID and comparing to OE specs is good policy as it measuring ID with the bearings installed. This minus crank journal diameter is the theoretical clearance, but this involves two measurements that can cause error buildup, so as a final check I recommend plastigage, and I think those readings are, on average, more accurate than bearing OD minus journal diameter.

Once the crank is installed and the caps torqued down, repeat the rotate and spin tests.

Duke

[Thomas H.]

Re: 283 Too Much Main Bearing Clearance

so as a final check I recommend plastigage, and I think those readings are, on average, more accurate than bearing OD minus journal diameter.
Duke

Tonight I checked each main journal using plastigage. Results from all 5 journals fall around the 0.002" mark on the measuring stick (see picture, typical of each journal).

I then removed the crank and mic'd the journals, used the mic to zero the bore gauge and measured each journal and came up with 0.0035 to 0.004" clearance (see pic).

So what do I trust? I don't believe I have enough experience to make that call so I loaded the block and crank into the Explorer and will visit the machine shop tomorrow. (I'd rather find out I'm either doing something stupid, or there is a real problem, then assemble it and have issues later)

In previous builds (mostly big blocks) I have never run into this. My past experience is that I can usually back up the bore gauge readings with the plastigage and feel comfortable. This build, not so much.........

This car has fought me every step of the way, why should this be any different!

20150623_175312.jpg 20150623_193504.jpg

Tom

[Steve G.]

Re: 283 Too Much Main Bearing Clearance

Here's what I do. Pull all the bearings and bolt the caps in place. Don't need to torque them, just tighten them down .

Are all the parting lines smooth? If so, whatever line up you have for the block is the same for the caps. If not, no need to go any further.

Next check the block for straight. Use the crank for your straight edge. If you want to double check the crank, put it in a couple of wooden v blocks and set your dial indicator on the centre journal and check your run-out. On a fresh ground crank it should be 0.000 unless you mishandled it on the way home.

Put the brgs in the block and put a couple of drops of light oil on each one. Set the crank in and give it a spun. Does it spin smoothly and easily? If so, your block is straight. Put the bearings in the caps with a bit of oil. Tighten them down one at a time spinning the crank as you tighten each one. If you get through all of them without an appreciable change in drag, you're good.

Lift the crank out and wipe out all the oil from the bearings. Clean the oil from the crank. You want the crank set in dry for the best plastigage measurement. Lay your plastigage across the full journal, frt to rear. Torque them all then remove them all. The plastigage should be a uniform width across the entire journal. If it is and all your readings are .002, you're good to go.

I would trust the plastigage long before I would trust your bore gauge reading. Just as you need to wiggle your gauge around in the bore and take the lowest reading to be accurate (finding the perfect perpendicular), you have to do the same thing when trying to set it to a micrometer. That's very hard to do with a mic.

I've held a journeyman's ticket for almost 40 years. I don't do engine work exclusively, but I probably do several a year. This is how I do it and it's never failed me. Plastigage has become a staple in shops because it does the job, accurately.

Steve

A little tip for removing the plastigage that sticks to the crank and the journal, lacquer thinner on a rag dissolves it and wipes away clean with just a couple rubs.

[Joe L.]

Re: 283 Too Much Main Bearing Clearance

I would trust the plastigage long before I would trust your bore gauge reading. Just as you need to wiggle your gauge around in the bore and take the lowest reading to be accurate (finding the perfect perpendicular), you have to do the same thing when trying to set it to a micrometer. That's very hard to do with a mic.

Steve and Tom------


I absolutely agree.

[Steve G.]

Re: 283 Too Much Main Bearing Clearance

One other thing that you can do to put your mind at ease as to which is the correct measurement would be to compare the vertical and horizontal readings of your bore gauge.

Assuming your parting lines are smooth, if your gauge shows the bore to be a perfect circle measured horizontally and vertically it is almost certain that the fault is in your transfer of measurements from the mic to the gauge.

When the bore "grows" it does not grow horizontally. There is very little load in that direction and you would have to spread the block to increase the size. So the horizontal measurement is pretty reliably at it's original dimension. If your bore gauge doesn't show an increase in size as you move to vertical that dimension has not changed either.

This is how we measure con rods to determine if they need resizing. We don't care about the actual measurement as much as the difference in the two directions.

Steve

[Joe L.]

Re: 283 Too Much Main Bearing Clearance

Tom------


By the way, the Chevrolet Overhaul Manual instructs to use Plastigage to measure bearing clearances.

[Thomas H.]

Re: 283 Too Much Main Bearing Clearance

Update:

Machine shop confirmed my dial bore gauge readings 0.0035 to 0.004" They inspected and "deburred" the main caps and now all clearances are 0.0025". I'll pick the block up tomorrow and continue assy over the weekend. Hopefully no more hiccups!

Thank you everyone for your input!

Tom

[Steve G.]

Re: 283 Too Much Main Bearing Clearance

Update:

They inspected and "deburred" the main caps and now all clearances are 0.0025".


Tom

I sure don't like the sounds of that. "Deburring" tightened them 1- 1.5 thou?

Not in my shop.

Steve

[Thomas H.]

Re: 283 Too Much Main Bearing Clearance

I sure don't like the sounds of that. "Deburring" tightened them 1- 1.5 thou?

Not in my shop.

Steve

Picked engine up today. Spoke with machinist vs counter guy that called Monday........ Said the caps most likely were not the ones that came on the engine originally. He said the parting lines were not flat like they should be for proper mating. He cleaned that up and then ran a hone down the mains. He was happy with the way it turned out. I checked the clearances when I got home and they are all between 0.002 and 0.0025 using the same measuring instruments I have used for years.

Tom

Tom

[Steve G.]

Re: 283 Too Much Main Bearing Clearance

Picked engine up today. Spoke with machinist vs counter guy that called Monday........ Said the caps most likely were not the ones that came on the engine originally. He said the parting lines were not flat like they should be for proper mating. He cleaned that up and then ran a hone down the mains. He was happy with the way it turned out. I checked the clearances when I got home and they are all between 0.002 and 0.0025 using the same measuring instruments I have used for years.

Tom

Tom

That sounds a lot better.

Steve