cylinder wall thickness discussion

[bcrazy]

When i was into vw i learned that "slip in" is a "bad" thing, and that it should be avoided.

I am now in the process of assemblying the 1720 cc engine for my Porsche 912. I will use 86mm cast iron cylinders, while stock size is 82.5mm biral.

The wall thickness at the head is 2.9 mm, just lik an 88 mm slip in for vw t1.

So how bad are they?

I did some measuring and found that the wall thickness on the "rest" of the cylinder is about 5.1mm. The 2.9 mm wall thickness is "only" the top 11 mm of the cylinder where it meets with the head. On the fin area it is 5.1 mm (of course without the fins themselves)

Comparable on a stock 85.5 mm t1 cylinder the wall thickness on the fin part of the cylinder is 6.1 mm and the head register part is about 4.3 mm.


So how important is thickness at the head register?
The cylinder gets thicker at just about the height of the top piston ring.

I do not have any 88 mm t1 slip in to measure, but i know that the cast iron part of a stock biral porsche 82.5mm is about the same as the overall thickness on my 86 mm slip in cast iron. Of course the cooling abilities is better with the birals, but the actual wall thickness of cast iron is about the same.

So is the 86 mm all cast iron porsche slip in any better than a t1 88 mmm slip in????

[bugguy076]

I have used 88 slip in's for drag racing and on the street. I have not had a problem with them. Don't get the compression to high and you should be OK.


I think the bad rap they get is from making the C/R too high(if it's even checked) with regular fuel. You know, too much heat.

They are slip in's, but you must adjust the C/R for the added CC's.

[bcrazy]

[bugguy076]

In the drag car, I went up to 10 to 1 C/R. On the street I would not go higher than 8.

[obadvw]

yes i hear people saying the 88 can be good if treated gently


is it true you can run 10:1 CR in a type 1 engine without pinging?!!!!

what grade of fuel?

[saulverde]

Sure, run ethanol for fuel.

[Sigurd]

[bcrazy]

Resurrecting this one..

Has anyone measured the thickness of a 88 mm slip in cylinder vs 88 mm machine in, in the fin area? Meaning about 12 mm down from the top of the cylinder?

For the sake of argument if you where to machine down the outside of a 88 mm machine in cylinder to 93.8 mm ( to fit the stock 1600 head), would this make it «as bad» as ready made 88mm slip in?

How important is the thickness of the top 12 mm of the cylinder if the rest is sufficently thick?

Does the 88 machine in cylinder have thicker cylinder wall all the way?
(I am talking about slip in case/machine in head)

[modok]

No, but kinda yes.
The head stud reliefs for the are probably about the same on all of the AA cylinders.

So, thickness at that point would indeed be inversely related to bigness of bore.

At the top I agree 98mm is excessively large.
96 would be OK,
or 95.5 like the original "machine in" 88

[mark tucker]

I used slip in 90.5's with zero issues.( slip into the machine in 88 holes) old berg stuff. and of coarse totalseals&fine sunnen hone job with plates..

[bcrazy]

just measured a stock 77 mm cylinder. At the sealing surface it has a thickness of 6,5mm, but it narrows down to a thickness of 3.5 mm, before it gets thicker
again at the fin area

Does this suggest that 3,5 mm is what the factory considered enough?
As long as the cylinders seals to the head does the thicker top of the cylinder really make that much of a difference?

[Ohio Tom]

88 slip in's will lose ring seal fast. Sure, it will run fine, but blow-by will increase over time. Your engine will blow oil out everywhere.

So will 92 thin walls..


I won't use either one on a motor. They just don't last.

All depends on the intended use....
Road racing.. forget it..

little trips around town, might be OK.

[modok]

Wall thickness is proportional to bore size
If 77 bore is is 3.5 thick

88 would be 4mm
100mm would be 4.5

And that would be just fine IMO.