Here we go again..... Another day, another customer with chronic head gasket problems on a big bore that was built by yet another "pro" :confused: What was the answer? Studs, copper head gaskets, O-rings, and copper spray?
So let's dive into engine building 101;
Everything on an engine needs to work as one harmonious system. The cam should be balanced with the intake and exhaust for best power, easy enough. Sealing combustion promotes maximum efficiency, while minimizing combustion by-products in the crankcase that cause engine component deterioration. This is all basic stuff.
Head studs and so forth do have their place, but are rarely needed. Studs using high torque values will absolutely pull a cylinder out of round causing increased blow-by, thereby increasing engine and/or ring wear. Ring wear in turn will eventually lead to more blow-by, power, and effiency losses over time.
The best studs will fully bottom out in the factory holes to distribute the added load over a broader area.
Copper gaskets are notorious for "cold weeping", where the gasket will leak coolant around the outside of the head until it gets warm enough to expand and seal. They are usually made of a thicker material, and will require extra machine work to get the piston to deck clearance back where you want it. They usually require more torque to do the job (see studs).
O-rings? Every O-ring setup I have seen on an ATV engine is grooved on the cylinder side only....... This absolutely defies any logic I can muster, so I won't even comment on it, except to say that turbo race cars use a reciever groove in the head.
So what is the answer?
Have the head and cylinder machined flat and as smooth as possible, just like you would for any decent rebuild. Use a flat piece of thick safety glass or granite that is nice and flat. Use a piece of wet/dry paper on the surface (I use 3M spray adhesive), and "lap" the head until it is smooth. I recommend a minimum of 400 grit final finish (600-800 is even better). Even the best machine work is not always smooth enough for Cometic (MLS) gaskets, they are extremely senstive to surface imperfections. Better safe than sorry.
The next step is to chamfer all bolt holes, which not only helps even out and spread the load, it also eliminates any "summits" that would pull up around the threads. Any type of riser around the bolt that would come with fastener torque, will lower clamping force across the rest of the gasket.
You will want to do this to the head and cylinder. What you'll end up with is a better performing engine that lasts far longer for a few bucks worth of the right tools rather than a few hundred dollars worth of "band-aid" fixes.
(http://i233.photobucket.com/albums/ee157/kennybasham/100_0947.jpg)
(http://i233.photobucket.com/albums/ee157/kennybasham/100_0995.jpg)
Great write-up...love the thread title.
Thanks.... I had to put some lipstick on that pig..... Spice it up a little :thumbs:
:thumbs:awesome info kenny. Is this done with stock bolts? or a special head stud?
Stock bolts, with stock torque settings. This whole thing came about when I sent the factory bolts to A-1 Technologies(an aerospace supply co) to be tested. I was originally having studs made. The engineer called me up and reported that the factory bolts have a tensile strength of over 150,000 ksi. He said that the factory bolts were definitely not the weak point, and would function at a much higher torque spec. The consensus was that there was another reason for the low torque specs. We found it as soon as we bolted a cylinder down on the boring bar. The factory specs are right at the verge of pulling the cylinder out of round. Of course this all changes when using a sleeved cylinder. A sleeved cylinder is far less sensitive to being over torqued. Far better tensile strength..... :thumbs:
Good info! How are you chamfering the bolt holes? Is that part of the head work you do?
yes, but you can buy a chamfer bit virtually any hardware store or farm supply. The only thing different about mine is a depth collar, that I made :) I also use a M9x1.25 tap that was bought at MSC industrial supply, and converted to a bottoming tap.
Thanks for the reply! :thumbs:
Great read Kenny 8)
Very nice :thumbs:
good post man.glad you told me i had to read it lol.once again Kenny is the man
your words are like a very good song in a differnet language, sounds amazing, but i dont know wtf it means. lol thanks for the write ups they are interesting:) i am sure people who actually do engine work get alot out of this.....
Is there such a thing as "TO" smooth ?
Nice work kenny :thumbs:
Quote from: Krandall on March 12, 2009, 03:40:12 PM
Is there such a thing as "TO" smooth ?
Nice work kenny :thumbs:
Probably, but I haven't found it yet. I usually go with 800 grit or finer. There will always be some porosity. Remember that the sealing material is very thin on the Cometic, and easily damaged. Add the fact that the steel gasket material expands and contracts at a different rate than the aluminum, so it needs to be able to do this without damaging the coating. I like to prep the cylinder and head with dry-film moly spray. You just have to be careful not to get this into any threaded areas.
If you notice what seems to be minor scoring on top of the cylinder in my picture...... Cometic gaskets will probably not seal that. They are extremely unforgiving. This all underscores the value of a really GOOD engine builder, that should have this and a hundred more routines in their bag of tricks. We will go into valvespring and valve prep, along with some minor rocker and shaft mods at a later date , that should add longevity and peace of mind, not to mention a possible power gain. There are many tricks that are added to a routine motor build. I am still torn between sharing this info with with consumers, and giving away all my tricks............... :lol:
Kenny, can I fly you up here and pick your brain over a case of Bud? I cant tell you how much I enjoy talking/reading this level of motor building!
hey kenny, i am anxiously awaiting you parts to arrive in canada. after rereading this post i have a few questions. will a stock head gasket seal nearly as well or better than the cometic gasket? and where do you get this dry film moly spray?
Kenny, I was looking at the chamfer bits and wanted to know what size you recommend. ??? I found 1/2, 5/8, and 3/4 at the local store. And what is the depth that you are chamfering? ???
Thanks again,
Matt
NaturalRaptor, the size of the bit isn't really important, as long as it comes to a small enough point to be used in all the small holes you want to chamfer and is big enough not to get the straight sides into the big holes you want to chamfer. I'd think the 5/8" would be good as long as it meets those criteria.
Kenny, if you ever want to explore the limits of "too" smooth and "too" flat, I have several lapping machines at work where we regularly achieve .00005" (yes I mean 50 millionths) flatness and can get an 8 micro or even a 4 micro finish. I could do one cylinder/head on the sly, but if you ever wanted to do a bunch, we could set up a P.O. and do it all legal-like.