The sintered metal or "powdered metal" parts have been, for quite a while, the darling of the automotive industry with mixed results. They're cheap to make -- just put the metal powder in a mold and hit them with some electricity. No expensive maching on more expensive metal pieces. But, in some instances like poor (or lack of) lubrication, these sintered parts fail -- where other (better) parts might continue to do their respective jobs without premature wear. It's a scenario of better quality: both in materials and machining, imo.
Sorry, but these comments about the powdered metal valve guides are a bit off the mark.
First, there is no "electricity" involved in the powdered metal process. The fine metal powder is poured into a mold and then mechanically compressed under very high pressure to make the green formed part. The powder is placed in a cavity and a mechanical punch closes the cavity and compresses the powder under hundreds of tons of pressure. That part is about the consistency of a hard cookie. You can break it with your hands. The compacted part is then sent thru an oven that sinters the part. The sintering oven heats the part to nearly melting and thus fuses all the single bits of powder together. Once sintered the part is as strong and durable as other metallic piece.
The powdered metal part has some "porosity" but if you look at the density of the part it is almost as dense as a cast or forged part so there is very little "air" in the part. Like as in almost none. The PM parts have this measured as densification and all parts have the density controlled by the compacting process.
There is no question that powdered metal valve guides offer some cost advantages in terms of machining required, but, instead of sacrificing quality and performance, the powdered metal parts will generally outperform any other type of valve guide material out there. Generally speaking, the PM material is actually much more costly than an equivalent cast or forged alloy but the increased material cost is more than made up for with the improved material properties and reduced machining costs.
The powdered metal process is practically a panacea for parts like valve guides. In addition to the strength and structure required with the material properties, any level of porosity desired can be built into the parts. Porosity as seen in PM parts is good in the case of valve guides as it promotes oil retention and lubricity.
The PM process also has the ability to allow the valve guide material in any alloy required. The valve guide materials are very specific and highly develped mixes of brass, copper, bronze, etc. powders that are thoroughly mixed prior to compacting. During the sintering process the PM material can take on whatever material properties required or desired based on the alloying and sintering times and temperatures.
Don't for a moment assume that you will get better performance out of any sort of "high performance" valve guide material that comes in a billet or is machined to shape rather than PM'd. Certainly it is plausible for a good machined valve guide material to outperform a relatively poor PM material....but.....no one is going to make PM valve guides out of the wrong material in all likelyhood. So, while it is possible to get better performing billet material in valve guides it is somewhat unlikely in my opinion.
We have seen repeatedly where PM valve guide materials easily outperform other valve guide materials. The issue with the tick may be due to poor quality valve guide materials....but.....I would honestly not put that eventuallity at the top of the list and I would certainly be very hesitant to assume that some shop is going to make valve guides out of some billet material that are going to be "better" than even inferior PM guides. Possible but not likely.
Head shops like the billet bronze or siliconbronze valve guide stock because they can easily make any valve guide needed for a head on a simple lathe and install them. PM guides that are used by the OEM's require tooling and sintering furnaces and process control and such and are much more expensive to make for just a few parts. But for the volumes the OEM's need the PM parts are very cost effective because they require very little machining AND they perform excellently. Most of the good shops actually have bar stock for valve guide fabrication that is made from the PM process JUST to get the superior material properties available from PM...i.e....the specific alloying, porosity and heat treatment from sintering.
Realize that "powdered metal" parts sound chintzy but PM connecting rods are incredibly strong and light and are used by most all of the major OEM engine builders with excellent success. If it is strong enough for conn rods then it certainly can be used for valve guides if strength is an issue.
Suffice to say that the problem may certainly lie with the choice of valve guide material or alloy Yamaha used but it is not because of the guides being made of PM. If I had to spec the best possible guides in the world they would be PM for starters.
From my experience even the poorest PM valve guides perform very very well ESPECIALLY when lack of lubrication is an issue. The slight porosity of the PM material makes the most of even limited lubrication as the material can hold and retain lubricatin better than "solid" material. The alloying properties of PM also provide a certain level of self lubricity that sold materials do not have. Copper particles in the PM material, for instance, provide inherent lubricity even in the absence of other lubricants and graphite and also be added to the powder alloy for additional lubrication. My point here is that the guides may have worn out even SOONER if lack of lubrication is the issue with the FJR valve guides if they were made of other materials. Even PM can wear under the worst of situations. That doesn't mean it is inferior or that something else might be better. Extensive testing is the only way to tell.
I wouldn't bet on some shop conjuring up a magic fix for this, myself.
