From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: custom made engine intake/exhaust valves?
Date: 7 Apr 1999 18:20:14 GMT

>From: Brian Evans <brian@uunet.ca>
>
>I just had guides renewed in my race head by having them reamed oversize,
>bronze liners installed, and then broached to size by stuffing a
>precisely sized ball bearing thru them. The race shop that did them
>swears by this method, and I've got two heads running this system this
>season, so I guess I'll find out how well it works first hand!


If those are the thin wall liners I see used over here then they will wear out
like the clappers. I never did see the point in dicking around with that system
when it is easier to press in a new guide anyway. I've seen those things wear
out after 3 races and then fall to bits as little slivers of bronze. Does
wonders for the crank shells too when they get into the oil ways.


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: cracked valve seat
Date: 27 Apr 1999 20:45:32 GMT

>From: rbuxton@pen.k12.va.us (Richard Hal Buxton)
>
>I asked a question about how valves burn and received many good
>messages, thank you!  Now I have a follow up question.  The
>valve in question has a cracked valve seat.  What is involved
>in replacing the seat?  Is it something I can do with a press?
>Can the seat be welded?  The other valves are in good shape and
>I hate to pay for a complete valve job, and that is what
>everyone wants to do.  I am interesed in what everyone thinks.
>Thanks again for taking your time to share your ideas with me.


Is this a cast iron head without seat inserts or a separate steel insert ? If
cast iron then chances are the crack will go right through to a waterway. If it
is very localised then you may be able to get the head bored and an insert
pressed into place and the seat recut. If it an ally head or iron but already
inserted then the old insert can be bored out and a new one pressed in.

If you have to ask the questions then chances are you can't do this yourself -
no the seat can't be welded !! Don't be such a cheapskate. The machining and
the amount of press fit has to be done very accurately or either the insert
will come loose or if too tight may split the head.

While the head is off get it all redone properly. You'll regret cutting corners
and then have another valve burn out later on.


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: burned valve
Date: 27 Apr 1999 21:05:52 GMT

>From: Carl Byrns <oxygen1@concentric.net>
>
>Rhbuxton wrote:
>
>>Hi All, I pulled the head off on old chey 350 and there it was, a burned
>>exhaust valve. I noticed the place it was burned was V shaped and got to
>>thinking that every burned exhaust valve I have seen has had this
>>failure shape. I haven't seen hundreds of valves but I must have seen
>>more than I wanted to that failed this way. If anyone has seen other
>>failure modes I would like to hear about them, at any rate, I was wonder
>>why this shape? What are the thermo concerns or material properties that
>>make it so? Any ideas?
>
>The most likely reason for the V shape is that is where the valve and
>seat were welded together and then torn apart.
>Other valve failures I've seen:


Nonsense - this is the standard mechanism for valve failure (particularly
exhaust valves) due to thermal stress. In 9 cases out of 10 it is associated
with a cracked valve seat. The hot spot where the valve and seat fail to
contact and transmit away heat leads to abnormally high expansion and a small
split forming in the circumference of the valve. This then overheats further
and speads into the valve head until the split is somewhat wider than the width
of the valve seat. This process continues due to thermal stress and the passage
of high temp exhaust gases through the split until it has reached a
sufficiently thick portion of the valve head for the heat to be handled by the
"heat sink" of the surrounding mass of metal.

Very common on exhaust valves on cast iron engines such as the MGB I have just
had in for the identical problem to be fixed. Much less common on heads with
steel seat inserts A) because they do not tend to crack and B) if they do crack
the crack does not open up and create a hot spot in the same way as with cast
iron.

If the engine is run for a sufficiently long time in this state the crack turns
into a rounded "bite" shaped hole due to erosion by combustion gases.


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: cracked valve seat
Date: 28 Apr 1999 01:46:35 GMT

>From: untmaintco@aol.com (UntMaintco)
>
>You said this was a Chevy 350? Small block chevy heads are (or were)
>Known to crack betwwen the valves. If thats where yours are split I dont
>think its repairable. It might be better to find a salvage "core head"
>for you to rebuild.
>
>On a side note I have seen heads welded in the seat area. I have a GM 6.2
>L Diesel that cracked a head. I bought rebuild heads (with a warranty)
>they are welded. My old heads were cracked in the same place that these
>were welded. I guess if you are setup and really know what you are doing
>it is possible to be done.


I am extremely dubious about that practice. If there is weld on the valve
seating area then you almost always get a material with a higher or lower
coefficient of thermal expansion and also a different wear rate than the parent
metal. This leads to high or low spots in time and then the valve burns out
again and yet another chance to witness that impressive V shaped split pattern
we all know and love.

I have seen cast iron heads nickel filled in non critical areas but it is a
dubious practice if there are easily available heads for that type of engine in
scrap yards.


