Cylinder base shim, how thick is too thick

[jgparker]

OK, I just did the deck height check on my 2.8 project, and I've gotten some surprising results. I did the measurement with the 1mm shims my machine shop provided, and I found a 0.75 mm deck height on both sides. By my calculations, I would need 1.5mm to 1.75mm shims to get a reasonable deck height. Is this too much? Does anyone stock shims that thick? What about stacking?

Also, two questions on the book:

Chapter 5 section 4 details the deck height check after the section on ring installation. Is this order mandatory? It seems like it is much safer to do the measurement without rings, since it will need to be disassembled after the measurement anyway.

Figure 4.7 implies the two compression ring gaps should be lined-up with the wrist pin 180 degrees apart, but the text states not to line-up the gaps with wrist pin. Could you clarify which is best?

[HawgRyder]

I always put the oil ring gap at the top...then space the compression rings at 120deg from that gap.
In the case of a 3 part oil ring....I put the 2 scaper gaps at 10 & 2...with the compression gaps at 4 & 8.
Bob

[kstylianos]

JP,

Just my observations from just measuring my deck heights....

You probably want to check the deck height with the rings installed. w/o the rings, the piston will have the tendany to flop around in the bore, giving you an inacurate reading, even if you are measuring on the wrist pin axis. Just be careful when sliding the cylinder off the piston to not damage the rings.

Figure 4.7 implies the two compression ring gaps should be lined-up with the wrist pin 180 degrees apart, but the text states not to line-up the gaps with wrist pin. Could you clarify which is best

good question!!


Can't answer 'how thick is too thick', but you do need to get your deck heights a little higher.....absolute minimum is 1mm, 1.25-1.5 to be on the safe side.

[cstreit]

I know plenty of racers who run between .8 and 1mm, but of course this will vary depending on plug config, fuel, compression, longevity, etc..

I'm running 1.0mm, .75 is too low. because the piston can move, you may try taking the measurements at each side and averaging.

[Wayne 962]

Hmm, good point. That probably qualifies as an error, or confusing contradiction in the book. In reality, it's not really a huge deal - the important thing is that they are not all lined up. The 120-degree separation from all ring gaps would probably be fine. The rings will constantly rotate around the piston when the engine is running, so this is more like a 'best practices' type of thing...

As for your particular deck height - that does sound excessive. How much machine work was done to the heads and/or the case?

-Wayne
Deck height - checking with the ring installed does prevent the piston from rocking, although you can do the measurement without the ring, and average the amounts from a few measurements to get a semi-accurate reading.

[WERK I]

Does the 2.8 use a head gasket? Don't forget to add the head gasket to your calculations if it does.

[jgparker]

Thanks everyone.

My 2.8's are actually bored-out 2.7 Alusils, reprocessed to Nikasil, so yes, they do have a CE type head gasket, but I do not believe this adds to the stack-up since it is in a recessed grove and the head still contacts the cylinder deck.

Yes, a lot of machining was done. 0.65mm in this rebuild, and probably some more on the previous rebuild.

So, has anyone ever had to use 1.5mm of shims?

[80sc man]

how much was the cylinder work?? I am thinking about getting my cylinders bored and plated with nicasil, but wondering how much it costs.. Did you have to get new pistons too, or are they interchangable??

[jgparker]

I don't remember how much it cost for just the reprocessing of the cylinders to Nikasil, but I'll check the invoice tonigh. I paid ~$2000 to bore to 92mm, the reprocessing, and a set of J&E pistons. My understanding is Alusil and Nikasil pistons and rings are not interchangable.

[Wayne 962]

That's correct - Alusil and Nikasil must be kept separate.

Your cylinder shims *should* be okay - although you may have chain housing clearance problems (may have to machine the chain housings). Also, you might want to think about your chain, and whether it may be too loose (because the heads appear to have moved closer to the block).

-Wayne

[kstylianos]

Alusil-->Nikasil replating through EBS is $150 per cylinder. The JE pistons (Nikasil compatible) are another ~$1000.

JE cut me a little deal since I bought the JE through them as well....$1800 for both the JE and the replating.

[Wayne 962]

I have a set of 3.0/3.2 cylinders on eBay right now that would make excellent candidates for replating if you don't have cores:

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&category=33617&item=2429702431

They are also probably goog enough to be reused again.

-Wayne

[jgparker]

OK, EBS only sells 0.25, 0.5 and 1.0mm shims, and I couldn't find any on PP. I plan to use 1.5mm of shims. So, is it acceptable to stack?

I'm betting this will bring my deck height back close to stock, so I wouldn't be surprised if the chain housings are OK, but I'll definitely check the alignment carefully.

Thanks all,

[emcon5]

Originally posted by Wayne at Pelican Parts
Hmm, good point. That probably qualifies as an error, or confusing contradiction in the book. In reality, it's not really a huge deal - the important thing is that they are not all lined up. The 120-degree separation from all ring gaps would probably be fine. The rings will constantly rotate around the piston when the engine is running, so this is more like a 'best practices' type of thing...

Well, I missed the part in the text about not lining the ring gaps up with the wrist pins, and assembled mine like the picture. Oil ring gap up top, compression rings 180° out, in line with the wrist pin.

I still haven't installed the heads, so Changing them wouln't be that difficult, but I would rather not risk breaking a (not easily replaceable 98mm) ring by messing with them again.

Should I be worried?

Tom

[Wayne 962]

Nope, don't worry...

-Wayne

[billncolo]

I missed the answer to stacking of gaskets.
Also, I assume these copper gaskets are one-time-use only, similar to copper wasshers.

[Wayne 962]

The gaskets shouldn't be reused - it's very easy to damage them. Yes, you can stack gaskets - I like to paste them together with a *very light* coat of the 574...

-Wayne

[Doug Zielke]

Originally posted by Wayne at Pelican Parts
The gaskets shouldn't be reused - it's very easy to damage them. Yes, you can stack gaskets - I like to paste them together with a *very light* coat of the 574...

-Wayne

Although many rebuilders do stack the base gaskets, it is *not* advised by Porsche (Ref: Pg. 47, Porsche 911SC/Turbo 3.3 Tech. Spec. book).

Porsche offers replacement gaskets in both 0.25mm and 0.50mm thicknesses.

I found siingle base gaskets of various thicknesses are also available from parts suppliers like Worldpac.

[Wayne 962]

Someetimes you can't find the right size that you need, and you need to mix and match...

-Wayne

[ChrisBennet]

Competition Engineering also sell different thickness base gaskets.
-Chris