Here's what you will see on...
Here's what you will see on my bench when I get ready to install and check crank-to-bearing clearances. At around $1,400, I am lucky to have access to the two internal mics on the left. This is out of reach for the low-buck racer, but there is a viable option in the form of Plastigage.
If the bores are really worn, then they must be remachined. This involves doing something drastic to the stock pistons so that they adapt to a bigger bore or new pistons. Of the two choices, new pistons provide the best results. The best place to look for low-cost, race-worthy pistons is KB's range of claimer and street/race pistons. If you have not yet perused this company's range of low-cost pistons, then hit some of their dealers' Web sites and be pleasantly surprised.
In a worst-case scenario you may not have the budget for new pistons. I have been in this situation more times than you may imagine. As long as the ring grooves were near perfect, I knurled the piston skirts and filed a suitable taper/barrel form on them. This is not what I would recommend for those less experienced in the art of either engine building or filing, but it can be done. In the past it was a cheap save-the-day option because I have always had some kind of lathe in the shop, albeit a beat-up one. A well-done knurl job can be a very effective stop gap until new pistons are in the picture. These days, very few shops have piston knurling tools. If you have to pay to get the job done, you may be paying a major proportion of the cost of new pistons, so weigh your options before going this route.
Lastly, when you pull the pistons from the block, check the wear pattern on the skirts. If it is not even and straight up and down on the skirt, the rod could be bent. A rod that shows any sign of being bent should be tossed, as it can easily sap 10 hp from the engine's output.
To measure the bearing clearance,...
To measure the bearing clearance, a thin, round piece of plastic is squeezed between the bearing and journal. The width it spreads is then compared to the gauge widths on the Plastigage packing paper.
The only way to know what your engine's crank-to-bearing clearances are with near certainty is to use the tools shown on the next page. Although a set of 1- to 3-inch outside micrometers for measuring journal ODs can be bought for a low price at Harbor Freight (sometimes on sale for less than $20), the same cannot be said for accurate bore mics. Though cheap, snap gauges and two-point inside micrometers are not the way to go for measuring housings and bearing IDs. Three different people will inevitably get three different readings. The best deal here is a three-point internal mic-but they are expensive. The two shown here ran to nearly $1,400, and that was supposed to be a deal.
With micrometers costing as much or more than the engine we are rebuilding, there is a real need to find a cheaper route to establish bearing clearances that are both functional and safe. Plastigage is the answer, and your local AutoZone or NAPA has it in stock. For just a couple of dollars, you can have the info on crank clearances to a degree totally satisfactory for what we are attempting to do.
Don't overlook checking the...
Don't overlook checking the crank end float, as this can sometimes be tight with certain brands of aftermarket bearings. A simple fix is to lap the thrust faces of the bearings on a machined surface using 400-grit emery.
As for the clearances to use, or to be more precise, clearances you can get away with, I could be opening a can of worms here-but it won't be the first time. First, let's make one thing clear. What works best for a $50,000 Cup engine is largely irrelevant here. Those close-fit bearings to minimize oil usage and windage are the result of a very expensive and accurate set of parts. That's a luxury beyond our means.
The big dilemma is often deciding whether to grind the crank. Although a crank will tolerate a little taper, even a few tenths of ovality can mean a short life. If it has worn a little undersize (but is still round) it may still be usable. Although clearances of 0.002 inch for the rod journals and 0.0025 inch for the mains are about the best for most applications, a little more in a budget build is not such a bad thing, as it compensates for small misalignments that may have crept into an aging block over time. If the main bearings show anything between 0.0025 and 0.0045 inch when Plastigage is used, you are good to go. The same theory applies with the rods-they can be up to 0.002 inch more than stock and still survive a season's racing.
If the crank needs a regrind, make sure the shop doing it maintains or increases the journal corner radii, as this will increase the crank survivability far more than the reduced journal diameter decreases it. If a regrind is inevitable, consider a Scat stock replacement cast steel crank. You can get a crank for about $200 that will deal with 500 hp and run to 8,000 rpm.