Blewdy Yaink
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- Joined
- Nov 10, 2018
- Messages
- 210
OK, a little background. Yes, BITD the supplier of the factory tools (these valve guide tools and many more) was MCA - Aston in Birmingham, England. I don't know if their involvement with Norton went back to the Bracebridge street days but in the 70s, they were very experienced and involved with these tools. They are still in business supplying many of the old tools but they're strictly "wholesale only". I don't remember the name of the rep (maybe even the company founder/chairman) who came over to Wolverhampton with a big box of tools for Norton Villiers to assess for supply through the dealer network but he was sharp as a tack and could immediatley answer questions like the diameters of tool guides, alignment features, clearances, etc. Many of the tools went into the NV spares system at the time (also, MCA had been supplying the four-pronged clutch diaphragm spring puller and other basic Commando tools -- some were even Dommie-twin carryover -- since the Commando's intro in '68). That particular set of sample tools was left with John Nelson, Manager of International Service for Norton at the factory, and he saw to it that most of those tools found their way into my toolbox once the supply of "in stock" tools was established -- yeah, he was a good guy like that.
The real issue with valve guides on a Norton is that the design is for a tight shrink fit. If the stability of the guide at cool temperatures is not assured, then the guide will be sure to move when the head is hot. Another factor is that the alloy used in casting Commando heads (actually all the way back to when aluminium alloy replaced iron heads in the Dommie twins) is a Rolls Royce aviation alloy that was developed for aircraft engine pistons shortly after WWI (yes, "I"). It was one of a pair of high-magnesium alloys (they were coded RR53A and RR53B - Commando heads were cast in RR53B) that were specifically developed for good casting characteristics , good heat stability, and ease of precise machining. It is not a low-expansion alloy and this means that the spec for the shrink fit must be for a very tight interference at cool temps. The high magnesium content provided a lot of the good charactistics but in a 50-year-old casting, you are likely to have some oxidation at the molecular level, leaving the metal weaker than originally produced as cast when new.
Oddly, for a piston alloy, if the metal in the heads is mistreated -- such as if the head is not heated enough to expand the bores for guides (same thing applies to the rocker spindles) -- then pushing or pulling the guides (or spindles) out or in will cause the bore material to "pick up" and stick to the guide or spindle in a way that seized pistons leave "picked up" metal sticking to a cylinder bore. That acts as an abrasive down the rest of the bore, scratching and gouging it. Many heads that must have oversized guides at the next rebuild were damaged in this way.
If you're going to push or pull guides into a Commando head, there must be enough heat to provide the clearance to allow the guides to go in and the alignment of the guide must be PERFECT. Especially for insertion, the edge of the guide cannot the canted at any angle; if this happens, you will immediately have gouged bores and almost inevitably the fit and stability of that guide in the head will be poor. WIth this in mind, I shiver to think of anyone who is trying to rush through a head job with a drift and a big hammer. Also, a hammer gives you little "feel", if you smack it and it moves, maybe OK, if it doesn't, maybe hit harder? With a threaded-type puller/inserter, you get the feel -- if you can feel the puller tightening up but the guide doesn't move, you'll get a feel for when the time comes to say "hmm, something's not right, let's check the heat with an IR gun or remeasure this fit when it's gool".
No matter what tool you use to remove or insert guides, Lord-preserve-us-and-protect-us if the crystallized carbon isn't carefully removed from the guide.
Care, the right process, meticulous attention to detail, and good tools make the difference between a precise and solidly performed guide change job and a hack job that leaves a damaged head. Please be careful with these heads.
