valve train GEOMETRY

[t150vej]

Can anyone give me a short story of the importance of Harley valve train geometry?

Assuming the valves are not too terribly "tall" or "short" in regards to how they sit in the seat and especially if you're running adjustable pushtubes, I don't see it's a big deal..... So is it and if so, why?

[MachoTaco]

Here's an article I found in the American Rider's magazine. (Love this mag.)
Here's the link: http://www.americanrider.com/output.cfm?id=1021969

And here's the article:

Valve train noise can have several causes and your engine's noise might be a combination of more than one of them. The more common sources, in order of frequency, are (except for the first example, these causes apply equally to Twin Cam, Evo and Evo Sportster engines):

* Pinion/cam gear fit (Evo only). The distance between the centers of the crankshaft and camshaft increase as the engine warms. The sizes (pitch diameter) of the two gears, pinion and cam, must be carefully matched if they are to remain quiet when the engine is warm. If the gears are too large (tight), they destroy the oil film and themselves. If they are too small (loose) they make noise but do no harm other than the rattling noise. The noise is mainly in the cam case and appears or grows louder as the engine warms.

* Aggressive or poor cam lobe design. Opening ramps with high acceleration rates can hammer the valve train parts and the noise is similar to that of a hammer falling on an anvil; not good. Closing ramps that are too abrupt essentially drop the valves onto the base circle of the cam lobe with the force of a hammer on steel, also not good. Closing ramps are supposed to slow the valve as it closes and deliver it gently to the valve seat. I have measured cams that have had no closing ramps at all; these cams were very noisy and destructive to valve seats, valves and tappets. The noise appears at both the top and bottom of the valve train.

* Stiff valve springs. Higher-rate valve springs intensify any noise source in the valve train. When combined with aggressive cams, the noise can be irritating and the impact loads may be very destructive. The "hammer" is heavier and swung more forcefully. The noise is louder near the valves. They may also 'ring' a bit.

* Incorrectly adjusted pushrods. If the mechanic does not adjust the pushrods correctly (long enough), the hydraulic tappets top-out before the engine is fully expanded. Play develops in the valve train and it becomes noisy. The noise may seem to come from both the cam area and the heads.

* Contact between the pushrods and pushrod covers. ---- Some combinations of cam, pushrod and rocker arm designs may result in the pushrods contacting the pushrod cover tubes. This noise seems to come from the heads. Wear on the pushrod diameters near their tops identifies this noise source.

* Rocker arms with needle bearings. Stock and most aftermarket rocker arms ride on hydrodynamic (plain) bearings. This is the best way to do it and the quietest. Needle bearings do their best work when rotating and not when oscillated as they are in needle-supported rocker arms. They also lack the noise damping characteristics of the plain bearing; hence they tend to make a bit of noise.

* Loose valve guides or loose valve seats. This is rare but can be very frustrating to diagnose. A loose guide or seat usually becomes noisy only as the engine warms. The best way to narrow the noise source is to start the engine cold, set it for a fast idle and listen as it warms. The noise develops as the engine warms and is fairly easy to isolate. In either case the mechanic must remove the head to make a final diagnosis. One can move a loose guide when cold, using a valve stem for leverage. A loose valve seat reveals itself by the appearance of the joint between itself and the head material.

A couple of these noises are benign, the rest are not. A loose gear set does no harm and can rattle reliably for the life of the engine. Needle bearing rocker arms do no harm but are likely to wear out sooner than the stock design. Pushrod contact with the covers isn't likely to do serious harm.

Any noise source that applies impact loads on the valve train is not good. In fact it is very bad. Poor camshaft closing ramps beat up the valve seats, valves and tappet bearings. Incorrectly adjusted pushrods are very rough on the tappet bearings. High-rpm valve float occurs when the valve springs are too weak for the cam design and can quickly destroy tappet bearings, valve seats and valves.

In short, be very, very suspicious of valve train noise. While sometimes harmless, it is not your friend.

I hope this article is helpful to you.

...M.T.