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Posted by: stephenaki Feb 13 2009, 02:04 PM

OK, first off, this motor is so much more complicated than my MG motors! So, I started looking at the piston assembly to the rod and subsequent assembly of the cylinder to the crankcase. It was then that I remembered reading on the forums about "deck height."

My understanding of this concept is somewhat rudimentary but the basics, as I understand it, is that deck height is the space between the highest part of the piston and the top of the cylinder head combustion chamber. Please correct my understanding if I am mistaken.

I have also read through several threads discussing how to measure deck height in order to shim it properly, change geometry, increase/decrease compression....my head hurts.

Since I want to get this thing put together properly I need to know the following.

1. how do I measure deck height (depth gauge is best from what I read).
2. how do I determine "IF" I need shims.

Now I know a lot depends on what I have so here is what I am working with.

1.7L crankcase
2.0L crank and connecting rods (no, I don't know the measurement of the rods. If needed, can I measure without re-opening the case? How do I measure them?)
standard 94mm bus cylinders and piston set (new set, pistons are dished)
1.7L heads that were machined to fit 2.0L piston and cylinder (I checked, they fit)

Rods and crank were balanced prior to my getting them and then the assembly, cylinders, pistons, rods, crank, etc. were balanced as a unit.

The PO had a 1.7L crank and rods with 2.0L jugs and no shims were present when I disassembled the motor.

I am venturing into a completely foreign area and concept for me and can use all the help I can get. I am not after anything spectacular, I just want the motor to run efficiently so I can drive and enjoy the car. I don't plan on racing it. HELP!

Posted by: messix Feb 13 2009, 02:12 PM

just a WAG but with dished slugs as long as they're not sticking up otta the hole you shouldn't have any trouble. dished pistons= low compression.

Posted by: Todd Enlund Feb 13 2009, 02:18 PM

Deck height is the distance from the piston to the "deck" of the cylinder head. The deck would be the flat part of the head where the cylinder seats. You can measure deck height without the head... assemble the piston/cylinder using something rigged between a couple studs as a clamp, rotate to TDC, and use a depth gauge to measure from the head seating surface of the cylinder to the flat area of the piston... if you have dished pistons, ignore the dish, if you have domed pistons, ignore the dome. Measure in several location around the edge to make sure that the piston is square in the bore.

Posted by: Todd Enlund Feb 13 2009, 02:20 PM

QUOTE(messix @ Feb 13 2009, 12:12 PM)

Posted by: Dr Evil Feb 13 2009, 03:55 PM

Some good general info here. Its corvair, but relevant.

Deck height is towards the bottom of the page.
http://home.hiwaay.net/~langford/corvair/trial_assy.html

CCing the chamber volumes is towards the middle of this page. Also shown is deack height and there is a link to an Excel spread sheet that will help you calculate CR and shims and stuff.
http://home.hiwaay.net/~langford/corvair/valvejob.html


You can easily CC the dishes on the pistons to help with the chamber volume. This is also provided for on the pages I referenced. If dished and you want a specific CR then I would recommend CCing at least one chamber and dished piston. Be greatful you dont have domed pistons as that is a bit more work. There is a good explanation of deck height and calculation in the second page as well.

AS for putting it close to the top of the cylinder, be careful and research this. Type 1 engines in particular as supposed to have like .060" or something close to keep from getting too close to the valves. (not actual measurements, only meant to be an example of concept)

Posted by: Joe Ricard Feb 13 2009, 07:39 PM

Aim at somewhere around .040 to .050 and the closer you can get each cylinder the better.

My last engine came out to less the .oo1 variance for all cylinders.

Yes it counts.

Posted by: Cap'n Krusty Feb 13 2009, 08:59 PM

Maybe it's just me and my machinists, but we've always measured "deck height" at the top of the cylinder. Same as water cooled inline engines (and Vs, too).

The Cap'n

BTW, I just built one of those 2.0 bus pistons with 1.7 heads motors and it worked GREAT.

Posted by: SLITS Feb 13 2009, 09:10 PM

Measure two places, in a vertical plane, 180 degrees apart to make sure the piston is straight in the bore.

I used to install one old ring in the top land for mockup purposes before final assembly.

Compression ratio is then calculated from the volumes of the deck height, piston dish, gasket crush (if used) and combustion chamber.

Posted by: MrKona Feb 13 2009, 10:52 PM

I've used this method from http://www.competitioneng.com/DeckHeight.htm. Easy enough after reading the article a few times.

Posted by: stephenaki Feb 14 2009, 10:20 AM

Thanks all, makes a lot more sense now. I am assuming that I need to install the cylinder with the gasket ring in place prior to measuring deck height, yes?

As I read through both the threads Mike posted and the one Bryan posted I thought about the tear down and realized that there were no head gaskets OR gasket rings in the engine when I tore her down.

Given the fact that several people have advised against using gaskets in certain areas that you get a gasket for in the kits, why WOULDN'T you use either the ring gasket or head gasket? Or is this the case of the machinist PO prior to the one I bought the car from had no idea what he was doing?

Posted by: Vacca Rabite Feb 14 2009, 11:18 AM

The ring gaskets (the ones between the cylinder and the block) are used for spacing the cylinder to get the correct deck height. You want your deck height to be the same between all of your cylinders or you will have poor compression and head leaks.

The head gasket... This is a whole can of worms with really smart folks on both sides. On one side is the VW tech notice saying not to use them on Bus motors. Given that bus motors are pretty similar to ours, and they do provide one extra surface to leak from, lots of guys choose not to use them.

However, lots of guys DO use them, and think that they are just fine and needed. They provide spacing that is critical for finding your proper compression ratio. And if you choose not to use them, you MUST have your heads machined to be perfectly flat against the cylinders. The head gasket was made to deform a little to make up for minute differences in shape between the head and cylinder.

So, there you go.

Personally, I did not use head gaskets when I made my motor, but I also was not making a stock motor.

Zach

Posted by: Dr Evil Feb 14 2009, 11:29 AM

I have also heard both ways, use no use. I have heard recommended that one lap their cylinders to their heads lightly to make the best seal possible. This has to be done correctly, squarely so you dont end up with a cocked surface.

Posted by: stephenaki Feb 14 2009, 12:07 PM

Thanks guys, I will assemble without the ring gasket first and check deck height and measure the thickness of the ring gauges and see where that will put me when installed.

Will also have a closer look at the heads and see how flat they are, I may install the gasket just based on the fact that I don't have a whole lot of confidence with the machinist PO's work. I didn't know the guys so don't know how anal retentive he was when doing machining work.

I recall reading a thread on the proper deck height for 2.0 motors, will look that up again to make the comparison.