Cracked valve seats

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  • #389950

    In the responses below to gearsets, Jim Birkner comments on leaning his engine out and getting six cracks across the exhaust valve seats. A 1934 SAE report written by a Studebaker engineer stated that when they ran their engines at higher speed and power they never had any problem with valve seat cracking unless detonation (knock) was present, so be certain not to run with any sign of detonation (which normally should be difficult to get with 87 Octane fuel). Another possible problem is if the exhaust valves are newer stainless/austenitic alloys, those valves have higher temperature strength to resist cracking, but they have much poorer heat transfer leading to higher seat temperatures and valve seat recession. The original ferritic Sil-chrome exhaust valves on Pierce engines were more prone to cracking but should be less prone to seat recession and high seat temperature due to better heat conductivity. Stainless valves seem like a good idea but aren’t necessarily. Valve seat recession without leaded fuels first became apparent in the 1930’s with the newer improved austenitic valve materials but was soon forgotten when leaded fuel became virtually universal.

    #393352

    Jim,
    I totally agree with your points. A significant, and somewhat overlooked function of exhaust valves is to carry the heat from the seat in the block and down the stem for cooling. I’ve seen some of the "new" stainless valves hogged from stainless barstock and I think that these can contribute to the block cracks which some people have seen when driving the engines hot and hard.

    Greg

    #393358

    Hi,

    Here’s an example of the stainless exhaust valves giving trouble. I had this happen a couple times with my ’29 after returning from driving to an Annual Meet. If I did it right there should be a photo link below where you can see my first four cylinders. Number 2 shows that the exhaust valve has gotten very hot and has begun to erode along the edge. The valves had about 2000 miles on them at that time and were adjusted to .008 cold before the trip. I was also driving on a 4.58 rear gear but did not exceed 55 mph.

    We theorized that the stainless valves were not transferring enough heat to the valve guide and were running too hot.

    I did two things. I found an original set of exhaust valves and installed a 3.54 rear end. I did this before driving the car to the Texas meet from Chicago in 2000. We’ve had no trouble at all since.

    With the 3.54 gear we have been able to cruise at 60 mph with no trouble. I think that both the stainless valves and the gear contributed to the trouble.

    Hope this helps.

    Bill

    http://home.wowway.com/~billmorr/valves.002.jpg

    #393371

    Bob Koch sent me an email to question my statement above where I wrote that the valve can actually carry heat away from the block. I was incorrect in my comments. The valves do not cool the valve seats in the block, rather the valve face is cooled by the block/seats.

    What I really meant to convey was that the poorer thermal conductivity of the stainless material will result in less heat travelling down the valve stem to points cooler in the engine. To cool down during the intake and compression strokes of the engine, the valve needs to dump heat down the stem, and through the seat. The typical austenitic stainless which we refer to as 304 or 316SS are about 50% as good at conducting that heat as the older ferritic steel valves which Pierce originally supplied. Bottom line is that stainless valves will run hotter valve head temps.

    I appreciate Bob’s comments and offer the above.

    Greg

    #393382

    Bob Koch has requested by seperate email copies or references of the engineering documents I use to support my assertions regarding valve materials, stating "You adhere to several positions that are at variance with textbook and repair manual literature that I have." The report I refer to cracking valve seats being caused by knock does not address valve temperature directly but is an interesting insight into engines of the 1930’s. It is "Problems in the Development of a High-Speed Engine" by Sparrow in the SAE Journal volume 36, No. 2, 1935. It is available for a copying charge from SAE.

    I didn’t intend to make this into a large technical dissertation, I just wanted to highlight the danger of knock being a cause of seat cracking and that the lower conductivity of modern aftermarket valves may contribute to the problem since hot exhaust valves promote knock. However, since questions remain I have written up a response that is too large to post on the forum, so anyone interested in my response as well as the original 1930’s engineering paper on valve seat recession can email me and I will fwd. My email is listed in the roster on this site.

    #393385

    An interesting thing is that my origional engine had the steel valves and had a crack across #3 exhaust valve to the cylinder wall. Not sure that steel valves are the answer.

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