“Harley-Davidson” name for reference purposes only. Not affiliated with Harley-Davidson Motor Co.
Selecting a Flathead Cam
by Jeffrey Diamond
Your Harley-Davidson flathead 45, flathead 80, or knucklehead engine will not develop its full power without improving the cam. Increasing the stroke length, intake valve size, porting & relieving, etc. all help increase power, but they do not extend that power into the upper RPM range that really gets results. Obviously, the choice of which cam to use depends on how your engine was already modified, and what you’re going to use it for. A hotter cam does not always mean more power, or even more peak power.
The original 45 & U-Series cams have approximately the same mild duration on both the intake and exhaust cams. However, for performance use, Harley-Davidson side-valve (flathead) motors need more intake cam duration than exhaust cam duration (the factory race motors used about 30-35° more). This is partially due to the fact that the intake ports are fairly small, badly shaped, and include many inefficient curves, while the exhaust valves are relatively large with much better ports.
When planning a street motor, or a motor for a heavy bike, a mild intake cam can be used with the stock exhaust cams. As the intake cams get hotter (more duration), the same bias should be retained - use an exhaust cam with less duration (milder than the intake cam). Never install hot exhaust cams with stock intake cams.
A high performance motor must use hotter intake cams to increase volumetric effiency (torque), and move the power higher up on the RPM scale. These should be used with somewhat milder exhaust cams as mentioned, supra, but this will not work with a restrictive exhaust system. A high backpressure muffler will completely confuse the carburetor and make accurate jetting nearly impossible. These cams only work well with completely open exhaust, or low restriction muffler such as Screaming Eagle, Cycle Shack, SuperTrapp, etc.
Straight pipes with internal baffles are an extremely bad idea for any motor.
No motor “needs” backpressure, there is no “minimum backpressure” required to make a motor work. If the motor runs better with more backpressure, the carburetor adjustment was incorrect. What is required is a minimum exhaust gas speed for the motor to operate cleanly, which is why larger diameter exhaust pipes are absolutely not needed. The tuned exhaust pulse produced by open pipes will interfere with carburetor operation at certain RPM. This can be reduced inserting a reflective object to “break up” the pulse. This can be done by drilling a ¼” hole 1” from the open end and inserting a 1” long bolt into the pipe secured with washers & nut, and reduce the effect without loss of power. A better shape is the old school “Snuff-or-Not” product. This is no longer available, but one can be made from small hardware parts. Note the spring and adjustment knob for selecting position: with or against the gas flow. Click the image for a larger view.
45 strokers using 4-7/16” or larger flywheels should use bigger cams than stock stroke engines.
No sidevalve cams require piston-to-valve clearance. Some will require valve clearance in the head (as noted). All hotter cams work better with heavier valve springs. All hotter cams require minor carburetor adjustment for best results. Linkerts and other carburetors with limited adjustment range may be difficult to tune with hot cams, especially with open exhaust. Mikunis, Amals and other carburetors with multiple circuits are easier to re-jet but will still require substitution of minor components especially to the idle/pilot system.
The re-grinding process re-shapes the original cam lobes. No welding has been needed in the past, because the roller tappet does not require a specially-hardened surface (as flat tappets do), and the new lobe shape can be made from the original with only minor metal removal (mostly from the base circle). This is why these profiles are limited to only slightly more lift than the original lobe.
Regrinding can also repair minor lobe surface damage (same prices), but not chipping, broken or missing gear teeth, worn or damaged journals, etc.
The process locates the new lobe center exactly at the existing point of maximum lift. The extra duration is divided equally between both ends of the cam event.
For example, an intake cam with 30° extra duration will open the valve 15° sooner, which extends the overlap duration, and will require idle circuit enrichment and slightly reduce low speed power. If a hotter exhaust cam is also used, overlap is extended even more. Higher idle speed may be needed.
An intake cam with 30° extra duration will close the intake valve 15° later, which reduces cranking compression and low speed power. This can and should be partially offset by higher compression ratio.
Alternate lobe centers are not possible. For those who wish to get the best results from their cam installation, minor adjustments to the installed position are possible by modifying the motor itself. Click here for more details: .
Cam specifications listed below (such
as “278º/.320”, etc.) give the 1
number as the duration in degrees @ .010” lift, 2
number as the valve lift in inches. These are the only grinds available.
The approximate duration at .010” of the standard 45 cam is 240°, the UL is 260°. The intake lobe center is at approximately 102-103°. The standard 45 lift and lobe height is .312”, UL is .375”, WR or K is .344”, KK/KHK is .375”, KR is .395” or larger. We have no other information about the stock cams.
For background and comparison, some factory KR cam data is shown here: . Some Sportster cam data is shown here: . Some possible Andrews cam alternatives for the 45 are shown here: .
Attention: KNS Cycle is the source of this product, not Victory. Questions sent to Victory will not be answered.
