Valve Seal Problems
A while back there were a number of comments related to insuring a good valve job listed on one of the 4 cycle forms. We took some time to reply by writing a few suggestions. As a result a number of karters showed interest and contacted us. Due to this interest this month’s Tech Tip article will be related to how to insure the best possible results when prepping the valve train on the Raptor engine.
Suggestions listed may differ from steps you may be accustomed to. We realize there may be other procedures but the suggestions that follow have proved to give successful results. Various types of tools mentioned are Rix produced shop procedures and tools but we realize other sources available may also produce a quality job. The main consideration should be to use the best affordable tools without sacrificing quality in the process.
Getting the best possible valve seal can pay big dividends in the horsepower department. A small leak can cause an engine to drop off power after running well for a few laps. A leak down tester may not even indicate the problem. Sometimes a leaky valve won’t happen until after the engine reaches normal operating temperature.
A number of in shop dyno tests conducted have showed a slight exhaust valve leak would not affect peak horsepower very much, but a big difference was found on rate of acceleration on inertia dyno test. How quickly an engine accelerates is where it’s at on most tracks.
Most engine builders are accustomed to lapping valves to keep an engine fresh. Many go to great lengths in selecting a series of compound grades and continual lapping sequences until a valve finally seats. In the long run this procedure has not proven to give our engine building program consistently good results. If a valve dosen’t seat after about 30 seconds of lapping, with # 500 grit compound only, there is a problem with the valve face, seat or guide alignment.
Lapping with an electric drill with 180-compound to cure this problem is deceiving. First of all #180 is too gritty and doesn't give an accurate indication of a quality face to seat seal. It also makes a rough surface between the seat and the face causing irregularities or air pockets. Rather than sealing under actual operating conditions the result will be a small leak, causing a hot spot to develop, that results in a larger leak.
Excessive lapping with an electric drill also causes a groove to form on the valve face. This procedure may seal when the engine is cold but once heat builds up the groove will not match the mating surface on the seat and face, yes you guessed it, resulting in a leak. This is another condition that will usually not show up when conducting a cold leak down test.
To true up valve seats many karters use a hand operated carbide-cutting seat cutter. This system works well but it will not give the best results when compared to a motor driven seat grinder- reconditioning unit. Our shop does use the carbide-cutting tool system for removing large amounts of seat material. The final seat prep, however, is done by a motor- driven grinding system to insure the smoothest possible surface finish.
Valve face to stem run out should be checked with a test fixture to insure that it has less than .0001" (one tenth of a thousands) run out. Up to 50% of new valves will not meet this standard. Face to stem run out can only be trued this accurately by using a good quality motor driven valve-grinding machine. After grinding work should be rechecked to with the testing fixture to insure accuracy.
Any and all valve guide work must be completed before the seats are trued up or given a final smooth grind finish. Reversing this procedure would require going back to the seat each time any guide reconditioning takes place. The reason for this specific sequence is because it’s extremely difficult to get a guide replacement exactly true to an existing unaltered seat. In plane English, it’s tough to get a guide exactly straight.
The best results come by doing all seat truing, grinding and lapping with a torque plate and head gasket properly installed. Aluminum around the seats will distort when bolts are properly torqued. Being concerned about valve seal, after the head is installed properly, will prove to be the trick in getting the best seal resulting in the best possible performance. To take advantage of this process most tools, stones, cutters and torque plates currently offered for sale may have to be altered.
Keep the valve guides tight during the seat reconditioning, grinding, and lapping process. If the valve seats with a tight guide the better the results.
After the first 30 seconds of lapping with 500-grit compound remove the valve, clean and check to see the gray ring formed on the seat. If there is a shiny spot the guide is not concentric or there is a high or low spot. Go back and re-prep the seat, then try again.
Once a continual even gray ring forms, finish-ream the guides to desired clearance.
Check your work by installing the valve train assembly, including springs, and slowly rotate the engine while observing the valve when it closes. Check to see how much valve timing changes comparing springs installed and not installed. The less shift or wobble observed on the valve head when it opens and closes, and the less variation there is in valve timing, the better the job is.
There may be other ways to do valve and seat reconditioning but we have experienced good results using this process.
We are currently in the process of producing VHS videos related to a number of engine building procedures and the use of tools produced by our company. We invite you to check back on our website regarding upcoming availability.
We hope this article may be of some help,
Rick at Rix