Sunnen Honing Secrets

SUNNEN HONING and engine rebuilding equipment has been around for more than 70 years in Australia and is the equipment of choice for the experienced engine builder. Thanks to Watson’s Specialised Tooling in Brisbane, the Australian Importers, Sunnen honing equipment has become well known in the industry for the exceptional finish it produces as well as it’s extremely high degree of accuracy.

Redgrave Motorcycles runs an engine building facility that is fully equipped with all the Sunnen honing equipment required to complete every aspect of Harley-Davidson engine building and performance modifications. Cylinder bores can be readily finished to + or – .0002”(1/5 of a thousandth of an inch). This is the accuracy specified by Harley-Davidson when resizing Evolution or Twin Cam cylinders.

Not only are the bores produced perfectly round, they are equally as straight. Crosshatch angle, length of stroke and stone tension are all adjustable to give the desired finish and accuracy. This all pays dividends with increased engine life and performance due to correct surface oil distribution created by the final honing operation.

Spiggot mounted cylinders fitted to most pre-1984 Harleys bolt into the machine for resizing much the same way as they do onto their engine. With Evolution and Twin Cam cylinders they need to be torqued to simulate the state they are in when sandwiched between the cylinder head and the crankcase and are assembled into a fixture which does just that. This produces the perfect bore.

Conrods are another component that benefits from the Sunnen principal and can be readily reconditioned to take advantage of today’s huge range of big end rollers and crankpins. A different machine is required here and a bench mounted Sunnen conrod machine which accepts a huge range of mandrels takes care of everything that the upright cylinder hone cannot do. This machine can also be used to refinish valve guides, timing gears, rocker arms, tappet guides, and in some cases, main bearings.

During their service life conrods can become slightly egg shaped in the big end. This is common in all engines. The Sunnen rod machine trues the big end up with an absolute minimum amount of material removal. A good operator can select a new crankpin and hone the rods to suit his choice of big end roller combinations giving the genuine H-D rods a new lease of life. This is a direct benefit to the customer who will retain his high quality original conrods and not need to rebalance his flywheels.

New bronze little end bushes require a different mandrel, as do most components. The mandrels used for these honing operations are very limited in their range and are pretty much dedicated to the components being machined.

One of the more difficult honing jobs but one that is certainly worth the trouble is valve guides. Very rarely are valve guides honed after their installation to remove the lip at the top caused by the valve guide drift and the wineglass effect in the centre of the guide which is the result of the interference fit in the cylinder head. At best guides are generally reamed. When guides are honed after installation and valves used with modern stem coatings, clearances can be kept to a minimum without the risk of the valves grabbing. Valve stem life is also greatly improved and seating is more precise.

A special mandrel and stones are necessary to hone the Harley tappet guides or lifter blocks as they are known. It is a clever device known as a keyway mandrel and features a double stone and shoe arrangement which rides over the cutout for the cam follower roller and prevents the stone catching in the slot. Damaged lifter blocks can be fitted with oversized cam followers restoring precise valve timing and perfect oil control where hydraulic tappets are being used. This also greatly contributes to a quiet engine.

Special mandrels are also available to hone new bushes in rocker arms. Sportsters, Shovelheads, Evolutions and Twin Cams all use the same Part #17428-57K bronze bushing and the special long series mandrel tandem hones both bushes together for perfect alignment.

Knucklehead rocker arms, although not fitted with bushes, can also be honed for oversize shafts.

By maximizing the quality and accuracy of the surface finish on internal components engine life can be dramatically improved. Some interesting facts relating to modern engines as quoted from the Sunnen manual.

“The piston and rings live in a very hostile environment and are subjected to great stress and heat. The rapid expansion of gasses during combustion suddenly applies forces of several tons to components on each and every power stroke. As the piston changes direction after reaching top dead centre (TDC) the combustion pressures force the rings against the bottom of the ring land at acceleration rates which can reach 2000 G’s. This sudden applied pressure occurs 30—40 times a second during highway cruising speeds, with combustion temperatures simultaneously reaching thousands of degrees. As many as 200 million of these punishing cycles will have occurred in an engine that has survived 100,000 miles and the rings themselves have traveled 50,000 miles moving up and down the cylinder. Think about that for a moment.”

If you’d like to know more about honing and general engine building, give Richard a call at Redgrave Motorcycles: 02-9484-9900. Redgrave Motorcycles.