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"Best"? exhaust valve guide material?

I'm trying to top a VW type 1 based 2276 av-conversion.
The exhaust valves are stainless, supposedly Manley.
According to the logs, the engine has less than 35 hrs on it, although it was "moteur neuf" in 2013. (could mean new, or could mean just "fresh") It is possible that the heads were re-used when the engine was removed for OH & re-barreled due to failed Nikasil cylinders; so # hrs on valves might be longer.

Anyway, 4 exhaust stems are barreled .0025" to .0035" under the center diameter, at each end of the travel.
The bronze guides are wallered some .020" to .035" at opposite ends over the center ID.

To replace the guides, there seem to be at least 3 options.
Scat sells "manganese" bronze. Other vendors sell "silicon" bronze.

Another very respected vendor sells "aluminum silicon bronze" as a performance up-grade, but for intakes only. When questioned why not for exhaust, the answer was that the performace derived from being shorter and better flow & also (implied) less friction, but that would limit durability in the exhaust app. It is unknown whether a full length guide in that material would wear better or worse than the other options.

This is a slow revving engine, 3600rpm tops, 3300 or so typical cruise. VW guides are simple.
Is there a preferred alloy for long term durability in the exhaust?
Is 64200 the alloy for aluminum silicon bronze guides, or something else?
How about the straight silicon bronze, vs manganese bronze performance and specific alloys?

Scat sells stainless "racing" valves with chromed stems.
Is there any reason not to use chromed stem valves?
It would seem that would be best with any bronze material, but then maybe the failure mode is not good?

Thanks!
smt
 

rogertoolmaker

Cast Iron
Joined
Jan 3, 2016
I'm trying to top a VW type 1 based 2276 av-conversion.
The exhaust valves are stainless, supposedly Manley.
According to the logs, the engine has less than 35 hrs on it, although it was "moteur neuf" in 2013. (could mean new, or could mean just "fresh") It is possible that the heads were re-used when the engine was removed for OH & re-barreled due to failed Nikasil cylinders; so # hrs on valves might be longer.

Anyway, 4 exhaust stems are barreled .0025" to .0035" under the center diameter, at each end of the travel.
The bronze guides are wallered some .020" to .035" at opposite ends over the center ID.

To replace the guides, there seem to be at least 3 options.
Scat sells "manganese" bronze. Other vendors sell "silicon" bronze.

Another very respected vendor sells "aluminum silicon bronze" as a performance up-grade, but for intakes only. When questioned why not for exhaust, the answer was that the performace derived from being shorter and better flow & also (implied) less friction, but that would limit durability in the exhaust app. It is unknown whether a full length guide in that material would wear better or worse than the other options.

This is a slow revving engine, 3600rpm tops, 3300 or so typical cruise. VW guides are simple.
Is there a preferred alloy for long term durability in the exhaust?
Is 64200 the alloy for aluminum silicon bronze guides, or something else?
How about the straight silicon bronze, vs manganese bronze performance and specific alloys?

Scat sells stainless "racing" valves with chromed stems.
Is there any reason not to use chromed stem valves?
It would seem that would be best with any bronze material, but then maybe the failure mode is not good?

Thanks!
smt
Is there any reason to use chromed plated stems is a better question. The valve stems are hardened and ground alloy steel. For a bearing material we use dissimilar metals. Bearing materials, Cast Iron and Bronze are a Crystallin Structure and are excellent choices for a bearing. A crystallin structure retains oil at the surface of the moving parts. Aluminum Bronze is a good choice. For that matter any bronze bearing material will work well. The old bronze guides wore .020 to .035. The result of extremely poor maintenance and lack of lubrication not the fault of the guides material.
Roger
 
Cast-iron?

Its a pretty short thermal path, steady state operation, I doubt it matters much what it is.

Is there any certification on this?

This type engine goes in airplanes with E/AB certification, so no, there are no certification requirements for the engine itself. E/AB = "Experimental/Amateur Built". FAA does certification of the completed airplane itself to have some oversite for what might happen on the ground. Your placard (part of the required certification) in bold letters warns any passengers that they are entering an experimental airplane with certain risks. Beyond that, the FAA figures it's your butt & they don't really care how you dispose of it so long as there is minimal inconvenience to other citizens.

AFA CI, the understood relationship is that valves running in silicon bronze last about "10 x" as long as the valve/CI combination. I don't know if the time factor is accurate, but no one uses CI because in practice it does not last very long.

Is there any reason to use chromed plated stems is a better question.
Yes, especially with some bronzes. There is practically no lube in the guide-valve combination.

The valve stems are hardened and ground alloy steel.

You would kind of wish that, wouldn't you?
Unfortunately, SS valves (unspecified alloy) are preferred, again, lots of experience shows they last longer. The stems are not very hard. Except some can be had chromed.

For a bearing material we use dissimilar metals. Bearing materials, Cast Iron and Bronze are a Crystallin Structure and are excellent choices for a bearing. A crystallin structure retains oil at the surface of the moving parts.

Again, that would be a good guess. Unfortunately, there is very little lube. In fact, some seal the guides as in automotive practice. Once oil starts getting into exhaust guides in any useful quantity, it bakes, and sets off an accelerating spiral of increasing problems including overheating & sometimes sticking.

Aluminum Bronze is a good choice.
You know this from practice, or it sounds good?

It sounds good to me, too. I love 954 bronze for bearings. Unfortunately it does not play well with un-hardened or un-chromed shafts. My question is what is the best combination, in actual practice. There could also be a heat weakness issue; or not? i don't know so am asking on here. If you have been using, specifying and comparing combinations of valves and guides, and have a log of numbers i want you to keep talking.

For that matter any bronze bearing material will work well.
That is sheer guesswork.
& known to be untrue.

