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Bearing clearances and oil pressure

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Morris

Morris

Yoda
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Pulled the oil pan and checked clearance on one of my mains tonight. It's at .003: High but in spec. It seems that such high bearing clearance could be causing my low idle pressure.

Is there any way that I can tighten that clearance a bit without re grinding the crank? I ground the crank less than 5000 miles ago, and the journals and bearings look fine.
 
No.
Other than replacing the bearing itself (whatever minor amount of wear on the bearing will be brought back to spec).

My racer has almost your clearance plus a used pump and it has decent oil pressure even after 4 Hours of racing. Look at your pump....be sure the side plate that traps the gears in the pump is flat. My side plate was worn so I cut it down a bit to flatten it and make the pump work better.

I think you said you're using 20W-50 oil (like me).

In the good old days, people would "shim" bearings with 0.001" shim stock, but that was in engines that turned maybe 2000 RPM max. I'm sure it would make the bearing "oval" and cause high and low contact points. Bad idea.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Look at your pump....be sure the side plate that traps the gears in the pump is flat.[/QUOTE]

There is some wear on the plate. In a previous post you described a pretty ingenious way of lapping that plate with a piece of glass and emry cloth. You also said it takes much time. I don't have a lot of money or time, but I have a little more money than time. If I buy a new pump, does it come with a new side plate?
 
Can you give me some more details on how you did it. You mentioned using fine Emory Cloth. Is Emory Cloth rated like sandpaper? If so, what number did you use? Did you glue the Emory Cloth to the glass?
 
Emory cloth comes in many grits just like sandpaper. Get several grits and start with maybe 120 down to 100 then 80.

Lay it on the glass and rub the item you are laping on it.


CLEAN SUPER WELL WHEN FINISHED, CARB CLEANER, SOAP AND WATER, ETC. THEN A LIGHT COAT OF OIL BEFORE REINSTALLING.
 
Or as an alternative, use fine valve-grinding paste and kerosene. you can finish up with toothpaste even
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Or as an alternative, use fine valve-grinding paste and kerosene. you can finish up with toothpaste even [/QUOTE]

Can you explain this process to me. I have a whole can of valve grinding paste sitting on the shelf!
 
Hello Jack,

"120 down to 100 then 80."

I think you have your grits mixed, 80 is the coarsest of those three.

Personally I would use wet and Dry paper (Silicone carbide) with a light oil. Emery cloth doesn't like being wet so for me the Wet and Dry gives a better action.

The quicker alternative is to get a machine shop to face it flat.

Alec
 
Take a sheet of approx 100 grit paper (the stuff used for wet sanding is good....that's what Alec means when he says "Wet and Dry paper").

Place it rough-side up on a piece of nice, flat glass.

Add a small amount of light oil or kerosene.

Push pump plate against paper lightly and rub back and forth. Rotate the plate in your hand every now and again to keep things even.

The worn plate will have two worn circles (from the sides of the gears). When the circles are almost sanded away, change to a finer grade (maybe 120 or 160) and keep going until circles are removed.

Clean really, really well.

You can put the lapping compound direcetly on the glass and try that instead if you wish.

And I've used toothpaste in a pinch too...but will take quite a while.

I agree that bringing it to a machine shop to have it planed off is the easiest and quickest.
 
aeronca65t said:
In the good old days, people would "shim" bearings with 0.001" shim stock, but that was in engines that turned maybe 2000 RPM max. I'm sure it would make the bearing "oval" and cause high and low contact points. Bad idea.
Click to expand...
In the old days, tapered shims were available. ie. a 0.001" bearing insert shim would be .0005" thick on the sides & 0.001" thick in the center. This removed the "oval" so that clearance take up was the same all the way around. To my knowledge, this tapered shim stock is still available.

Still, as you say, not something I would use on a high rpm engine.
D
 
Tipicaly I was backwards, sigh. Senior moment.
 
And, if hand lapping, a "figure eight" motion works best to insure good even contact.
Ever do the rocker box covers on a radial aircraft engine, Nial?
Jeff
 
OK, here's a subject I have a great deal of knowledge on, if you have .003' clearence and you are seeing low oil pressure, would .003" clearence cause this ???

You better beleive it!!!!!!!!!!!!!


