Cambox gasket thumbnail analysis

[Paul Valente]

My spirits were buoyed after my recent post on liquid gaskets and some of the excellent discussion by other XWEB members. People generally seemed to like this insight, were respectful, no one hijacked it or told me I was all messed up. So, pressing my luck, I thought I would take a closer look at the cambox joint as it was brought up in the last thread. It isn't going to solve anything but it is fun to study.

I must say, that I don't think mine is leaking at the moment with its paper gasket with Hylomar Blue dressing, but I know it is a problem for many.

This is a half-a$$ed analysis at best because I really didn't measure the surface area of the gasket or put Fujifilm (aka pressure measurement film) on joint to see what the pressure pattern really looked like but as crude as it is, it shows some of the issues with this joint.

The basic problem is that there are not enough screws holding it on and they are not ideally placed from a sealing point of view. About 80% of the load (force) that a screw exerts on the joint is concentrated around the head*. It drops off to a low point halfway between it and the adjacent bolt. So you don't want the bolts too far apart and you don't want the gasket hanging outside of the bolt pattern. You can see there are three really bad places where the gasket won't see much load.
*Brink, Robert V., "Handbook of Fluid Sealing," McGraw-Hill, 1993

There are a few marginal areas too that I suspect would be low load as well.

So, it looks like the bolts aren't in the ideal locations which may have been unavoidable to some extent. The other part is there isn't enough load from the bolts to do the job. About 750 psi of pressure is needed to seat a paper gasket**. I calculate that the joint sees less than 300psi. This is really rough because I am just looking at computer images of gaskets and making a guess on the dimensions and calculating an area...but I doubt I'm off more than 10-20%. You can't simply crank the bolts down more because like I said, the load is concentrated around the head. Over-tightening will not appreciably change the load in the bad spots and could overload other areas.

Ironically, the bolts do not generate enoguh force to make a good paper gasketed joint but it would have been a good candidate for an RTV joint had they decided to go that way. It would have had metal-to-metal contact so no re-torque, excellent bolt torque retention, the valve clearance would not changed if you had to pull the cambox off and put it back on for some reason. All better than paper. Sadly, if you read my liquid gasket post, you'll know that the joint lacks the features needed to make it a good RTV joint so merely applying RTV instead of paper is not recommended*. It would also have been a good candidate for a press-in-place gasket but they can be expensive. Keep in mind that this motor was designed for what was at one point the cheapest car you could buy in America (the 128 was 10% less than a Super Beetle!) so cost was a very important consideration.

*Like Hussien posted in the last thread, there was a Volvo AWD gearbox application in the 2000's where the fix for the leak filing a 30° chamfer into the casting to give the RTV a place to form a bead. I suppose it could be done on the Fiat head but I'm not doin' it to my car.

 

[Pete Whitstone]

Good info, thanks for posting. I also read the other thread, never knew that about RTV joints so very informative.

Does part flexibility play a role in why this is a common leak point? You've got a cam (which I assume has some degree of imbalance) zinging around at 3500rpm or more in an aluminum structure, and you've got hundreds of lbs of resistance force from the valve springs trying to keep the valves on their seats while the cam lobe fights for the opposite. I wonder if the part was a bit beefier, would leaking be less of a problem? Of course, this would not solve the lack of clamping PSI you describe... or would it? If the part was less flexible, could not more clamping load be applied?

Pete

 

[fiatfactory]

Cambox base gaskets come in several different materials.

The usual 'standard paper gasket found in most kits (usually folded up) which is quite a narrow gasket (not home right now to put a measure across one) which during my apprenticeship years I was told to lightly oil the gasket (to get it to soften and swell) before installation.

The most common aftermarket (made by Payen and Tako) gasket is quite a lot wider, and is made from graphited paper. This is what I now use most regularly, and find they seal very well.

Some more high end gaskets (Goetze and Spesso in their more upscale kits) supply a gasket that has a sealer applied in a thin layer on each side. Most often found plastic wrapped individually inside a gasket kit.

