Head CC kit question plus deck height question

[DjMatio]

First question:

I bought a compression checker kit for my 90.5mm pistons, the from rocky mountain with the circular plastic disks and the syringe.

How do I get these to fit inside the head? The diameter is too large to fit inside the bore on the head. I'm not sure why this is, the sticker on the package says it's the right size kit.

Second question:

After measuring deck height, I don't have to check the CC's of the area in the combustion chamber from the top of the piston to the top of the cylinder, do I?

thanks. I got about .056" of deck height all around with no shims. I'll start with that and work my way to 7.5:1 compression. I forget how long my rods are, either 5.325 or 5.394...damn

[Guest]

what heads do you have, have you had them cut to fit the 90.5 cylinders?

you don't have to check the cc's manually. calculate the deck height cc by using the formula pi x (cylinder diameter/2)^2 x deck height. don't forget to keep the units consistent.

stock vw rods are 5.394.

[DjMatio]

they are cut to fit the 90.5 already. know cause i've used the heads already. I got them ported and polished.

[turboblue]

Sounds like you got the plexiglass disc for the 94's. The 94 cylinder measures about 3.980 at the cylinder head end. If it is too big, use a sander to reduce the OD to fit your heads. It isn't critical to get it perfectly round, just as long as it covers the combustion chamber. I use a thin film of grease around the edge to seal it and some light weight fluid to measure the cc's.
If you need help with the calculations, post back the head cc's, deck heighth and engine size and we can give you the numbers.

[Marc]

Sounds like you got the plexiglass disc for the 94's...

Either that, or your 90.5s are the old-school variety which are 1mm smaller at the top than 92/late 90.5. If so, you need a disk made for late machine-88 size (97mm, ~3.8")
.060" deck is 9.803cc with a 90.5 bore, you can simply ratio what you have to that to get your volume - (56/60)/9.803 ≈ 9.15cc

[DjMatio]

Thanks guys. I will post my findings shortly so I can keep rechecking my calculationss with you guys.

I've used a feeler gauge and the bolt method. I kept getting bad number because when i tightened the rods nuts and torqued down the deck height guage, it would bend slightly and be higher in the middle than on the edges. I didn't even torque them down much. just tightened them by hand slightly. wierd.

[turboblue]

Rod nuts?? You mean head studs? I use a piece of 3/4" square aluminum and bolt it diagonally across two head studs. Then a depth micrometer at the edge of the cylinder in line with the wrist pin. That gives a pretty accurate measurement of deck.
A good dial caliper will suffice too.

[DjMatio]

yea.. head studs. oops. i got mixed up there for a sec.

[57Drag]

http://home.earthlink.net/~jalby/puter.htm

[DjMatio]

I just finished measuring the deck height and head CC's. When my motor died, I had to sand the tops of pistons #3 and 4 because of pits and nicks caused by something getting into the combustion chamber (probably a valve guide). I also had to purchase a new head, the same type and brand as the other of course.

So here are the numbers... tell me if they make sense.

Deck height cylinder #1: .057"
deck height cylinder #2: .055"
deck height cylinder #3: .056"
deck height cylinder #4: .0515"

Head CC's =========

Old head:
chamber 1: 57cc
chamber 2: 57cc

New Head
chamber 1: 55cc
chamber 2: 52cc

As you can see one head has less volume than the other. What am I to do, and how will this effect my putting the engine together? I'm looking for 7.5:1 compression. I'm running Kadrons now, but have a K26 turbo on hand for when i get the fuel system ready. Thanks

[fredybear]

90.5s x 69 id guess

55 ccs

.075-6 deck

7.6:1


the new heads should b 55 check again with marvel mistery oil good stuff for that...

than ya need a .020 shim to bump it up

[Marc]

Has the 57cc head had a valve job done that increased its size or did it just come that way? Ideally all four chambers should be as close to identical as possible, which'll require some combination of reshaping and flycutting to achieve and you may not be that anal about it (for a mild engine it really isn't going to make much difference anyway provided the C.R. comes out fairly even).

3cc difference between the chambers in one head is too much. You didn't say which is the smaller one - if you put the new head on the left will the 55cc chamber be on #4? If so, here's my recommendation (a pragmatic solution involving the least work, not going to get into the pros & cons of "tight deck" right now):
First, you'll have to open up the 52cc chamber to ~54.25
You'll need shims for the left that are .017" thicker than the ones for the right to keep the C.R. even; for 7.5:1 that'll mean .030" on the left and .013" on the right.

If the smaller chamber is on the left as you look at the head from the bottom, that head should go on 1/2 and the 57cc head should go on 3/4.
Left side will need .015" shims, right will need .027" for 7.5:1 average.

Obviously shims aren't sold in these exact thicknesses but it's somewhere to start - say you had .010" and .030" for the first scenario, the RH chambers would only need to be opened up by half a cc to compensate.

[DjMatio]

What I also don't understand is if I had sanded off the tops of pistons #3 and 4, because of the pits and nicks, how could the deck height on those pistons be lower than the untouched pistons??? I checked this a few times, and made sure each piston was a TOD.

ex.. piston with deck height .0515" was ground down the most! wtf did I have a brain fart?

[Marc]

Assuming the case is decked evenly and the crank is exactly centered & straight in the case, the crank is exactly indexed with the same stroke on all throws, the rods are all exactly the same length and the compression height of the pistons and installed heights of the barrels are identical, the deck heights should all be the same - but you seldom encounter such a perfect scenario unless the parts have been blueprinted by a machinist, the stackup of real-world acceptable tolerances can easily cause a spread like you have. That's why shims are made
Buggsy's tip over in the Forced Induction forum is a good one, make sure that the cylinders are all even across the top when snugged down into the case (you may need to use different-thickness shims under two adjacent jugs to achieve this if the case wasn't decked evenly or the barrels differ).
A long straightedge placed over the cylinders at any point/angle should make contact at four places; the head can conform to slight imperfection but I'd use .002" as the maximum acceptable variation. You can probably get away with more by using copper head gaskets but that seems a little sleazy to me.