<VV> Re: sleeved valve guides, and bronze

[djtcz@comcast.net]

>> From: "N. Joseph Potts" 
> I pulled the head off. 
> The VALVE GUIDE on that intake valve, which had been resleeved > >with  bronze, had broken, evidently near the end that's in the >combustion chamber.  I am missing that inch, more or less, from that >end of the guide (which  pulled out of the head by hand). This head had >been professionally  refurbished in May 2003, and installed in February >2004. Less than 5,000  miles. Teflon valve-stem seals were installed >then. 
Are the inserts thin wall sleeves, or the spiral inserts (like a bronze helicoil) ?  How much wall thickness is left after machining for bronze?  What color is the OD and ends of the loose guide?  Discolored from heat?  The OD is probably nasty from moving around in the now-buggered-up head hole.

First I would question the installation of the bronze inserts, in regards to the remaining guide wall thickness.  The inserts I am aware of are swaged or burnished after installation by forcing an oversize tool down the ID to tighten them in the guide bore, but before the bores are reamed/honed to size .  If the remaining guide wall is "thin", it would be EASY for the swaging operator to crack the portion of the iron guide not supported by head material and maybe not even know it. I'd look carefully at the surviving guides with a bright light and a magnifying glass. If they look OK I  might even go to a welding supply shop and get  Spot-check dye penetrant kit.  My paranoia would force me to pull the other head to inspect its guides.
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Whether the guide integrity is good or bad I'd investigate the valve-to-guide clearance. Note the 1965 manual specifies 0.0010 new minimum on the intakes, and 0.0014 new minimum on the exhaust, with no difference for the reported bronze guides in the Turbo engine. 0.002 inch is solidly in the middle of the new spec.  I'd like to think one benefit of synthetic oils is they will maintain valve stem clearance much better than dino oils.
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A Bronze as a bearing rant follows, for anyone interested.
My only close-to-first-first hand experience with the iron-guides-with- bronze-spirals was in an older aircooled Harley (knucklehead).  I worked in a shop that did engine machine work. As part of a thorough "valve job" for a pepped-up engine An experienced and very particular tech installed some new valve guides  that came with the spiral bronze inserts. Those guides were made by a well known manufacturer (Manley).  Within a few hundred miles the engine ran terrible.  I don't recall The autopsy results very well.  Basically some of the valves had seized in the guides, and I believe portions of the bronze inserts were missing.  Manley said they were baffled, but sent some new parts for the repair.  The tech chose to install iron replacements.
Jump forward 15 years. I am a mechanical engineer working at a company that makes precision spindles for machine tools.  The shaft speeds areWAY too high for any lip seal, so the standard is a non-contact labyrinth arrangement.  The traditional seal material is usually a bronze of some sort (highly leaded alloy for marginal lubrication survival), chosen because it was believed bronze could rub on a steel shaft and not damage the shaft.  The seal is made as a ring pressed/shrunk into a steel endcover, and machined to have about 0.004 inch radial clearance with the spinning steel shaft (which has zero radial clearance by way of preloaded angular contact ball bearings or roller bearings) .  3 or 4 times  (each with a different spindle design) spindles failed in the seal area. One time a customer (about half dozen nearly identical spindles) reported one bronze insert was visibly spinning and drifting in and out.  We assumed the bronze seal ring's OD was damaged during the pressing operation, and updated the design to include some tiny flat head screws installed as "dutchmen" to help retain the bronze ring. Later another customer (with a much different spindle) called to  report that when they were "running in" the spindle and approached top speed for the first time "sparkles came out" of the front of the spindle, and it stopped turning.  After it sat a while, the spindle would turn, but the bronze seal ring (that had be press-fitted into the stationary cover) turned with the shaft. We got that one back, and the bronze ring was seized to the shaft.  The seal ID was badly discolored from heat, smeared, and full of radial cracks (heat checks). The end cover bore (where the bronze ring was press fit) and the seal OD showed NO signs of damage at all. The perfect appearing seal OD was several thousandths smaller than the specified dimension.
The last episode got me to thinking that our combination of a heavily restrained bronze ring, accidentally rubbing (whether due to immmense side loads or assembly/machining mistakes) on a 3 or 4 inch diameter shaft spinning at 6000 to 12,000 rpm was generating a LOT of heat.  With Bronze's high thermal expansion it had to grow somewhere, and, since the thick steel housing had the seal ring in a headlock, it had to grow inward, encouraging an even tighter grip on the spinning shaft, generating even MORE heat. Maybe enough so the seal brazes itself to the shaft.  When the stresses and temp gets high enough the seal material "upsets" (just like heat shrinking sheet metal) and deforms permananently to the restained dimension while still hot.  When things cool off the OD is smaller.