Common Head and Cam Mistakes

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  • November 21, 2018
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Disastrous Head and Cam Installation Mistakes

Most cylinder head problems can be avoided by understanding the cause of failures. A head that is properly assembled and machined with valve train components engineered to handle the RPM of a particular engine will seldom fail on its own. Fastheads accurately machines, assembles, and tests every head. If the head fails, it is usually due to one of the scenarios below. We can’t express enough the fact that there is zero room for error during installation and in the continued maintenance of your motocross head.

You don’t have to be a skilled mechanic as long as you take the time to follow the instructions outlined in your owners manual. There are many online videos and picture instructions that can be very beneficial. The list below are common mistakes associated with the assembly of motocross type heads. Mistakes are never found in the owners or service manuals and are only mentioned here as a warning and food for thought. You will need a torque wrench both inch and pound, feeler gauge, metric socket, and wrench set. New valves will not break or bend unless they hit the piston, each other, or lose contact with the cam. Valve float occurs when the valve lifts off the top portion of the cam lobe due to improper spring rates for the weight of the valve and the opening speeds. In many cases the valve can slam shut on the valve seat without touching the back side of the lo be.

1. If you removed the piston, make sure the new one is installed in the right direction. Valve pockets are machined into the piston for intake and exhaust valves. Installing the piston backwards will bend the valves! What may look obvious may not always be correct. Some pistons have an arrow or dot that indicates the exhaust side. When installing modified cams, oversized valves, or pistons it is a good idea to check the piston to valve clearance. The best way to do this is put a thin piece of soft clay across the top of the valve pockets, torque the head to specs, set the timing and valve clearance and turn the crank one complete revolution. The closest possible
contact with the valve is a little before the valve opens or closes, not top dead center. Remove the head and check the thickness of the clay. Most engine builders suggest .060″ / 1.5mm building in a safety factor for heat expansion, crank bearing wear, and carbon buildup. Lube the head bolt threads or nuts, and both sides of the washers with engine oil before being torqued. Dry or dirty threads, and washer can add up to 10 lbs of resistance and improper torque reading. Always use new gaskets.

2. Cam journal caps must be tightened to the correct torque. Over tightening can cause the aluminum to bulge inward and bind the cams. Lube the cam journals before installing the cam. It is a wise idea to kick the motor over with the kill button applied ten or more times to fill the oil galleries. This is one of the reasons motors should be warmed up at low revs. Note: The early KX250F and Suzuki manuals stated 109 inch lbs and it was later changed to 86 inch pounds.

3. Cam timing to crank position must b e set as prescribed by the manufacturer. If timing is off a tooth or more you may bend a valve. If your cams previously witnessed rotational resistance while the motor was running or being kicked over, the sprocket may have slipped on the cam shaft as they are pressed on with no indexing. This can happen due to a broken lifter bucket, cam journal seizure, bent valves from slipped timing, etc. If you think this happened it would be wise to degree or replace the cams. The manufacturers usually don’t scribe index marks between the cam shaft and cam sprocket. It is a good idea to scribe your own marks on known good cams for future reference. If your cam sprocket did slip you will still be able to align the marks at TDC but the cam lobes will open the valves at the wrong time. If this happens you may end up with bent valves the first time the motor is turned over. Usually when the sprocket slips it is a noticeable amount so if you can find another like model to view the lobe location at TDC it may help. If slippage is questionable we usually suggest replacing the cam. Otherwise, unless you have the tools (degree wheel, dial indicator and special holders) and know how to physically check the degree at which the valve opens, it is usually cheaper to buy a new cam.

4. KX, RM, Y Z and most KTM ’s motocross heads have cam journals that are bored directly into the head. They do not have replaceable plain or ball bearings. If the cam seizes in the journal due to a lack of oil, oil pressure, or cam caps have been over tightened the head may have to be rep laced. Most of these heads run over $600 without the valve train. We can fix some slightly seized journals buts its never as good as original. Cam caps are only sold with the head and cannot be purchased separate as they are mounted in the head when line bored. It’s easy to break a cam cap if the bearing locating half washer is not centered when the cap is tightened. Using the wrong valve cover bolts or seal washers with the wrong spacing can also break the cap on models that bolt directly to the cam caps.

5. The CRF heads have ball bearing on both ends of the cam so there is less chance of ruining the head from a cam seizure. If these heads run low on oil, the lifter bucket may seize in its bore, as on any lifter bucket style head. If you bought a new head or had yours repaired due to this reason, you will need to check the rocker arm bearing, cam, and lifters. Parts that have gotten hot enough to bake the oil have become red hot and their molecular structure and heat treatment has been changed. A indication of this is if the part has turned black or discolored compared to stock. Using these parts is a sure way to failure. Lifter bucket bores can be oversized and
fitted with larger diameter buckets on some models. If the head is worn and needs rebuilt along with the lifter bore work it is usually cheaper to buy a new head. Lifters are also called tappets or buckets.

