Installing Cylinder Assemblies?

[orangebug60guy]

I've been deep into a top end rebuild on an old 1800cc type 4 engine that had a bad 1&2 cylinder head that damaged the number 2 cylinder.

So new cylinder assembly and heads are going on it. I've been reading the book by Tom Wilson and the Bently. I have a couple of gray areas I need clarified for me.
Part 1:
I purchased a set of these NOS NPR 93p NPR cylinders and pistons

When I go to check the "DECK HEIGHT" do I measure from the domed or flat part of the piston.


Part 2:
I have a few questions about installing cylinders.
My gasket kit came with what looks like 3 different sets of cylinder "shims" or metal gaskets. One set is in cardboard, the other two in plastic and one set was bent in shipping....

I think I should use this set between the case and cylinders -

Does that seem right?

[vwjetboat]

you can not check deck height till after you have mounted them..you can put one piston on then slide cylinder on.. then rotate crank till piston comes to highest point.. then put a strait edge across cylinder and use feeler gauge to check distance.. and use the right shim to get where you need

[busman78]

Measure from the flat to get deck height. CC your heads to get there volume. On line you should be able to find what those piston dome takes from the chamber volume or calculate it out yourself. Once yo have all of that there are several sites that can calculate your compression, from that you can leave it as is bolt it back together or lower the compression by use of base shims, they will be the smaller diameter of the shims you have. Do not use the aluminum gasket between the head and cylinder, do all adjustment from the base.

Nice set of pistons, NPR made some nice VW P&L's

Appears that engine has been running very rich.

[Wildthings]

Get yourself a piece of 1x1" tubing or something like 1/2x3/4 bar stock and drill two holes in it to match two of the opposing head stud, install the bar you made using pipe to make spacers for the studs so that you can tighten the bar against the cylinders down using the stock head nuts. Now with the piston at TDC use a feeler gauge to determine how close the flat of the piston comes to the bar. You want something around 0.040". More than 0.060" will kill your efficiency and less than around 0.025 will lead to the piston hitting the head when the engine is revved up.

Do not use the head gasket "shim" but shim between the block and the cylinders if need be to get the deck height right.

[orangebug60guy]

[orangebug60guy]

[Max Welton]

Reread what Wildthings posted. Measure to the flat part of the piston top, not the top of the dome.

Max
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[TDCTDI]

From those measurements, it sounds like the piston set might be for a shorter stroke/con rod length (maybe for a 1.7?). Compare the wrist pin height between the old & new pistons, this might be the reason why this piston/cylinders need set sat on a shelf so long.
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[busman78]

Measure from the flat of the piston to the cylinder rim, not from the dome.

[busman78]

I looked up the part number for that set, it lists for 74-75 1800cc Type 2 Bus and Porsche 914. Pin height is probably correct, just not measuring deck height at the right location.

[Wildthings]

Sorry I missed the "domed" part.

With domed pistons, you will have to come up with a more complicated jig to measure the deck height, one with a cutout for the dome.

With domed pistons, you should test for valve to piston clearance with Playdoh if you are running anything much more aggressive than a stock cam.

With a stock cam you need to be very aware of the possibility of engine knock and may find you need to run premium fuel.

[SGKent]

when you calculate compression you will need to take the domes into consideration. Your engine will be pushing a bus around not a light Porsche so it may not last long if the compression ratio is too high.

Once you think you have the correct deck height at the wrist pins on the flat area, (you cannot add up shims to get that, you can only use one shim as they come in different thicknesses) then you can lay a disk of modeling clay across the domes, bolt and torque both the heads on, check clearances and for interference from the heads and valves. You'll also need to adjust the valves to be sure you really see if there is interference.
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[orangebug60guy]

I will check again tomorrow with only one shim on the "flat" part of the piston. Thanks guys. Sorry for not being specific enough before/my own ignorance to read a post.
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[busman78]

It is actually easy, those domes have been around for thirty plus years, google them, the 1700 NPR ones I had displaced 7cc. But for fun make a putty impression then cc it.

