They are not in San mateo but close, It's too bad that Elgin move to Santa Rosa, They were in Redwood City FOREVER! Here are a few that I and my friends and family have used.
Jim's Machine Shop 929 Main St Redwood City, CA 94063 650-366-9394
Dave's Engine Machining 37530 Enterprise ct #4. Newark, CA 94560 510-797-9536
S & S Automotive 14127 Washington Avenue, San Leandro, CA 94578-3324 510-357-9041