Let me tell you a little story about rod clearences that wil shed some light on this subjest. About three years ago, I had a two time SCCA HP national champion and his crew chief approach me at the runoffs, they wanted me to build them a bottom end for 1275 race engine, they had a spec sheet telling me every spec on the motor and wanted me to deliver to them this blueprint in the engine. Their spec was .0025" for rod bearing clearence, I normally bult to .002" for a race motor, and .0015" for a street motor, but I fiquired, what the hay, they are national champions, and really smart guys, they must know more than me. At the same time I built their bottom end I started on a HP 1275 race engine for mytself and followed thier spec sheet and the .0025" clearences on the rods. Got the engine built, it ran great, but I always struggled with oil pressure, ended up regulated the pressure reilef valve higher by making it adjustable, and using 9/16" ball bearing in place of the cup, with the adjustment pegged, all I would every get was 60 psi cold, at 45-50 in the heat of race, much lower than I was accustom to seeing. After picking alot of people's minds we came to the conclusion that the rod clearence was too great, and even as little as .0005" could make a big difference in pressure, now mind you during all this I tried several oil pumps with no gains in pressure, and side thrust clearence was dead on the money. Finally I decided to bite the bullet and take the engine down and do whatever I needed to do to get it back .002" on rod clearence. I did so, rebuilt the motor, fired it up for the first time, with the pressure regulator still pegged, and immediately pegged the oil pressure guage, then had to back off the adjustable regulator big time, now the engine has a nice 85-90 when first cranked cold, and then settles into a nice 75-80 for the entire race. Turns out my customer who also had a engine speced this way had been suffering the same problem, did what I did as well and saw the same sucess. So the moral of all this is rod bearing clearence is DIRECTY reated to oil pressure in a big way, and if you have .003" clearence time to rethink it down there and on a street engine get it back to .0015", you are twice what I recommend for a street engine, and I'd bet good money that this is your problem without knowing anything else. If you think you see red flashing lights and hear sirens going off in my post, you are correct!
 
Fasenating as always Hap.

No clue what mine is but seem to have about 50 hot running so guess rod bearings must be preaty darn close.

Thanks for the insight.
 
or if you know of a hydraulic repair shop in your area, stop by & see if they have a lapping machine, I used to work at a shop that did, used it for the plates/gears in hydraulic pumps - no set up or anything, just turn it on, and set your plate onto the surface, have a coffee & your done :smile:
 
Thanks Hap. At this point, oil pressure is not bad enough to pull the crank. I can shim the relief such that I get 85 cold and 60 warm. It's just the idle pressure that distresses me. If I do a long highway trip, idle pressure can drop below 10. Youch!

I'm just going to run it into the ground and plan on changing my bearings (easy to do on a 1500 with the engine in the car) in about a year. I have an extra crank sitting on the shelf. So in the extremely unlikely event I have some extra time and money I can send it to a much better shop (Acme Speed Shop perhaps...) than the one that turned the crank I am currently running.
 
Hello all,

I can't believe that an engine with only 5,000 miles since a rebuild has lost oil pressure unless there was some error in assembly, insufficient cleaning maybe, or the crank was ground under specification size for the bearings.
The other point is that high oil pressure per se is not necessarily a good thing, it just adds a load on the engine. I certainly would be quite happy with 45 psi hot at say 3,000 rpm or more. AS long as the engine maintains that then I wouldn't worry, particularly with a road car.

The only engine, in my experience, that was very sensitive to bearing clearances is the 6 cylinder Aston Martin engine that has three grades of shells per journal size, with I think from memory about 1\2 a thou difference per grade. Bear in mind that this is an aluminium block so the expansion rate is higher than with iron.

Alec
 
Well, I throw this out there as well, on of the biggest problem I saw with the race motor with .0025" rod clearence is when it got hot, it had nearly no oil pressure down low, after the race when I would go to impound, I always try to blip the throttle otherwise the pressure would drop to below 20, now we can't get it to drop below 50, the only change in the motor is the .002" clearence instead of the
.0025" on the rods, that's it.
 
The machine shop that ground the crank for me over did it. I gave them a spec sheet that said .001-.003. They gave me .003. I would have to say that they messed up just about everything I had them do. The story about how they messed up my head is almost too painful to repeat.

Anyway... this engine has fine top end pressure. It's just the idle pressure that suffers. Whatever... I'm over it.
 
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