The old bronze guides wore .020 to .035. The result of extremely poor maintenance and lack of lubrication not the fault of the guides material.

Do you know these engines? it could be the result of time in service.
Which in the VW conversion tends to be relatively short. probably not to the 1,000 hrs the bottom ends can make.

So back to the actual question: in your practical experience, what valve and guide alloys are known to be superior? Among the 3 common commercial choices listed above, which would you choose and why?

smt
 
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Ampco 18.

Gbent, that would play to my prejudices. :)
But do you know for a fact if it is used in hard service IC exhaust valves and lasts longer than other combinations? Also of typical available valves, what alloy and condition stem would be the best to mate with it? I can make the guides though Ampco 18 would not be fun. I can't (practically) make valves and have to use one of the commercial offerings.

smt
 
Michigan - I'm pretty sure that is what is currently in my heads.
Wondering if there is a better combination, since there are 3 commercial offerings of alloys.

silicon bronze as you show, is available from most suppliers.

aluminum silicon bronze is available from some.
Unfortunately, the above are short, the "performance" is through better flow, not necessarily durability. But i could make a longer set if the alloy is superior? (Thinking 64200?)

Scat is well respected, and supplies manganese bronze:
Their primary focus, too, is automotive "racing" performance. So perhaps the alloy has better durability, or perhaps it has other attributes that racers like? Or maybe it works better @ 9,000 rpm and has no advantage at 3,600? (don't know what alloy)

smt
 

scsmith42

Aluminum
Joined
Jul 28, 2020
Location
New Hill, NC
Valve guides commonly wear at each end of the guide due to the side loads put on them by the rocker arms. This wear pattern also transfers over to the valve stems.

Back when i had my machine shop and was doing competition valve work (1980's) , silicone bronze was the replacement guide material of choice.
 

gbent

Diamond
Joined
Mar 14, 2005
Location
Kansas
Gbent, that would play to my prejudices. :)
But do you know for a fact if it is used in hard service IC exhaust valves and lasts longer than other combinations? Also of typical available valves, what alloy and condition stem would be the best to mate with it? I can make the guides though Ampco 18 would not be fun. I can't (practically) make valves and have to use one of the commercial offerings.

smt

Place I was working made a lot of heads that got Ampco 18 guides. NASCAR, drag racing, and street rod applications. Even put them in a few Vdub heads machined from solid. That was a lot of years ago, but I doubt there is anything better. I could supply you with enough Ampco 45 for a set of guides, but that is mostly a helicopter transmission material.
 

chipss

Aluminum
Joined
Jul 19, 2020
Just my .02. Bronze guides are typically used for racey applications as they have a better thermal conductivity than standard guides. (cast iron)
Having said that, fine grain cast iron guides do see very high rpm in street applications. Cars, motorcycles etc.

Here's the determinate that I haven't studied. It seems the bronze guides wear quickly although they assumedly offer good performance and heat rejection. Of course high lift cams do put more angular thrust on the valves via the rockers increased angles and the guides also carry higher spring loading that must be overcome through that rocker geometry. So they wear faster.

As this old air cooled VW is only going to be buzzed to 3600 maybe the cast iron guides will be ok.

I say this based on a motorcycle I turboed years ago. The turbo company wanted bronze guides so I did it. After a couple years the guides were shot. I rebuilt the head with stock iron guides and it was running fine several years later.
 

steve-l

Titanium
Joined
Mar 2, 2012
Location
Geilenkirchen, Germany
Stephen,
I think you are overthinking this. In truth I don't think it matters much because these engines are really only good for about 100,000 miles anyway because they are air cooled. Air cooled engine cylinder heads run very hot. You should consider valve seats, guides and valves consumables and use anything you want and be prepared to replace them all every 100K. The engines are easily removed and replaced.
 

DDoug

Diamond
Joined
Oct 18, 2005
Location
NW Pa
Stephen,
I think you are overthinking this. In truth I don't think it matters much because these engines are really only good for about 100,000 miles anyway because they are air cooled. Air cooled engine cylinder heads run very hot. You should consider valve seats, guides and valves consumables and use anything you want and be prepared to replace them all every 100K. The engines are easily removed and replaced.
In the OP's application, it is running at at least 75% load at all times, and failure is a very bad thing.
 

michiganbuck

Diamond
Joined
Jun 28, 2012
Location
Mt Clemens, Michigan 48035
likely you already read this material.
It sure seems a toss-up/coin toss between SS, cast iron, and bronze.
I rebuilt a Jaguar twin OHC -6 and the race-engine specs were way loose compared to a street machine. Perhaps home-made of hardened tool steel with chrome stems made .0007 loose (or target .oo27 to 0030) would be good for a 200K street engine. I suspect a hot race engine stem might grow .oo1 (?)

(That Jag engine was neat. you could change the valve timing.)
 
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steve-l

Titanium
Joined
Mar 2, 2012
Location
Geilenkirchen, Germany
In the OP's application, it is running at at least 75% load at all times, and failure is a very bad thing.
Doug,
There is absolutely no way any automotive engine will survive very long at a 75% loading. They were never deigned to do this. Automotive engines are designed for limited duty applications. Compare any automotive engine design to a continuous duty engine. There are huge differences. This is specifically true in Stephen's case. Please compare his over bored and stroked motor to say a Lycoming aircraft motor.There is a very good reason automotive engines are only rated at peak brake using power pulls rated in seconds as opposed to an SAE rating of work done over time.

Secondly, wear on valves, guides and seats rarely cause catastrophic failure. Even in stock engines, that kind of wear as Stephen specified is very unusual. He should be seriously looking at valve train geometry IE valve length, push rod length and rocker arms. Stephen did not specify if his engine is using after market heads, rockers or cam All of which can drastically effect valve stem loading..
 








 
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