The 913 head is a lower compression version of the later model 848 head which came out in about 1956 or so. It can be milled .090" to get the same compression ratio as an 848, but a few other modifications need to be done, also. The intake valves need to be recessed the same distance to protect the edge of the valves from the combustion flame front, and the rocker arm shaft stands need to have 3/32" shims placed under them. Then a lash cap is added on top of the exhaust valve stems. Those mods allow the original pushrods to be reused without running out of adjustment height.
Jerry
