Harley Crankcase Lapping Tool and Usage
Article by Mark Trotta
A fundamental part of a Harley Big-Twin or Sportster engine build is checking/lapping the main bearing races. The goal is to have the crank assembly sitting dead-parallel in the cases, since this is where the flywheels will be spinning thousands of times a minute.
Lapping the main bearing race assures precise alignment of flywheel shafts, allowing maximum power and engine life.
The tool required for this service is called a Crankcase Main Bearing Lap, also referred to as a Pinion Shaft Bearing Lap Tool. This article covers tool usage, as well as it's several main parts.
Before Harley discontinued this tool, it was sold at H-D dealerships as part # 96710-40. It covered Big-Twins 1955-1998 and Ironhead Sportsters 1957-1976. Also fit on Harley Flathead UL models 1937 through 1948.
The Harley-branded tool was actually made by Kent-Moore, which they no longer make. Fortunately, they are still available in the aftermarket (more on that below).
There are three main parts of the lapping tool, an arbor, pilot, and lap head.
The long shaft of the tool is called the arbor. It goes through the pinion bearing race and into the pilot, or guide sleeve.
The part that does the actual lapping are called lap heads, or expansion collars.
The expansion collars are tightened/loosened with appropriate-sized spanners.
When you buy a Harley Crankcase Lap tool, you get two different size pilots, three different size lap heads, a crank handle, and spanner wrench to tighten and loosen the lap head collars.
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Lap Tool In Use
Part of the rebuild on this 900cc Ironhead Sportster included lapping the pinion shaft bearing race and refitting the pinion roller bearings.
The line-lapping procedure will check case alignment, as well as remove any traces of wear at the sides of roller paths.
Line Lapping vs Line Boring
If the pinion race needs to be bored, lapping is required afterwards. It's done with the same tool, but with a finer compound. Line boring uses coarse abrasives, lapping uses a fine compound, which removes less metal and leaves a smoother finish.
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Crankcase Lapping Procedure
A pilot is temporarily installed in the left-side case.
There is a 1.00" ID pilot for Harley Big-Twins, and a .75" ID pilot for Ironhead Sportsters.
With the expansion collar fitted, the arbor goes through the right-side case and lines up with the pilot in the left-side case.
Apply a thin coat of fine lapping compound to the collar and install tool into case.
I'm using Clover 280 grit fine compound.
Turn the arbor handle by hand only at a slow to moderate pace. To avoid any grooves or tapering, slowly and methodically work lap back and forth in race.
As you turn the tool, the lapping compound on the collar finely sands the bushing.
Pull the tool out occasionally, wipe compound off of bushing with a shop towel or rag, and check progress. A smooth and dull solid color should begin to appear.
The end result will be an even gray color across the bushing surface.
If you need to make another pass--before putting on more lapping compound--clean the tool with brake parts cleaner or other cleaning solvent, then dry with compressed air.
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Harley 96710-40 Tool Set
If you're looking to buy a Harley engine lapping tool, you may find a good used one, or you can buy an aftermarket kit offered by V-Twins.
The V-Twin tool set fits Harley Big-Twins 1936 through 1998, as well as Ironhead Sportsters 1957 through 1976. Also works on Harley Flathead UL models 1937 through 1948.
Shop: Harley Crankcase Lapping Tool
If needed, pilots and lap heads are available in different sizes. Can also be used to re-size four-speed transmission bushings.
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Notes and Misc
When done, you'll likely need oversize pinion roller bearings. These are readily available in different sizes. The factory service manual explains how to calculate what size you'll need, but I just fit roller bearings by trial and error.
It's possible for the race to be worn beyond limits, in which case there is no alternative but to replace it. Heating up the case will help the race come out easier. A shop press is helpful. And, of course a new race will need lapping to confirm trueness.
After using the tool, clean off all the lapping compound with brake parts cleaner or other cleaning solvent, and dry with compressed air.
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