Biggest issue I have found with cambox base gaskets leaking is small surface divots and lumps on the sealing faces, easily cased when handling. A fine sharpening stone run over the faces shows these quite readily. Same process and reason for most leaks on transmission casing halves.

When I get home I'll snap a few pics.

SteveC

 

[Paul Valente]

Does part flexibility play a role in why this is a common leak point? You've got a cam (which I assume has some degree of imbalance) zinging around at 3500rpm or more in an aluminum structure, and you've got hundreds of lbs of resistance force from the valve springs trying to keep the valves on their seats while the cam lobe fights for the opposite. I wonder if the part was a bit beefier, would leaking be less of a problem? Of course, this would not solve the lack of clamping PSI you describe... or would it? If the part was less flexible, could not more clamping load be applied?

Pete

Flexibility does play a role. If it were infinately stiff, you'd only need enough bolts to generate the force to seat the gasket and they could be anywhere. You really only need enough load to generate the seating pressure....well I guess if just stick to theory, you only need the pressure inside the gasket to be higher (at all times and all the way around) than the pressure you are trying to seal.

So as it is, parts are flexible to varying degrees and the loads are concentrated. You will see manufacturers using as long a bolt as they can so that there is a thicker the section of metal (as viewed perpendicular to the axis of the bolt) in the part that is being clamped. This spreads the load out better. Think about the extremes of a piece of sheet metal being clamped or a 2" tall casting. The 2" tall casting will spread the load out much more. In the cambox's case, it has longer bolts than the other so that side will have the load spread out much more than the side with shorter bolts.

Another trick designers do (that Fiat did not in this case) is to make the sealing surfaces thinner (in the plane of the surface) to compensate for there being less bolt load. In other words, you can get the same pressure with less load if you reduce the area. For the same reason you often see thicker (again in the plane of the surface) around the screw holes to reduce pressure in those areas so as not to rupture the gasket.

 

[Paul Valente]

Cambox base gaskets come in several different materials.

The usual 'standard paper gasket found in most kits (usually folded up) which is quite a narrow gasket (not home right now to put a measure across one) which during my apprenticeship years I was told to lightly oil the gasket (to get it to soften and swell) before installation.

The most common aftermarket (made by Payen and Tako) gasket is quite a lot wider, and is made from graphited paper. This is what I now use most regularly, and find they seal very well.

Some more high end gaskets (Goetze and Spesso in their more upscale kits) supply a gasket that has a sealer applied in a thin layer on each side. Most often found plastic wrapped individually inside a gasket kit.

Biggest issue I have found with cambox base gaskets leaking is small surface divots and lumps on the sealing faces, easily cased when handling. A fine sharpening stone run over the faces shows these quite readily. Same process and reason for most leaks on transmission casing halves.

When I get home I'll snap a few pics.

SteveC

I suspect that the thin paper gaskets deform very little when clamped and in the areas I highlighted as NFG, there may be barely any deformation of the gasket at all. Meaning just about any speration for whatever reason will result in leaks. A thicker gasket or a gasket with a sealer on it (either printed on or applied from a tube like Hylomar) adds some amount of tolerance to seperation and surface imperfections and as you point out (and I'm absolutely certain you have taken apart way more motors than I have) this could be the root of most leaks.

 

[Dr.Jeff]

The basic problem is that there are not enough screws holding it on

it looks like the bolts aren't in the ideal locations

there isn't enough load from the bolts to do the job.

the bolts do not generate enoguh force to make a good paper gasketed joint

As I read Paul's first post in this thread, the information I've quoted (above) made me think "what about using a different gasket material?" My thoughts related to something I've experienced working on other makes of older vehicles, where the manufacturer had kept the same parts' design for many years but changed the gasket material between those parts to something more modern. Therefore when working on the old stuff you can utilize the modern gaskets for better results.

Then I read the Steve C's post...

