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Sign up for our E-newsletterHONING THE CYLINDERS
Following a recommendation from our buddies at Sunnen, we visited Gressman at Gressman Powersports in Fremont, Ohio (Gressman has built a number of 632 motors, in addition to countless big-number drag, sprint car and road-race engines). Naturally, the Ohio winter weather forced us to drive in sub-zero temperatures. Even with the block boxed and wrapped in the bed of my dually truck, it looked like a gray Popsicle when we arrived at Gressman’s (it may have been cryo treated as a result of the two-hour drive … just kidding … sort of), so we allowed the block to warm up for a while before we began messing with it.
With the block at room temperature, we measured the Dart block’s bores, noting a raw diameter of 4.590″ (Dart leaves enough material to allow the builder to obtain desired diameter). Since our Diamond pistons measured 4.59375″, we needed to remove a total of 0.01075″ in order to achieve the desired 0.007″ clearance.
After confirming that our main caps were snugged to 100 ft-lbs, Gressman mounted a pair of BHJ deck plates (tightening these to 70 ft-lbs).
Before honing, Gressman used a hand grinder to knock a bit of material from the webbing surfaces below the center three cylinder areas where the bores Siamese (small humps are featured in the casting) to prevent damage to his honing stones. This took only about 10 seconds per spot, so it was no big deal.
He then began honing with 500-grit diamond stones with a high load setting to hog out the bores to an initial 4.599″, followed by a final honing pass to remove the remaining 0.00175″. All cylinders were then treated to four passes with silicon carbide brushes at 30 percent load for a plateau finish. Because of the amount of material removed from the start, the entire honing process took about an hour and a half. Gressman noted that the high-quality Dart blocks feature a high nickel content (he mentioned that these blocks are about 30 percent harder than “common” iron blocks), which requires increased honing time as compared to most OE blocks.
By the way, Gressman recently purchased an RMC/Sunnen V40 multi-axis CNC machine, which the shop uses for a variety of operations, including block clearancing, cylinder boring, lifter bore correction, and block lightening (for race applications). I wish that we had the luxury of time to take advantage of a block lightening job (if for no other reason than to make the block look even cooler), but we were on a tight deadline, so I left without having a chance to see the CNC in action.
By the way, Gressman’s shop was a thing of beauty. We definitely plan to revisit in the future. This is a very nice operation and clean as a whistle.
Our pistons began life as high-density slugs.
Domes were cut flat and pin bores were roughed in at the beginning of the transformation.
A view of the unfinished pin boss area.
The outer body was machined to desired diameter and the rind grooves were cut. Notice the big chamfer cut on the top. As you can see, quite a bit of material will be removed during the machining process.
The skirts were shortened and the domes were cut to height, along with the valve pockets. Notice the huge hunk of aluminum that was removed at the pin bore sides, to remove unneeded weight.
Gas porting holes were drilled along the perimeter of the top.
The underside was finish-machined for additional weight savings.
Skirt diameter was verified for each piston before shipment.
That’s a bunch of pistons. Diamond has earned a solid reputation for quality.
Closeup view of a piston underside.
A view of a finished piston as we received it. While weight was shaved everywhere possible, no compromises in strength were made.
This view shows the deep weight-saving relief at the pin bore side.
The pin bores were polished with a mirror-like finish and pins were carefully fitted at Diamond’s facility. Note the deep location for the spiralock grooves.
Gas ports will aid in top ring stability and sealing.
Tags: CNC, CUSTOM PISTON DATA, CYLINDER HONING, DIAMOND PISTONS, GRESSMAN POWERSPORTS, SUNNEN
