PUMA RACE ENGINES - CAMSHAFT FITTING & RUNNING IN

It seems that one of the best kept secrets in the engine building world is how to fit and run a new camshaft in properly. This is despite the information being printed on the sheet that accompanies nearly every cam kit sold and in every supplier's brochures. Most of the failures that take place when a cam is fitted are not the fault of the design, manufacture or materials but of the person who fitted it and first started the engine.

The camshaft is the most heavily loaded component in the engine. It has to cope with far higher point loads than the crank bearings, pistons or conrods. An average valve spring needs a person's full weight to compress it and each cam lobe applies that much force thousands of times a minute. All that force is transmitted through an infinitesimally thin contact line between the cam lobe and the follower. The contact force is many thousands of pounds per square inch. To have even a hope of surviving the cam has to be made out of the right material, fitted properly, lubricated properly and run in properly at the start of its life.

Never fit a new cam on worn followers or vice versa. If there is wear on one of the two components then the assembly will never bed in properly. The only exception is with overhead cam engines with flat faced bucket lifters made of hardened steel. These, if unworn, will usually be ok to use with a new cam. With pushrod and rocker type engines or engines that have finger followers like the Ford SOHC, the follower MUST be replaced with the cam. The Ford CVH is notorious for cam wear and many an owner has tried (and soon regretted) a cheap fix to quieten it down by fitting just a new set of followers on the old cam. The new followers have no chance of bedding in on the worn cam lobes and go the same way as the old followers in very short order.

Always use cam lube on the lobes and follower surfaces whenever a cam assembly is fitted. Even if the same components are being reused during a rebuild it is still good practice to do this. Most quality cam kits will come with a sachet of lube in the box. If not then you can buy lube from engine reconditioners or cam companies.

If an old but still good cam and followers are to be refitted after a rebuild then ALWAYS keep the parts in order so that the same follower goes back against the same cam lobe.

If a cam is being replaced because of gross wear then stop and think for a moment about where all those worn off metal particles are. Down in the sump playing havoc with the crank bearings and oil pump. At the very least, flush out the engine with flushing oil and use new oil and a new filter before fitting the new components. Also make sure that all oilways in the cylinder head and block are clear or remove the head and get it chemically cleaned at an engine reconditioners. Replace the oil pump if in any doubt about its condition. If oil pressure is still low then consider the need for a crank grind and/or new crank bearings.

The most critical part of the rebuild process is starting the engine for the first time and this is where many well meaning mechanics go fatally wrong. You MUST NOT let a newly fitted cam run at idle speed for the first 20 minutes of operation. Always keep the rpms up at about 2000 during this period. At idle speed the loadings on the cam 'nose' (that's where the cam is at full lift) are at their highest and the new cam can suffer permanent damage if it is allowed to idle before it has bedded in and work hardened properly. Sitting on the driveway with your foot on the accelerator for 20 minutes can seem like an eternity. Also if the engine starts to overheat it may be necessary to switch off, let things cool down and split the process into several shorter intervals.

So why are these loadings highest at low speeds? This may seem contrary to common sense but I'll try to explain. The cam lobe throws the valve open and it is the valve spring that has to apply enough force to keep the valve and follower in contact with the cam lobe as it passes over peak lift and back down the closing side. As rpms increase, it takes more and more spring force to stop the valve flying off down the cylinder bore which is where its initial velocity is trying to send it. If the spring rate is not high enough we get "valve float" which is where the valve loses contact with the cam nose and catches it up later in the cycle or even after the lobe has reached the closed position.

So imagine we stop the engine with a cam lobe in the full lift position. The valve spring is fully compressed and applying its maximum force to the cam lobe. As we run the engine at higher and higher speed, more of this spring force is used up in controlling the motion of the valve train. So the contact loads over the cam nose actually fall with increasing rpm. The loads at the start of the cam lobe where it lifts the valve off the seat work in the opposite fashion. Here the lobe has to do work both against the mass of the valve train and also against the force of the spring as it opens the valve. So loads increase as rpm increases. In other words:

Contact loads on the peak of the cam lobe are high at low rpm and low at high rpm
Contact loads on the base of the cam lobe are low at low rpm and high at high rpm

When we run a new cam in, it is matter of finding an engine speed that is a compromise between high loadings on the cam nose and high loadings at the start of the lobe. A speed somewhere in the middle of the rpm range is what we need to achieve this. Everyone has a different opinion on the best speed and time to do this running in process. Some manufacturers specify 2,500 rpm, some 2,000 rpm etc etc. The key thing is not to let the engine idle. Letting the engine idle can wear the peak of the cam lobes away before they have had chance to bed in. The bases of the lobes will not be affected though so the wear pattern is very distinctive.

On race engines with strong valve springs and high lift cams, this running in process can even be a two stage affair. The engine is initially built with weak springs (or just the inner springs if dual springs are fitted) and run in for 15 minutes or so to start off the bedding in process without suffering excessive contact loadings. Then the proper springs are fitted and more running in done to try to get the cam surfaces to their final hardness and bedded in condition.

Back to Main Menu page