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: cracked valve seat
Date: 28 Apr 1999 02:02:06 GMT

>From: kd6jdj@aol.com (KD6JDJ)
>
>>Hi Dave, Thanks for your view, I do not know about what happens to heads
>>when these problems arise. I am comfortable with interference fits as
>>well as cooling and pressing techniques, what I am interested in
>>learning is, are these the same skills that are applied in this case. If
>>this was the family car I would not be such a cheapskate, but it is my
>>"toy" and I enjoy trying things and learning along the way. It is not so
>>much to save money, although I like to do that, it is more like if I can
>>fix it myself, I would like to give it a go. Given these conditions,
>>what do you think? What do others think? Thanks again for your insight!
>
>If no 'professional mechanics' reply to your request, you may want to
>know about my experiences. The seats can be purchased from an auto parts
>house. Your experience with thermal expansion seems to prepare you for
>the machining of the head to accept the new seat. My experience with
>seats in alloy heads (big valve Corvairs) makes me very cautious. Water
>cooled alloy heads (in my experience) dont have extreme loosening
>problems. If you do the machining yourself, consider 'reverse tapering'
>the hole in which the seat will be inserted, and shape the seat
>accordingly. I suggest that this 'shaping of the socket and seat is
>overkill (except for air cooled heads) . You will need a pretty big oven
>to preheat an alloy head.


Tapering the seat sounds good in theory but in practice forget it. It is hard
enough to get an insert in at the right fit with a straight bore, nevermind a
reverse taper. And how would one bore a tapered hole to any degree of accuracy
- or at all ???

I don't use heat when I fit inserts either. I know the vast majority of the
rest of the world will disagree but f**k em. I do what I know works and I'll
stick to it. In cast iron heads I use about 2 to 2.5 thou fit and on ally heads
about 3 to 3.5 thou fit on the average sized inserts I use over here (35 to
45mm).

I measure VERY carefully with internal bore guages and then turn the insert to
an exact fit after boring each hole. I find that it is very hard to bore to
within a thou but much easier to turn something to size. The average machine
shop buys inserts that are pre-sized to the fit over and above a std boring
size. They bore all the holes and then freeze the inserts or heat the head and
whack things in quick. It means that tolerances can be all over the place.

I make sure there is a really nice radius on the leading edge of the insert and
then press in cold with a coat of oil on the parts using the knee feed of my
milling machine. That gives me much better feel than my fly press and if an
insert is a tad loose I can feel it as I wind it in.

The thing to remember is that with a cast iron head the coeff of expansion of
the insert will be greater than the head so the insert won't come loose and
doesn't need a high interference fit. With ally the situation is reversed and a
bit more fit is needed.

In the old days when I heated heads like other people I got half the inserts
stuck half way in as they absorbed the heat and then it's God's own job to get
them back out or right in again. Since pressing in cold not a single problem.

Then all you have to do is cut the perfect seat on the insert you have just
fitted. But I ain't going there at this time of night :)


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: cracked valve seat
Date: 28 Apr 1999 10:15:48 GMT

>From: frankbooth@ix.netcom.com (frank booth)
>
>On 28 Apr 1999 02:02:06 GMT, pumaracing@aol.com (PumaRacing) wrote:
>
>>I measure VERY carefully with internal bore guages and then turn the
>>insert to an exact fit after boring each hole. I find that it is very
>>hard to bore to within a thou but much easier to turn something to size.
>
>I was with you all the way till you said that. You can't bore within a
>thou? My 5 year old can bore within a 1/2. What planet are you from?

I didn't say it was impossible I said it was hard. My engine block borer runs
consistently to a couple of tenths. My valve and seat insert work however is
done on a floating cutter type device running on a fixed pilot in the guide to
maintain concentricity. The downside of this is a tolerance on diameter. If all
I wanted to do was make an exact size hole then a non floating cutter would be
far superior but would also take much longer to set up for concentricity. In
any event the basic premise holds - it is far harder to maintain a tolerance in
a boring type operation with a fixed cutter size than to be able to work
towards a size with several cuts in a turning operation.

Your 5 year old is welcome to come and see if he can do better with my
equipment than I can. I hope he doesn't grow up to be too much of a smart arse
though like his dad. Sadly I expect he will acquire the same traits over time
due to exposure.


