The subject of KR heads has come up in several different threads on this forum, and although the forum information on the topic is rich, it is spread thinly throughout many threads. Was hoping to consolidate more of it here under a new thread focused solely on KR heads, so hope people with KR head information will chime in and post.
For my own curiosity, I asked in a previous thread how many different KR heads were produced by the MoCo, and sifting through that thread and others come up with 52R, 59R, 60R and 69R. Is this correct and complete or are there many more? Also, our forum experts teach that not only were there these major part numbers, but also the single integers 1 through 7 stamped on the heads that indicate something about that particular combustion chamber? Here’s a teaching moment for someone in the know – did the integers 1 through 7 correspond directly with head part #’s or were these integers separate indicators that were totally independent of the part #’s?
Another thing I’d like to see are images of the unmolested combustion chambers of the various heads to see firsthand how the shape progressed over time and cam development. I’ll start it out by posting the ubiquitous chamber shown by Jerry Branch in the 1965 Cycle World article by Gordon Jennings on KR development, along with an image I have but don’t recall where it came from. I’ll also post an image from the starcain thread about “KR Parts and Questions”.
So a couple of images and that is enough to start me off on a whole new series of questions. Note that the combustion chamber in the starcain image starts beyond the middle head bolt from the IN side toward the EX side of the chamber, whereas in the Branch and unknown sourced image the combustion chamber, or as I'm fond of hearing Erik call it, "the snowcone", starts above the middle head bolt. And when you start the shelf beyond the middle head bolt you can't create a nice symmetrical snowcone. So to those in the know, was the poorly shaped snowcone an early factory shape or that of a privateer? Also what occurred along the path as the heads transitioned from integer 1 to integer 7.
The Jerry Branch image also raises another question, in that he is showing a stock K model head and saying it has to be converted into the shape of the finished head shown next to it. I thought the KR heads came ready to use as cast with the proper shelf angle and combustion chamber depth already established. Is that true or false? Did you actually have to start with a stock head, weld it in, then carve out the correct pattern?
Directly related to the discussion of combustion chamber shape is the image below found in the Competition Racer Manual for KRs, showing the shelf angle for a #7 head and cautioning that "Location of Shelf Angle Very Critical". I always wondered why this was in the manual and figured it must mean one of 2 things. Either it was an instruction of how to modify your existing head to this conformation, which never made sense to me since I thought the KR heads would already be made to this specification if it was VERY CRITICAL, or it was a cautionary statement that the shelf angle is very critical, your head already looks like this, and don't even think of trying modify it or you will loose power. So gentlemen, why was this image included in the manual - was it to assist in performing a Jerry Branch job on a stock head or was it a cautionary statement by the MoCo to inform that "we've got this" don't worry about it and don't mess with it? Does anybody know the vintage of the Competition Racer Manual this image came from or the approximate vintage of the #7 head?
Another question I have relates to the brass or bronze spark plug insert used in KR heads. I appreciate that the insert makes the thread robust and affords effortless plug inspections as often as one would like, but always wondered about the change from a 1/2" plug in standard K models to the 3/4" reach plug employed in KRs. I see in the manual that KRs were shipped with a Champion N60-R, which is a very cold plug to begin with, but with the additional notation that "These spark plugs are for warm-up and break in only". Thus I suspect they needed a cooler plug yet for racing and that such a cool heat range plug did not exist in 1/2" reach so they required a 3/4" reach to access such cold plugs?
That's enough for now, and hoping people have additional images, insight and information to share on this interesting topic. Thank you.