Briggs 21 HP barley runs , no power.

Like Walt, never had any long term success replacing guides in a mower engine.
Do it all the time on motorcycles no problems.
Tried machining up my own oversize so they have better interfearence but they always come loose again and new heads are not that expensive.
Staking has worked well as a short term fix so the customer can continue to use the machine while I get the new head in.

bertsmobile1,

Well I didn't say they couldn't be put back but I knew he didn't do it right and the first thing he needed to do is cure why they moved. Valve guides do not move for no reason. I have been fixing and supplying fixes for these for several years and never knew of one failing afterwards.

Walt Conner

PS: I can tell you IF you are interested, if you find my address. Only costs a head gasket.

I will play my old familiar solo here. I agree with Bert. When my Intek pushed an exhaust guide I carefully measured the guide od and guide bore. About a .002"fit which seemed ok, but the replaced old guide moved again.

My theory is that the head alloy had reached what engineers call a phase shift, also know as creep , aka as softened. The head cannot hold on to the guide. That is why Briggs will not sell replacement guides, only head assemblies. The phase shift may occur out around 390F (or before). I have measured 305F at the plug on my running Intek in summer. I know it is a long way from there to 395F, and I tried to measure the area around the guide, but could not get a good setup.

In another thread I suggested we all chip in and get a hardness test done on assorted cam lobes. We could also have a batch of heads done which had pushed guides and see if they are at proper temper, (Bert should not read this):laughing:

Too late.
Having a late Saturday lunch & I have already read it,,:licking:
Metallurgy 101
As a rule you do not get phase shift in aluminium - silicon casting alloys because there are no high temperature phases that can be trapped by heat treating nor triggered by overheating.
To get a phase shift, you have to cross a phase boundry line in a phase diagram like the one underneath I pinched off the web

Most aluminium casting alloys used in mower heads will be in the 4% Si to 7 % Si range and if you draw a vertical line anywhere between those 2 percentages you will see these is only 1 phase, till it melts and you have 1 solid phase & 1 liquid phase.
This one phase has 2 parts, aluminium with a tiny amount of Silicon dissolved in it and Silicon.
As the temperature drops the amount of Si that can be accomodated ( dissolved ) in the aluminium drops from 1.65 to 0.5 % and this variation is what the heat treatment of Al-Si alloys utilise.
If you quench it from 577 deg then the 1.1% extra Silicon gets trapped inside the aluminium and causes it to be hard.
If you heat it up to 577 deg then hold it there for a while and allow it to cool very slowly then the extra 1.1% Si will migrate to the grain boundries and it will be full soft, which is what happens when the engine overheats.
It is also additive so consistent overheating will eventually make all the alloy fully soft.

I used the simple Al - Si example, for clarity .Cu, Be, Fe, Zn, Mn & Mg will create substantially different diagram,
In foundrys each one of those elements gets converted to a % Si called a silicon equivalent in the same manner as elements other than Zn in brasses get allocated a zinc equivalent.

Next.
Mechanical properties of metals do not have a direct & calculatable corelation to each other so a hardness of X does not mean a tensile strength of P or a creep resistance of B
However over time for a particular casting, and limited to that casting alone it will be found that if the hardness is between X & Y the creep resistace will be between A & B.
This means that on the production line you can do a hardness test on every part and provided the process is exactly the same every time be 99% sure that the creep resistance will be within spec.

Sheese, gives me a headache. My fix is simple requires only average ability and no special tools. It includes two other proven fixes furnished to me by other people which do require some skill and some specialized equipment.

Walt Conner

I love it when Bert delivers the metallurgy 101, and indeed has very rounded experience. But after the smoke clears on his and my constructive discussions I am always left with the same questions.....

(1) Why do valve guides move and why in a upward direction?

(2)We know aluminum can be annealed (softened), why not in repeated cycles of heating and cooling on an overheated AC lawnmower?

(3) Why cannot guides be reused on mower engines, but can on motorcycles?

(4) Why would a relatively small chunk of aluminum be exempt from softening when the industry is full of guidelines for softness in aluminum , like a scratch test


Observations:

A. There are (hundreds?) of aluminum alloys. Presumably with different characteristics.
B. The action of the spring returning the valve stem may contribute to the upward movement of the guide?
C. Can we compare the cfm (air movement} from fan vs the e.g., 25 mph air movement of a bike.?
D. So called critical temp where softening might occur always 570F?? Auto heads run 570F?? With liquid cooling?

Respectfully submitted for everyone's continued enjoyment and without rancor or ego. :smile:

Edit: Long ago such baffling questions involved a handsome monetary prize for solution. Here , alas, only attaboys from the forum .

"(1) Why do valve guides move and why in a upward direction?"

In my experience, the valve movement has been due to a cooling problem, plugged cooling fins/cooling shroud. Don't forget the passage running completely thru the head around the valve guide castings. Some say ethanol fuel is causing the valves to stick resulting in movement, I have no opinion on that. Ethanol fuel gets blamed for everything people can not otherwise explain. Why always upward, I have no idea but Bert will. I have only heard of one instance where a guide moved downward. Note that B&S Vanguard engines do have replaceable valve guides.

Walt Conner

No I have no idea why mower guides always go up.
I even cut a shoulder on one & it still went up
Motorcycle guides can go either way and guides in iron heads oft do just that and the way you pick them early is because the valve lash continually changes from tight to loose .

Sorry about the lessons but I am one of those blokes who hates seeing incorrect things on the web, because they stay there forever.
Just look at all the garbage about oils and our favourite product , ethanol.

Many years ago we invented a fake brand of motorcycle the "Spagthorpe"
Google it now and there are thousands of posts about them and 90% believe they were really made.

Yes I may have been a non -ferrous foundry metallurgist, but on the web, there should never be a call to authority.
After all I could just be a smarty 12 year old with advanced searching skills.
You have to be able to convice people with the strength of your arguement and i can be just as wrong as the next guy.

"No I have no idea why mower guides always go up."

Well let me throw a wild idea out there then. When the exhaust valve is moving up, closing, the valve stem and valve guide have just experienced their highest temperature as the exhaust gases have just passed thru. When the valve moves down, opening, the valve stem and guide just passed thru their coolest time as they have rested thru the intake, compression and ignition strokes.

Walt Conner

Tinkerer200 said:

"No I have no idea why mower guides always go up."

Well let me throw a wild idea out there then. When the exhaust valve is moving up, closing, the valve stem and valve guide have just experienced their highest temperature as the exhaust gases have just passed thru. When the valve moves down, opening, the valve stem and guide just passed thru their coolest time as they have rested thru the intake, compression and ignition strokes.

Walt Conner

Click to expand...

Sound good enough.