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: cracked valve seat
Date: 28 Apr 1999 19:56:21 GMT

>From: raweich@my-dejanews.com
>
>In article <19990427214635.19245.00000393@ng26.aol.com>,
>  pumaracing@aol.com (PumaRacing) wrote:
>
>>>From: untmaintco@aol.com (UntMaintco)
>>>
>>>Hi You said this was a Chevy 350? Small block chevy heads are (or were)
>>>Known to crack betwwen the valves. If thats where yours are split I
>>>dont think its repairable. It might be better to find a salvage "core
>>>head" for you to rebuild.
>>>
>>>On a side note I have seen heads welded in the seat area. I have a GM
>>>6.2 L Diesel that cracked a head. I bought rebuild heads (with a
>>>warranty) they are welded. My old heads were cracked in the same place
>>>that these were welded. I guess if you are setup and really know what
>>>you are doing it is possible to be done.
>>
>>I am extremely dubious about that practice. If there is weld on the
>>valve seating area then you almost always get a material with a higher
>>or lower coefficient of thermal expansion and also a different wear rate
>>than the parent metal. This leads to high or low spots in time and then
>>the valve burns out again and yet another chance to witness that
>>impressive V shaped split pattern we all know and love.
>>
>>I have seen cast iron heads nickel filled in non critical areas but it
>>is a dubious practice if there are easily available heads for that type
>>of engine in scrap yards.
>>
>> Dave Baker at Puma Race Engines (London - England)  - specialist flow
>> development and engine work. .
>
>
>There are outfits that test the iron to find out exactly what kind it is and
>get filler material to match. So no problem.

I defy anyone to put that much localised heat into a job and still end up with
the same metallurgy on the welded part as the rest of the seat - regardless of
what type of filler material is used. Theory is fine - in theory :) - I've
actually been there, done it, got the teeshirt and it doesn't work. Each to
their own of course.


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: burned valve
Date: 28 Apr 1999 20:04:01 GMT

>From: Carl Byrns <oxygen1@concentric.net>
>
>OHC engines with timing chains have some other problems. On some, the
>chain tension is high enough to pull the front of the head down, slowly
>crushing the head gasket. Really old Datsuns were famous for this and
>they would leak water like crazy by number 4 cylinder because the rear
>of the head would lift so much the head bolts would be stretched.

<snip>

What a load of nonsense !!!

The compressive load of EACH cylinder head bolt is in the order of 3 to 5 tons
on an average engine. The load exerted by the timing chain/belt is in the order
of a couple of hundred pounds. There is no way on God's green earth that any
type of cam drive system can have the slightest influence on a head gasket.

Where on earth do these old wives tales spring up from ? Do you believe the
moon is made of green cheese and that babies are brought by the stork as well ?


Dave Baker at Puma Race Engines (London - England)  - specialist flow
development and engine work. .

From: Dave Baker
Newsgroups: rec.crafts.metalworking
Subject: Re: Don at Puma read this
Date: 30 Jun 1999 20:23:06 GMT

>From: mulligan@advinc.com
>
>In article <19990630141458.08641.00006628@ng-fa1.aol.com>,
>  pumaracing@aol.com (PumaRacing) wrote:
>
>> Suggested fits [in Al] are even tighter but involve heating
>> the head/freezing the insert which I don't like.
>
>Could you please explain what it is about the shrink-fit
>technique that you dislike?  This is a common theme in german
>bikes, and while I have never had to re-do seats someday
>I might.  And you seem to have a good handle on the issues,
>so I would be curious to hear of your experiences along
>these lines.

I decided on the fit to use in my road/race heads after a lot of time studying
technical references on the temperatures reached in those parts of the cylinder
head under operating conditions. Then I calculated the differential expansion
rates of various head/insert materials and decided on cold fits that would
still give sufficient fit at temperature.

It depends though on being able to measure very accurately the size of the
holes bored for the inserts and then turning the inserts to an exact fit. If
you aim for 2.5 thou and mess up by 0.5 thou on both hole and insert you could
end up at 1.5 thou - not good. As any engineer knows, it is much harder to
accurately measure a bore than an O/D.

The recommended fits are IMHO designed to allow for "low grade labour" doing
the job in question and plenty of tolerance so that you at least don't end up
on the loose side. Problem is if you end up on (or over) the tight side of the
recommended fit you stand a good chance of cracking the head - especially if
you are only fitting both inlet and exhaust - you have to do one first (by
definition) and then there is nothing to push against on the other side of the
divide. Unless you bore out one, refit the new insert and then set up again and
do the other side.

Also I rarely believe anything in print or otherwise until I have thought about
it, calculated it out etc etc. Call me cynical - but if I built my engines the
same way most people say to then they wouldn't win as often as they do.

Finally I don't have a special oven or liquid nitrogen. Simple but pragmatic -
I needed a way to do the job properly without wasting money on equipment I
decided I didn't need.

It takes me 3 times as long to do inserts though as a high volume shop who just
bore to size and bang em in. Horses for courses - when I do a set I know that
every one is fitted at exactly the tolerance I decided on to within a couple of
tenths. I wouldn't try to offer the service on its own as a cost effective job
- as part of a race engine I am happy to do it properly and in my own time.


Dave Baker at Puma Race Engines (London - England)  - specialist cylinder head
work, flow development and engine blueprinting. Web page at
http://members.aol.com/pumaracing/index.htm