Tuesday, December 11, 2007

Oversize Valves in a Knuckle

(some of this post is taken directly from an answer that I posted on the FlatHeadPower bulletin board in response to a question about what was the customary way to modify stock Knuckle heads for a 2" intake valve. I thought I would add a little more info and make it a post here)

Vintage hot rod Harley's ....you gotta love 'em! And one of the most popular motors to "hot rod" in times past was the venerable Knucklehead. If you have been around Harleys long enough, you have likely heard the stories, some might even say legends, of somebody that had a Knuckle that would "whup" anything around. Seems that every locality has at least a few of these stories that the graybeards can relate with a little prodding. Like all legends, the stories likely had their roots is some degree of fact.

So, what made these hot rod Knuckles so fast? Like every good motor, it was a combination of the right parts and the right modifications. Some of the most popular mods included installation of Flathead flywheels for an increase in stroke. Along with the flywheel change, often the flywheels were lightened to increase the rate of acceleration. A surprising number of Knuck heads were modified to run dual carbs. Cams could be re-ground for more lift and duration. And then there was porting and polishing along with an oversize intake valve. This last modification, the oversize intake valve is what I would like to focus on in this post.

A vintage modified dual Linkert Knuck

Back in the old days, the usual way to install a big intake valve in a knuck was to remove the old seat insert and grind the new seat right into the cast iron of the head (like an iron head XL). When you do it this way, a 2 inch valve is barely big enough, and many of them used a 2 1/32" or larger valve. For good flow you want the "choke" under the 45 degree seat to be a maximum of about 90% of the valve head diameter. If the stock seat insert is 1.875, then you can see that even the 2 1/32" valve would leave too large a diameter hole (90% of 2.031 is 1.828). On the other hand, low lift flow will likely be a bit better with the "too large" diameter under the valve, and lets face it, street knucks are for the most part limited to low lift. A bigger problem with using a 2" valve directly onto the cast iron might be any misalignment between the guide and the machining for the seat insert. If they are not close to concentric, there may not be much of the valve "catching" the head on one side.

valve seat cut onto cast iron of head after seat insert removal

A 2 1/32" (2.031) intake valve may have been the most common size used in the "old days" but there are some other considerations also. The valve pocket in the piston may not be large enough to give clearance around the edge of the valve. Even though you will likely be using a Shovelhead piston, remember that those will have been manufactured with a 1.940" (1 15/16) valve head diameter in mind. That means that if your intake valve and piston are coming close enough to each other (remember the intake valve will begin to open before the piston even reaches TDC) then even if there is plenty of depth to the piston valve relief, it may not be of sufficient diameter. The edges of the valve reliefs in the pistons can be opened up easily enough with a die grinder, but machining them on a mill will give a much more professional look.

A 2 1/16" (2.0625) intake valve will come closer to the ideal size for cutting the seat directly into the head (at least from a flow standpoint), but if you are still running a 3 7/6" bore, then opening the diameter of the valve relief in the piston will put you perilously close to cutting into the top ring groove. With anything larger than a 1.94"intake valve it would be a really good idea to mock up the motor with clay in the valve reliefs on the pistons. That way after you have gently rotated the engine a couple revolutions, you can see (and measure) the actual valve to piston clearance via the flattened clay. Don't forget that it is best to use light springs on the valves to avoid deflection of the valve train during this check.
Many of the large valve conversions done in times past used what I assume to be a tractor valve. That involved cutting down the valve head diameter as well as cutting a new keeper groove to shorten the valve. If you have to shorten the stem, then you should use a lash cap so that the rocker pad has a hardened surface to ride on. The added thickness of the lash cap should be considered when determining valve length. Whatever valve is selected should have a fairly minimal tulip or you will have to shorten the bottom of the guide for clearance (something a knuck can little afford; the knuck guides are already too short for good longevity!)

An example of a valve that will work (though far from ideal) is a Perfect Circle (brand name) 211-2455. It is a 2 3/32" with a 3/8" stem and not much of a tulip. The stem length is plenty long for mounting in a lathe to cut a new keeper groove. This valve also has a 30 degree seat angle rather than a 45. If you plan to use a 30 degree seat, then you are already there, but with the extra diameter there is enough material to allow you to grind it to a 45. One of the drawbacks to this valve is that the stem diameter is actually .3725". A stock Knuckle intake valve will have a stem diameter of approximately .375" so unless you are using guides that leave a little extra meat in the I.D. for fitting, you will likely wind up with more clearance than you might want.

When machining an oversize valve for use in a Knuckle, your first steps would be to cut the head diameter to size, grind the 45 degree seat on the valve, and then reduce the margin (which will have become wider due to cutting down the diameter) back by cutting the face of the valve. Once you have all of that taken care of, you are ready to cut the new keeper groove. You will want to mount the valve in the lathe by the stem end and cut the groove as close to the chuck as possible to keep everything concentric. The groove only needs to be "squared off" on the top edge, since that is where the spring pressure will apply force to the keeper. The very last step is to actually shorten the valve. This can be most easily accomplished by cutting the stem an appropriate distance above the new keeper groove with a cut-off wheel, leaving a small amount of extra material to face off on your valve grinder's valve stem tip refacing wheel. Don't forget to take into account the height of the lash cap, and where it will ride in relation to the valve keepers, when determining where to cut the valve.

note the keeper groove is only squared on tip end

Rowe replacement Knuckle intake has overall length of 3.575" with about .090 of that being the margin. I have cut 2.060 intake valves for use in Knucks using 3.585 as the overall length with .050 of that being the margin. This length does not take the needed lash cap (Crane # 99422-16) into account; it will add .060" to the effective length. If you are working from scratch, the 3.585 length (or 3.645 if you are having custom valves made: 3.585 + lash cap) may be a good place to start. If you start with the same length as a stock valve you will have to sink the valve quite a bit to get the stem protrusion in the ballpark for proper rocker arm geometry, and that will lower the compression ratio and tend to "shroud" the valve hurting flow.

Another option would be to install an oversize seat insert and use a 2" iron XL valve. This valve is readily available as a 1/16" oversize for a '70 to '84 XL. It is approximately the right length, so you don't have to mess with shortening or lash caps. The drawback is that you need to come up with a 5/16" I.D. valve guide and suitable springs, top collars, or keepers. The FHP knuckle heads used an "off the shelf" 1 15/16" XL intake valve, but I never did see where they offered the guides for 5/16" stem in any oversizes. Their spring kits should work for such an application, though the lower collar may be a problem.

But is this type modification worthwhile at this late date? That depends. By far the majority of Knucklehead owners today are just thrilled to have one to ride. Many would probably be horrified that anyone would consider modifying an otherwise stock Knuckle head, despite the fact that this particular mod can't be detected externally. Of course all of these modifications can easily be applied to any of the reproduction Knuckle heads, such as those from V-Twin and those from Flathead Power (now part of S&S). And, there are always those people who just can't leave things alone. I am one of those. There is hardly an engine I own that has not been modified for more power, and just being a priceless antique is not enough of a deterrent to keep me in check. Are you one of those people too?
If your goal is to build a Knuck that will dominate everything around, just as those of legend did ....well, that probably isn't going to happen. On the other hand, there are a whole world of bikers out there who will be totally blown away by just how strong your Knuckle can run! In my book, that is just one of life's little pleasures.

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