Ask Away! with Jeff Smith: Bearing Clearance, Cam & Oil Pump Advice for SBC Build

(Image/Jeff Smith)

I have a 350 c.i.d. small-block Chevy that I’m rebuilding. It has Wiseco forged pistons with a 1/16-inch ring package and 10:1 compression, good I-beam rods, and a good steel 350 Chevy crank that I bought used that’s in great condition.

I’ve read a number of your stories and others it seems like the idea is to use a standard volume and pressure oil pump and I don’t need a high volume pump for this mild street engine. I’ve always gone by the standard of 0.001-inch of bearing clearance for every inch of journal size. Will this work well with a standard oil pump?

To keep the cost of this engine down, I’m limited to a flat tappet camshaft. I’ve got a mild hydraulic XE 268 cam from COMP that has 224/230 degrees at 0.050 and 0.477/0.480-inch lift. I know I need a break-in oil for this but then afterward, are there any restrictions on using a synthetic oil? Has the break-in procedure for flat tappet cams changed with these new break-in oils? Thanks for your help. — J.T.

Jeff Smith: You have lots of questions, so let’s jump right in. Your 0.001-inch per 1-inch of journal diameter bearing clearance approach is close.

On a small-block Chevy with a large journal main bearing size of 2.100-inch, this would put your main bearing clearance at 0.0021-inch. This clearance spec assumes that everything is completely square with the main bearing housings, there is no taper or eccentricity in the crankshaft main or rod bearing journals, and that the engine is very carefully fitted to this spec.

In our experience, it’s not unusual for main bearing housings to vary +/- 0.0005-inch. It’s also common for a good used crankshaft main journal to vary by at least +/- 0.0003-inch.

So if you had a main journal on the crank that was +0.0002 and a housing bore that was tight by 0.0005-inch, this adds up to 0.0007-inch tight on the main bearing clearance. If the goal really was 0.0021-inch, this instantly puts the actual clearance at 0.0014-inch which is too tight.

It will work, but this clearance does not take into account any situations where the crank may move a bit — like under max power perhaps with a little shot of nitrous.

A much better idea is to allow for some of these inconsistencies by using the same 0.001-inch per inch formula but then adding 0.0005-inch.

So with a 2.100-inch main bearing journal size, this means 0.0021 + 0.0005 = 0.0026-inch. I generally round this off to 0.0025-inch as my goal for both rod and main bearing journal clearances for a small-block Chevy.

Assuming the crankshaft journals are close to the mid-point on the tolerance, then the main area where bearing clearances begin to vary is with housing bores. We’re talking about the main bearing housing bores and the big-end of the connecting rod.

If the housing bore is slightly larger than spec, the clearance will be loose. If the housing bore is tighter than mid-point on the spec, the bearing clearance will be tight.

Some enthusiasts think the reason bearing clearances change is because of inconsistencies with the bearings. This is not the case. Over 90 percent of the time, the inconsistencies will be with the housing bore or journal diameters.

Often what will happen is a tolerance stack-up. As an example, the housing bore will be tight by 0.0003-inch and the journal diameter will be 0.0002-inch too large. This can add up to losing close to 0.0005-inch of clearance — taking a 0.0025-inch clearance down to 0.0020-inch.

This is where you may have to mix half of a 0.001-inch larger inner-diameter bearing pair to regain the clearance. Companies like Speed-Pro, Clevite, King Bearings, and others offer these bearing shells in both plus and minus 0.001-inch increments.

You can add a half shell to one side of the main bearing for example to increase the clearance to your desired spec.

All of this assumes that you are measuring these clearances with a dial-bore gauge that is accurate to 0.0001-inch. If not, you are wasting your time.

This, of course, also means you are not using Plastigage to measure these clearances. This wax substitute for accurate measurement tools is like playing horse shoes with hand grenades — except that close is not good enough. The only way to know is to use accurate measuring tools.

So assuming you have set the rod and main clearances to somewhere near 0.0025-inch, now we can address engine oil break-in.

Its best here to use a non-detergent or low-detergent oil that has higher levels of zinc and phosphorous (also called ZDDP) to allow break-in.

Synthetic oil is never a good break-in oil because it is too slippery which will delay or prevent actual break-in. What we desire on the cylinder walls is to wear off the microscopic peaks slightly to establish a proper surface for the rings as well as a proper surface on the lobes of the cam.

COMP has recently changed its recommendation for break-in from the old standard which was to run the engine at high idle (1,500–2,000 rpm) for 20 to 30 minutes while varying the rpm between those two points.

The revised approach is to perform this break-in process in three, 10-minute stages allowing the engine to cool in between. However, in speaking with the Total Seal people, they say that this does the rings a disservice.

A better approach would be to run the engine for the first 5-7 minutes to initially break in the cam and then drive the car under load for the remaining time.

Piston rings require a load on the rings to break-in. The only way to achieve that with the engine in the car is to perform multiple mild to heavy accelerations. This loads the rings and performs the proper break-in. Mild rpm acceleration will also allow the cam lobes to break-in with sufficient load.

There’s no need to run the engine much past 3,500 to perhaps 4,000 rpm to accomplish both tasks.

The last point is choosing the proper break-in oil.

There are tests that indicate that there is a wide disparity in formulations of break-in oil. It’s far too complex a subject to deal with here in detail. We’ll keep it simple by saying the best oil is one with high levels of ZDDP and low levels of detergents. This combination works very well.

Among the better break-in oils is Driven’s BR-30, a 5w30 conventional oil that would work well. Driven also has a GP-1 Break-In oil.

Some enthusiasts prefer diesel oil because this oil used to have high levels of ZDDP. Unfortunately, that oil formulation has changed dramatically in the last few years with reduced levels of ZDDP and increased concentrations of high detergents. This is not a good combination for proper break-in.

Better choices would be COMP’s 10w30, Edelbrock, Lucas, or Summit Racing’s new Break-In oil.

Once the engine is broken in with at least 500 miles, you could use a semi-synthetic oil as a good compromise between lubrication and cost. The issue is finding a semi- or full synthetic that still has good ZDDP numbers. Do not use API “donut” oil – those oils are intended for late model cars with roller lifters. You need an oil like a Driven performance oil with 1,500 parts-per-million (ppm) or higher of ZDDP to protect those flat tappet lifters and cam lobes. Among the “hot rod” oils are those from Summit Racing, Comp, Edelbrock, Driven, or perhaps a race oil. The problem with race oil is you have to change oil more often because they offer very low detergent values that a street engine needs.

I’d also recommend a full PCV circuit with the engine like on a stock engine. Keep the oil clean by running a very good air filter. An oil engineer once told me “The best oil filter in the world is a really good air filter.”

He’s right.

Tags: , ,


  1. Pingback: Ask Away! with Jeff Smith: Bearing Clearance, Cam & Oil Pump Advice for SBC Build

Leave a Reply

Your email address will not be published. Required fields are marked *


This site uses Akismet to reduce spam. Learn how your comment data is processed.