Cambox base gaskets come in several different materials.

aftermarket (made by Payen and Tako) gasket is quite a lot wider, and is made from graphited paper

high end gaskets (Goetze and Spesso in their more upscale kits) supply a gasket that has a sealer applied

Exactly what I was thinking. Seems that using one of the optional gasket materials should be a definite improvement. I know it is on the other makes where a better substitute is available. Steve, when you get home and have time please give us more details.

Thanks to Paul for bringing this up and thanks to Steve for letting us know there are options.

 

[Ulix]

Thanks for another geeky tech post Paul, very much appreciated! Right up my alley.

I guess the only thing we can do in response to this suboptimal design is to pay special attention to this joint by making sure the surfaces are plane and undamaged and using a good gasket.
There are some really terrible ones out there. Super thin paper, terrible fit, no ribs. I had to throw some away, they were so ill-fitting, they could not be installed without breaking them.
The one in this kit is probably of the thin paper kind:


I have spent some time hunting down this much nicer one out of better material and with stabilizing ribs in the interior:


I would hope that it seals better as well.

Regarding paper gaskets: a friend who was a mechanic at Fiat in younger years claims that paper gaskets are suppused to be soaked in water before installation to swell them.
Personally, I am also a fan of using Hylomar on paper gaskets. I have not used it on the cam box gasket yet though. The gasket thickness is critical to valve clearance, and I guess I was worried that a Hylomar covered gasket would take more of a set.
I once measured valve clearance on a freshly installed cam box and came back to measure it again the next day. The difference in valve cleareance was measurable. So probably it is good practice to let the gasket set before valve adjustment.

 

[Rupunzell]

Been using Hylomar on paper gaskets for decades, generally works OK.
Hylomar is not "Hylomar" as they are a UK based sealant and related supplier.
http://hylomar.com/hylomar-product-range/gasket-jointing-compounds/

Note the wide variety of "Hylomar" products.

There was a time when either Permatex or Loctite (not sure which one, been too long) marketed their version of "Hylomar" did not perform as the original Hylomar blue.

Fiat OEM paper gaskets for the cam housing tends to shrink over time if not used rendering the gasket difficult to use.

Personally, there is a strong aversion to using RTV silicon based gasket sealant or gasket replacement in engine sealing due to the overly common event of applying too much RTV. This is very easy to do. Excessive RTV will cure in oil resulting in gobs that can cause problems like plugging up oil passages. Gasket dressings like the first version of Blue Rolls Royce spec Hylomar tends to dissolve in oil reducing the risk of gobs plugging up oil passages.

Folks who build racing rotary Mazda engines favor Original Hylomar blue as gasket (Many Mazda rotary gaskets are paper just like Fiat) sealant. Been there, done this and it works on Madza Rotaries.

Mazda Miata 1.6L & 1.8L specifies their brand of RTV to be used in very specific amounts at the corner intersections of the cam end plugs on the valve cover gasket (moulded gasket that fits into a cast groove on the valve cover). If this is not done properly, oil leak is assured.

Saab B2x4 series engines used anaerobic sealers in the oil pan, timing cover and a host of other locations. Saab is very specific as to the type and amount of anaerobic sealer used. Their sealing surfaces have a machined groove with a very specific shape optimized for this type of sealed joint. Another common sealing method in modern auto design is the machined or cast in groove for an O-ring.

This does NOT mean RTV cannot or should not be used in engine building, if it is used, extreme caution must be observed to limit the amount of RTV used and assure none of it can squeeze out resulting on potential problem causing gobs.


Bernice

 

[fiatfactory]

Quick look on my computer yielded this picture of two cambox base gasket types.

The Payen gasket is quite thin paper, but slightly thicker and also quite a bit wider than the stock gasket, and it's also made from a graphited paper.

The Spesso gasket has a thin pre applied adhesive (doesn't really show up in this pic) and again is wider than the stock gasket.

