Welcome to ModelEngineMaker !If you have problems registering or logging in, please use the contact menu option to request assistance.
For model engines some builders prefer the valve cage/seat configuration. The theory is that the whole insert can be cut at one time, guide, pocket and seat so that everything is concentric. There are two catches to this method, one being that a center drill is used to start the hole followed by an undersized drill then a reamer. First of all a reamer only follows the drilled hole so if the drill doesn't drill a perfectly straight, concentric hole then the reamer just follows what is already there. Now the seat is cut with lathe tooling. In most cases the seat is perfectly concentric with the axis of the lathe but the guide and seat might not be concentric to each other. The second problem is that when the valve cage/seat is pressed into the head it can distort and further amplify any misalignment with the seat and guide. I know builders who have had good luck using this method I'm just saying that I prefer the following.
Maybe this is a good point for a dumb question I've been harboring. What is it about bronze that makes a good valve seat in our application?
A conventional, multi fluted, countersink cutters will produce perfect holes when used with a milling machine with controlled down feed. The chatter marks are more likely to occur with manual downfeed in a drill press....
Quote from: petertha on May 05, 2018, 02:52:46 AMMaybe this is a good point for a dumb question I've been harboring. What is it about bronze that makes a good valve seat in our application? Hi Petertha, That's a very good question, Why do we use bronze?I guess for the majority of model engine makers use bronze because that is what our drawings tell us to use, rarely have I seen a specific grade of bronze specified.In our miniature engines, a combined valve cage incorporating the valve seat and valve guide as a single item, is a very practical approach. It is so much easier to achieve the all important concentricity of the valve seat to the valve guide bore, in a single bronze valve cage compared with juggling three separate parts (the head, the seat and the valve guide). The combined valve cage is also easier to install and retain in the cylinder head, no shrink fit valve seats to come loose and wreck the engine. Also the high copper content of the bronze efficiently conducts heat away from the valve components and into the cylinder head.What grade of bronze should we use? A much more difficult question. The valve guide requires excellent bearing properties, it needs to work at high temperature without additional lubrication. It needs to endure these poor conditions for it's life time. This suggests the use of a bronze with excellent bearing properties. Machinability is another criteria, the valve guide needs to be reamed and finished to fine tolerances. I, for one, have had some scary experiences with some grades of phosphor bronze pinching and seizing on a drill or reamer. so I select a bearing bronze with known machinability.The valves on my Mercedes Benz W165 engines are only 7.5mm in diameter. There are four valves per cylinder and eight cylinders per engine. So I needed a minimum of 64 valves for the two engines. I designed a combined valve cage in bronze for all the reasons identified above. I selected C932 (SAE 660) bearing bronze because of it's bearing and machinability properties. However, I feel that a "harder" grade would be better for the valve seating. Too late now, I have made them and have no intention of repeating the exercise.This is what the screw in Mercedes bronze valve cages look likeThis is what 6 foot (2 metres) of bronze bar becomesThis is the four valve cages filling the combustion chamber. There is just enough room in the center for the spark plug.You will notice that I have not yet cut the 45 degree valve seats. I plan to cut the narrowest possible seat with a piloted seat cutter. That will wait until I have made the 64 valves to fill those holes.The bronze is softer than the steel valve, therefore the lapping compound will want to embed itself into the softer material. It cannot be removed or washed of with a solvent. Therefore we need a lapping paste which breaks down quickly, we want a lapping compound which effectively self-destructs. I plan to use 'Autosol' metal polish, which contains a 'soft' abrasive, it's probably pumice or something similar. The abrasive quickly breaks down into smaller and smaller particles and stops working. It should be possible to polish the bronze valve seat for a good gas seal without long lasting side effects. This soft abrasive break down is one reason why metal polish takes so long to polish hard steel valve seats, bronze seats should be quicker.Hi Zee, How's the retirement going? You could now find the time to change your signature from "to work, to work" into "to play, to play". Just thinkin.Mike