TheSamba.com :: View topic - Newbie Piston Cylinder Head Build Question?

Wed Feb 14, 2024 9:59 am

Hello again, just me asking irrelevant questions... :)

I'm going to make an assumption. I assume that all rods/pistons will be exactly the same distance from the crank centerline, or at least should be. Correct?

Next, how accurate are the cylinder lengths?

What I'm driving at is this, I've read several threads about deck height and how "sometimes" the deck height from cylinder to cylinder may vary a few .000's. The heads of course are solid one piece and from head seal to the other head seal is fixed, non-adjustable.

So, when I'm indicating in my deck height and I'm off a few .000's between two side by side cylinders, does that mean that one cylinder's top sealing surface is further away from the crank centerline?

If so, how much is too much? How does the head align to two different cylinder heights? Are there thin enough base shims to get the deck height dead nuts? How do you know if the cylinders are truly the same exact length? Do people measure the cylinders, or do we just assume that they're all the same length? If they aren't the same exact length, hardly no one could be building motors without a full-on machine shop. I never see build videos of guys measuring their cylinders.

I know, I know, WAY too many questions. But even at my extended age, I tend to question things that may not need to be questioned. I'm just worried that when I'm putting my new motor together that I end up with good head to cylinder sealing and the heads do NOT rock from cylinder to cylinder.

Thoughts? Or, just tell me to keep my thoughts to myself... :)

Dan

Wed Feb 14, 2024 10:08 am

Dan_Lockwood wrote: I assume that all rods/pistons will be exactly the same distance from the crank centerline, or at least should be. Correct?

Dan
You can not assume.

Stock VW rods a 5.394", aftermarket ones are 5.4/5.5/5.6/5.7. Pistons come with different pin heights, the distance from the centerline of the wrist pin to the top of the head. There are A/B/C heights, someone will post them.

If you have a difference from side to side, you should have the case "decked".

Wed Feb 14, 2024 10:22 am

Yes, you need to measure the cylinder lengths, but lately, they have been pretty good out of the box. I mark them all, and if they are off, you can group them together on one side. I also make sure to set them in the case, and then use a good straight edge across the tops. The straight edge should have zero gaps across the tops of both cylinders.
You also need to make sure the cylinders sit perfectly flat on the deck surface. Zero rocking or movement. If they are rocking, stop and fix the problem. Case savers, casting flash, or a burr on the case are usually the problem.

If the cylinders are NOT flat across the tops, they wont seal.

Deck height differences can be from different stroke length on the crank, different rod lengths, different pin heights, different case deck heights, offset crank center line, etc. And it can all stack up. A .0005" difference in all the parts can be a .003" difference in deck heights.

You can usually work your way out of it by swapping parts around. If its more than about .008" side to side, you can have the taller deck side cut again to even it up. I usually try to get all four holes within .005" deck on engines with rebuild crank and rods, and an old case. All new parts engines should be around .003" variance on all decks.

Brian

Wed Feb 14, 2024 10:48 am

The crank centerline is not always machined parallel to the case halves mating surfaces. One end of the crank centerline can be a couple thou off the axis in any direction.

I have never built an engine where all 4 deck heights were exactly the same. They are always a few thou difference.

If you are that anal you can simply shave a few thou off the top of the piston so all 4 will match.

Wed Feb 14, 2024 1:49 pm

The dimensions of all parts can vary.
You can find out pretty quick if you get some good dial calipers and learn how to use them.
I will admit the case is most tricky to check, but everything else can be checked quick. If you have old rebuilt rods don't be surprised if they vary in length .005 or more, and pistons can vary a bit one brand to another, and stroke can be off if the crank was reground.

Wed Feb 14, 2024 1:59 pm

In a perfect world, everything would be the same dimension. However in practice, there's 5+ parts that determine the deck height (crank, rod, piston, cylinder, and case) and even with a relatively precise tolerance of +/- .001" on each part, the tolerances can stack up and potentially produce a variance of .010" between cylinders.

Best practice is to measure everything as accurately as you can, and if you want to get it perfect, spend a few hours swapping parts around and trying every combination until you find the least variance.

At the end of the day though, the most important thing is that the top of the cylinders on each side are flat with each other.

Wed Feb 14, 2024 2:23 pm

Dan, I think it's a fair question for us new guys. I have had so many oh sh!t moments that turned out to be nothing because there is a lot to juggle at times and if you're used to working at any level of standards it can seem like a mess. I think having each cylinder its own separate entity makes things a little weird compared to other engine builds.

Wed Feb 14, 2024 3:49 pm

Dan_Lockwood wrote: Hello again, just me asking irrelevant questions... :)

So, when I'm indicating in my deck height and I'm off a few .000's between two side by side cylinders, does that mean that one cylinder's top sealing surface is further away from the crank centerline?

Dan

A straight edge across your adjoining cylinders will tell you if they are sitting level with each other and straight in the case. Once you've determined the cylinders and cylinder seating surfaces are good you you know the deck height variance is coming from somewhere else. I had a welded German crank with number 3 journal 0.09mm longer than the other 3, chased my tail for ages before finding the problem with the crank.

Wed Feb 14, 2024 5:55 pm

The cylinders are usually pretty good, within .0005" in a set.
Or at least, they have been in the past, in the future it could all go south.
New fancier aftermarket rods are usually very good for length, a good 5.5 rod is really 5.500 long, within a thou or two.

AA pistons are a bit random. I don't recommend "trusting" AA pistons when it comes to the dimensions and clearances of the piston and the piston pin and the rings, but the pistons are basically good.

in the old days a piston and cylinder set was matched components, as far as, you could trust the rings and the piston and the cylinder and pin all fit eachother, but these days they are often separate parts just combined into the same box and the end user gets to find out if they fit.
the pin not fitting the piston is a more common problem all the time in general, with some companies, even JE or Mahle, but UEM is still absolutely nailing it every time, so that's good.

Thu Feb 15, 2024 6:48 am

Some pics of checking cyl. heights for equal and length.
Checking decks for evenness (flat) and square to split, they should be very close to ensure seal at head, a little bit off here multiplies fast by the time you go up 3 in. to head. If case decks are tilted to rear of case a bit and head decks (sealing surface) are tilted to the bottom of case a bit... you can imagine what happens. Flat is flat!
As said, keep your cylinder sides,1-2 and 3-4 equal, crank bore not always centered in split and sometimes not parallel.