Pic of the Goetze gasket to follow when I get home in a weeks time, I'm not sure what Goetze part number is, I've only seen them in full gasket kits.

I will also mic these all up for thickness comparison, and get some better pics.

Part numbers are on the packs ... and I stock both types...personally I like the Payen gasket the best.

SteveC

 

[Paul Valente]

Yeah, the Hylomar (Universal Blue in particular) as a gasket dressing tip from you was about the best thing I learned on XWEB, Bernice. I had been using the spray Permatex Hi-Tack before that (it was just ok).

 

[MikeHynes]

This is an interesting discussion, but is this joint a big problem - leak wise? I know some folks experience leaks (I bought a car with a bad leak), but I've never experienced a leak on any of the engines I've built. So I wonder if leak problems are caused by the gasket used, or something else? I've always used the thin paper gaskets (the same ones that Fiat used) - and I always coat both sides with an extremely thin layer of RTV on both sides. The reason I use the RTV is to make disassembly easier. (Yes, I know, excessive RTV is bad, I don't use an excess of it.)
Back to easier disassembly - it is very easy to damage the cam box trying to get glued on gaskets off. The head is much more robust, but if you use excessively abrasive methods to remove gaskets it's very easy to make any flat surface - less flat. Of course if the surfaces of the joint are no longer flat, it's a lot easier to get leaks. No matter what gasket you use.
As I said above, this is an interesting discussion - the proper application of gaskets is a very basic part of component assembly. But for the most part the joint being discussed is mostly not under pressure. (That is unless the crankcase isn't properly vented.) So, if it leaks there is a significant gap, or the crankcase is under excessive pressure. I'm not aware of any redesign Fiat made to the joint to fix leaking problems (just kinda guessing here, but I'm not aware that anything changed over the many years). I would think that the Fiat design is good (thin paper gasket), at least it's good enough. Good enough as long as the surfaces are flat, the engine isn't experiencing excessive pressure, and the proper clamping force is used. If a leak develops it would be good practice to make sure the mating surfaces are flat, the crankcase is vented properly, and the cambox is tightened properly - before using a different gasket.

 

[Pete Whitstone]

This is an interesting discussion, but is this joint a big problem - leak wise? I know some folks experience leaks (I bought a car with a bad leak), but I've never experienced a leak on any of the engines I've built. So I wonder if leak problems are caused by the gasket used, or something else?

I don't think it's shadetree mechanic solutions that are at fault here. My 86 (bought new) leaked in this area from the factory.

Pete

 

[MikeHynes]

Pete,
It seems as if everyone has different experiences. Did you ever figure out why you had the leak?

 

[Pete Whitstone]

Pete,
It seems as if everyone has different experiences. Did you ever figure out why you had the leak?

Mike,

Not sure about the root cause, but the reason it was leaking was that the paper gasket had separated under the cam box cover. There was a gap of maybe 1mm or so in the gasket, fairly clean edges. I don't know if the gasket was defective, or it was damaged on installation, or it was fine on installation and separated later (the car sat on the dealer lot in hot Texas for a year before I bought it). I also don't know if the cam box was properly tightened at the factory, I didn't try to get "un-torque" measurements. So a lot of unknowns. Except for the fact it leaked, I don't really have a lot of info.

Pete

 

[MikeHynes]

Pete,
Hope its fixed now.

 

[lookforjoe]

I have had my cambox off more times than I can recount

The only time that was for a leaky cambox gasket was shortly after the original install of the shaved cambox & 35/75 cam from MWB. Their boxes are (powder?) coated, and if you don't remove the coating from the cambox bolt seat areas, the box will loosen along the rearward edge, allowing oil to soak the paper gasket, at which point it will crack & leak.

In my experience, as long as the surface is thoroughly cleaned and free of oil (which means allowing the cambox to drain for some time, and removing oil film from the inner surfaces), the thin (.012") paper gaskets ( I prefer the green ones) don't leak. I usually have had two on hand for each install, because Murphy's Law.