6. Adjustment shims may only touch the top of the valve stem and the shim pad in the ceiling of the lifter bucket or the pad on the underneath of the rocker arm. If the spring retainer gets depressed rather than the valve stem, the keepers can dislodged and the valve could drop. We see this a lot and often get blamed for the failure. The most common problem is when the lifter bucket is lifted above the width of the shim and it sticks to the lifter and then slides to one side. When the lifter is pushed back down the shim rests outside the retainer bore and depresses the retainer. Always take the shim out of the bucket and install it in the retainer before installing the
bucket. The same can happen with the rocker on the CRF’s. The shim can fall out or get cocked side ways on the retainer. Every time the cam shaft is reinstalled and after the cam caps are torqued, re-check the valve lash. If the gap is super loose or way tight you will know that the shim has become dislodged. The keepers, also called locks or cotters, are wedged into a groove near the top of the valve stem. If the retainer gets pushed down rather than the valve stem you should visually look to make sure they are in the groove and then set them by tapping lightly on the retainer with a socket that is larger than the shim bore. Aftermarket lifter buckets often have larger diameter shim pads in the ceiling of the bucket. If the surface of the shim sits below the top of the retainer the lifter may come in contact with the top of the retainer rather than the shim and will dislodge the keepers. Some after market retainers are taller above the valve stem and the same can happen. Even if the keeper doesn’t completely dislodge they will not wedge and eventually wear out the stem grove and the valve will drop. This can also happen when a seat has been cut deep, raising the stem in the head which requires a much smaller shim. If you think this may be a problem, check the valve lash without a shim in the retainer. You will need at least .010″/.25mm more clearance than the required maximum valve clearance. (i.e. If your valve clearance should be .006″ maximum you should have at least .016 ” when the shim is not installed.)

7. If your head was assembled and tested by Fastheads and we adjusted the valve shims with a stock or aftermarket cam you can skip this section. If you are installing a cam with higher lift or a stepper ramp than stock you will need to strictly follow the cam manufacturers spring and installed height instructions. If the seats have to be cut more than .010″, spring shims should be installed under the spring locators in order to correct the installed spring height. All of the above scenarios have to be adhered to.

8. If the cam chain is stretched, worn, or kinked, replace it! Cam chain tensioners have a greater tendency to fail with loose or kinked cam chains. When the tensioner can no longer do its job, the timing may slip causing the valve to contact the piston or a change in power. If you hear excessive noise in the top end, check the chain tension and cam timing and then valve lash.

9. Be careful not to let dowels, shims, bearing retainers, head bolt washers, or dirt fall into the cam chain area. These parts usually stick to the magneto and can cause the cam chain to skip or dam age the stator and trigger coils. If you can’t find a part while reinstalling the head remove the stator cover and hunt for it there.

10. If you are installing your own valves and assembling the head please keep the following in mind. Many heads have different size intake and exhaust valve stem diameters and use different size keepers, retainers, sea ls, and springs. Don’t get them mixed up. Apply a thin amount of anti-seize or engine assembly oil on the valve stem whenever new guides or valves are installed. Tap on the retainers a few times to wedge and seat the keepers. Read all the above for a better understanding of potential problems. Never use lapping compound on a valve that has had a coating applied to the face. This includes all Titanium valves and some stainless. The hard coating is only microns thick. Once the coating has worn off, you’re into the softer alloy. All modern valves have been surface hardened using various techniques. Grinding valve faces on modern valves is not feasible and the valve will wear out in a few hours. Some machine shops offer valve grinding but it should only be done on old low RPM cars before unleaded gas. We repair a lot of heads that have been recently rebuilt by shops that tried using old school seat grinding and valve guide work along with a poor understanding of the close tolerances required in high RPM race motors! These heads are not like cars or utility vehicles nor do they compare with motor cycle heads of just a few years back. Concentricity has to be in the tenths of thousandths to avoid premature wear, proper sealing and possible valve breakage. Concentricity is the angle difference between the guide axis and seat face and the roundness of the valve face diameter to the seat face diameter. For instance, every time a guide is installed the concentricity has to be corrected by re-cutting the seats. There is no getting around it. We see hundreds of heads come in ruined due to improper guide installation. Even porting methods are changing; removing to much of the casting around the guide and cutting the guide flush with the port can affect concentricity when hot. These heads have very thin castings to reduce weight and increase port size. If the porter has cut the guide support casting out for better flow the side of the guide with the greatest amount of casting will expand and tip the guide to the leaner side. Be aware of this if you send your head out for machining or porting. Most of the catastrophic and expensive problems we see in the 4-stroke race motors are from improper assembly by someone that did not follow the instructions as
described in the manual.