Done this before, the flat should come in at .024 to .025, with the VW head gasket which crushes to .009 to .010 that would give a deck of .034 to .035, with a stock 1.8 head (never molested) in theory your compression would be 8.2:1 which is a nice compression range for a bus. You scrap the VW head gasket put in a .010 steel barrel shim and life will be good. Again that is from several years ago, always measure, calculate then verify.

Correction: Need to clarify the above, this was for a set of US Spec 914 dish piston, they were NPR. Also works for most stock, again always measure, calculate then verify.

[raygreenwood]

Domes are pretty easy to work with once you know how.

As others have stated....measure from the "land" area or ring around the dome ...not from the dome.

Also...as noted...calculate the dome first. How to do that?

1. first get a caliper and measure the dome...just the major diameter where the base of the dome meets the top of the piston. You can calculate the angles and exact volume of the slope on the sides of the dome...but the difference is very slight...like about 1/10th of a CC at most. keep it simple.

2. Round that off in your mind to a "square edge"

3. Measure the height of the dome over the surrounding ring or flat land area around the dome. You can do this close enough with the piston out of the cylinder with a machined straight edge and caliper. I would not have the pistons in the bore yet...because you will just need to pull them out to shim the barrels.

4. So now you have a round "disc" with square edges. You know the height and diameter.

5. Use the standard volume for a cylinder calculation to find teh VOLUME of that disc.

6. take that "disc" volume and write it down.....and NOW...calculate the compression of the engine using as if the piston is a FLAT TOP PISTON.

7. Take that "disc"/dome volume...and either subtract that volume from the "deck volume" you calculated for the flap top piston or subtract it from combustion chamber volume. That makes no difference which you choose...it still subtracts volume from the combined total volume of deck and chamber.

I have seen far too many mechanics make this mistake with both 1.7L and 1.8L domed pistons in type 4 engines.

YOUR ACTUAL DECK HEIGHT MEASUREMENT FROM THE TOP OF THE CYLINDER TO THE FLAT PISTON EDGE THAT SURROUNDS THE DOME...WILL BE NOTHING LIKE THE DECK ON NORMAL PISTONS.

I shouted that to make sure you get it. We typically speak of "nice tight deck" on high compression engines as being....less than .050"....about .045" very good...and a minimum of .035".

!!!!!! The domed pistons on type 4 for both 1.7L and 1.8L...had a factory deck to the outer edge of the piston of about .075" average. The tightest I have found are about .068" and the loosest were at .079"....about .073" to .075" are where most are.

This is because the compression is augmented by the DOME subtracting volume from the deck and combustion chamber and NOT from a tight deck from the piston edge.

If you try to shim the type 4 domed pistons at the cylinder base to achieve a deck like .035" to .040"...you can easily end up with about 8.3 to 8.5:1 compression.

Some examples...from 1.7L mind you....is that the 5mm dished (IIRC) pistons...produced about 7.8:1 compression. The rare flat tops in 1.7L...produced right at 8.0:1 compression. The stock domes...produced 8.2:1 compression.

ALL OF THESE THREE COMBINATIONS HAVE THE SAME ACTUAL DECK HEIGHT AT THE PISTON EDGE...OF .075"

EDIT:

Sorry...I forgot to convert from cubic inches back to CC's in the last picture so I corrected it.

Ray

[busman78]

Nice write up Ray, did an edit on my earlier post clarifying that was for a set of US Spec 914 dished NPR pistons, also works for stock.

The dome setups I did were close to twenty years ago and I do not remember what the initial deck was. When you have a one pound+ hunk of putty in the shop, making a impression works real well, also handy when the limit of compression is being sought to verify the valves play nice with the piston.

Even for stock engines you have to measure, calculate then verify for that will eliminate surprises later on.

[raygreenwood]

[Wildthings]

[raygreenwood]