Questions on shipping, turn-around time, or how to purchase a cam core, e-mail KNS Cycle: .
You must make your own selection. Generally if you can’t decide between 2 cams, the smaller one is the safer choice. Please read the following detailed and comprehensive information. Do not e-mail and ask the questions answered below. There is no answer to “Which cam is best for my motor?”.
For detailed information about jetting and adjusting the Linkert Model “M” brass carburetor, we suggest “The Linkert Book”, click here for details: , or for the Mikuni “VM” round-slide carburetor “Mikuni Tuning for Harley-Davidsons”, click here for details: .
Intake Cam Set, 45; 294º/.377”, hot, best for motors with improved breathing (relieved, ported manifold, big valves) & high compression. Requires minor (.060” maximum) valve spring & valve pocket clearancing especially
with big intake valves, should be used with heavy valve springs and 25515-10R exhaust cams. Still streetable!
This cam is slightly milder than 25506-10R (above), except for more maximum lift. The higher lift requires a bit more room in the chamber, lowering compression slightly.
Excellent for strokers, especially where a broad power range is more important than peak power. Heavier valve springs strongly recommended. Exhaust cam recommendation: 25515-10R .
Cams can be slightly advanced for best cylinder pressure, which will give more overlap; for experienced engine builders only. Click here for more information on re-timing cams: .
Warning: Linkerts and other carburetors with limited adjustment range will be somewhat difficult to tune with this cam, especially with open exhaust.
To order cams: print this page, fill in your information in the box below, circle the cams you want above, thoroughly clean and dry the cams, then box your own cams, add the correct postage to the price listed, include full payment and ship to KNS Cycle at the address below.
My motor is a: 45 UL 1936-47 OHV
Cams enclosed for re-grinding: #1 #2 #3 #4
Amount of payment enclosed: $
You cannot purchase these cams outright from us, they must be ground on your own good cams, which must be shipped to us for processing. The grinder rotates the cam on the journal centers. The accuracy of the new cam profile depends in part on the condition of your own cams and journals (shafts). If you are sending us cams from more than one engine, please permanently mark them (“A” and “B” with a scribe on the end of the journal, for example) so that the same set goes back in the same engine.
Delivery back to you takes about 6 weeks. If lost in transit, your claim is against the shipper, not us, so insure for the replacement value (we suggest at least $200. per set for 45, $400. for U-Series, Chief & Scout), and keep your receipt. If necessary, we can verify the value.
It is your own responsibility to see that the cams you send us are in good condition, and are carefully packed to avoid damage in transit. The individual cams must be separately wrapped and isolated from each other before the box is closed.
Warning: if the cams you send us are un-grindable (in such bad shape that they cannot be re-profiled) they will be returned at your expense. The potential damage includes:
»» journals badly worn undersize
»» gear teeth chipped or missing
»» excessive rust, pitting or corrosion on the cam lobes
Payment: use a money order or bank draft. Checks will be held for clearance which may take 2 weeks or more. Cash is acceptable, and is safe. U.S. customers: add $22.00 postage & insurance for a set of 4 cams (2 cams: $20.00).
Canadian & Foreign customers: for a postage quote, look up a 3 lb. package from New York zip code 11726 to your home address here: , add $20.00 to be sure, and add this total to the price. Send the cams to the address with a note of the payment amount and date. If the shipping is overpaid, it will be refunded it to your PayPal account. E-mail for a postage quote on different weights or insurance value only: . Ship your cams & payment to:
174 Lake Drive
Copiague, NY 11726-3520
Your 45 cams will be shipped back to you insured for $350. ($175. set of 2, etc.). U-Series cams will be shipped back to you insured for $550. ($275. set of 2, etc.). If this is not enough coverage, you MUST let us know when you ship them to us. There will be an additional charge for the extra insurance. If the cams are lost, you can NOT get replacement cams from us, only the insurance coverage.
KNS Cycle can now accept your U.S. or foreign credit card for payment with PayPal. To order, total your items, postage (and sales tax in New York), click the logo on the right, and make payment with a brief note listing the details.
Due to the high fee that PayPal charges the merchant, please add this small service charge: 3% (U.S. orders) or 4% (foreign orders) to all PayPal orders. Please add this amount to your total including postage. If not sure, e-mail for instructions. This is still faster, less expensive & more convenient than your bank.
H-D flathead 45 and UL cams are numbered in order of placement in the motor, beginning with #1 at the left (back) side
of the cam chest. H-D intake cams are #2 & 3, exhausts are #1 &
4; be sure you send the right ones. Click here for more information on Harley-Davidson cams: . Here they are, as seen from the right side of the motor (with the cam cover removed), with the #1 rear exhaust cam on your left:
1 gear + pump drive tangs
2 gears + ignition drive spiral
Indian cylinders (front & rear) each have their own cam: you must send both.
can supply you with American-made high performance stroker flywheels, click below for your motor: