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Discussion Starter · #1 ·
A common problem with EVO's is base gasket leaks. Many times just replacing the base gasket doesn't solve the problem. The base of the cylinder can change shape over time. Heat, cylinder stud tension and cylinder compression can all take its toll. Lots of base gasket leaks can be attributed to this. The cylinder studs firmly hold the cylinders down in the 4 corners. When the cylinder heats up it expands and the centers between the cylinder studs distort. When changing the base gaskets on an EVO it is good to put the cylinders on a lapping plate and check the bottoms for flatness. Many times you will see that only the stud holes make contact with the lapping plate.

This is a lapping plate for EVO cylinders. You can see lapping compound on it.





For this example I used a blue sharpie to cover the cylinder base. The cylinder base looked flat to me.





I set the cylinder on the lapping plate. While pressing down, I turn the cylinder and the lapping compound "sands" the bottom of the cylinder base.





After wiping off the excess compound, you can see the high and low spots. The blue sharpie is left in all the low spots where is doesn't hit the lapping plate.





You can see the low spots are between the cylinders. At these low spots there is less pressure against the gasket causing a poor seal. Note the low spot here is all the way from the edge to the cylinder sleeve.





I covered the base with blue sharpie again and repeated the process. You can see I removed more of the high spots but it is still not flat.





I repeated the process until all the low spots were gone. The cylinder base is now flat again. This will allow it to apply equal pressure on the gasket and reduce its chances of leaking.



 

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Well, lapping is up my alley, but not just on HD mainbearings.
We also do high pressure hydraulic pumps, so we have Crane Lapmaster machines.
Lapping is the most extreme and precise way to generate a flat surface. So flat, that to measure such a surface, a optical quartz flat and monochromatic light is used and light bands is the measurement. Its in the millionths of an inch.
I have seen these cylinder laps and they have several problems. One is that it would take too long, the other is that its nessesary that the lap and the work needs to constantly be moving or the lap will wear rapidly.
So, this type of lap will need to be trued up often by precision surface grinding or using two other identical laps and working them together by lapping each other.
The cylinder base problem is hard to deal with, we used a lathe for many years, but its not all that easy to set up.
Now, we use a very expensive automatic facing head in a large milling type machine and center the cylinder with a digital readout. We do alot of cylinders for alot of shops.
I will try to post a picture of that operation if I can figuer out how.
 

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Discussion Starter · #3 ·
The object is to get the cylinder base gasket surface flat. As described in another thread, we turn them using a lathe and typically correct the deck height and bump the compression at the same time. Not everyone has access to a lathe and the surface doesn't have to be flat to a millionth of an inch. The surface plate shown can be purchased from aftermarket parts distributors at a reasonable price and is made to do exactly what is shown. After following the above procedure there is still no guaranty it won't leak but the chances are more in your favor than just pulling the cylinder and slapping a new gasket on.
 

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Spring, I LOVE your tasteful and artful display of "Product Placement!" Nicely done by the way. Question: Are Twinkies as prone to distortion? I know the difference between the big "O" ring and a base plate gasket. In theory, they can distort too.
Thanks!
 

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IronButt
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With a evo many times the best thing to use is a metal base gasket, As well many times the laking is not always from a base that is not true. With the amount of heat needed to get teh cylinder to grow to apply more pressure. The most common cause of leaks on a evo is not allowing them to warm up. And the case pressure that is creat looks for the path of least resistance. So instead of going through breather gear up through the pushrods tubes and out through the head breathers it posp the lower gasket. If is paper it is a easy way for the engine to create a leak. Use of a metal gasket is much better as the gasket will not tear or allow oil to seep through it. And even if they do the wrong thing " no warmup" and is leaks a bit cold it will seal when warm. " worse case deal" But lapping a cylinder that is not tq'd is a issue as we know the cylinder is going to move around when Tq'd. Use of the laths same thing. So really all we can do lap of lathe but still no way to say " you will never have a leak"

I have tried many mnay things to se if they can be stopped 100% I really thing the biggest thing is letting them warm up. On a evo that is cold and they hammer on it , you will have issues.

The T/C o- ring really takes care of this by allowing the cylinder to expand or contract and still stay sealed. Nice improvment over a evo.
 

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Do TC cylinders bases warp? Yes they do, but not as much. The reason is, the TC uses no base gasket.
The Evo base gasket allows for more distortion by its give. The TC metal to metal contact tends hold the distortion down. Just dont see many TCs with leaking cylinder bases
The Evo cylinder bases machined correctly will not leak using a plain dry gasket.
Evo cylinders can be machined to eliminate the base gaskets by being cut for orings, though is better the cut the cases as per TC.
All cylinders that use an iron insert or cast in sleeve in an aluminum body have this problem. After a cylinder has been run and gone through this initial distortion, they tend to stabilize.
Needles to say perhaps,the cylinder bases need to be true before resizing them.
 

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Discussion Starter · #7 ·
Geezer-Glide said:
Spring, I LOVE your tasteful and artful display of "Product Placement!" Nicely done by the way. Question: Are Twinkies as prone to distortion? I know the difference between the big "O" ring and a base plate gasket. In theory, they can distort too.
Thanks!
I try to include it in most pictures now because I found someone using my pictures without permission and representing them as their own. This helps keep that to a minimum or at least gives us credit for the work.



As for the issue of base gasket leaks, this is by no means represented as the "cure all" fix. What I have found is this helps considerably and I had someone asking about this very problem, this is what lead to the post.
 

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You have to factor in a few more things, cylinder and case mating surfaces, the two have conformed through expansion and contraction from fluctuations in engine temperature, stud torque, and mechanical + harmonic vibrations. If you are going to lap the cylinders, ideally it should be done on the case, thats technically what lapping is, you lap valves to the valve seats you lap a check ball into it’s check ball hole/orifice/cavity...unless you have a lapping plate that is compact enough to flip over and lap the case bore hole decks. One more thing, an aviation/marine machinist’s finishing touch, handed down to me, chamfer the head stud holes on the bottom and top of each cylinder, it eliminates malformation where the force from the stretching studs pull up on the case, and bolt flange contact area will not form a raised lip that causes the bolts and surfaces to run out of square when they are re installed and torqued.
ARP and all extreme condition manufacturers employ this extra step just in case, even if there was never an issue, here we have a common issue, ounce of prevention.
All in all, a Very Good Point, most guys would just stick on gaskets and install the jugs, things happen for a reason, use deductive reasoning to determine it, and any other contributing conditions, correct them and not until then should you re-assemble unless you “GOTTA GO BRO!! I JUST CAN’T MISS THIS BEEF & BEER!!”
Then you can just do it right on the next tear down, and hope the issue hasn’t compounded!
Preciate the tip!
 
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