kbb6644

I will be overhauling my 1985.5 944 NA motor. It has 350k miles on it and I wanted to know information about the cylinder bores. Can they be bored oversized? What does the factory do to the bores to prep them for the pistons? If they are plated, what type of plating? Can they be re-plated and honed to accept new rings? Anyone in the Seattle area capable of reworking the block?

Thanks,
Keith

87 944 C

the original engine was aluminum with a silicon layer. the pistons have iron skirts to help not wear the cylinder bore too much.

if you bore the factory oversize, you have 2 options; 1: line it with a cast iron layer to bring it back into factory diameter 2: buy some oversized pistons and have the cylinders bored out to fit the new pistons in them.

any decent engine performance shop should be more than capable of relining or boring thr cylinders to match the pistons.

daveo90s4

Kolbenshmidt (sp?) have on the internet a technical document for what was / should be done. Google that name plus Alusil and you'll probably find it. If not email me / PM me your email address and I'll send it to you.

kbb6644

Thanks 87 944 C & daveo90s4, I hope to rebuild it and get another 350k out of it. It is still running pretty dependably. Thinking about pushing it to 400k before tearing it down. Collecting parts and doing my research. If this motor can go 350k on stock specs, I'd like to refresh the motor with new OEM parts and try for another 350 -400.

V2Rocket

http://www.uschrome.com/

SeaCay

This continues to be one of the most misunderstood areas of the 944 line...

The silica in the block/bore is NOT a layer that can be worn or machined off, the silica is integral to the casting of the block and using the correct machining techniques will result in a factory fresh (if not larger by .5mm or so) finish. The correct procedure for machining the bores is clearly spelled out in the factory service manuals, but a very quick guide would be: Machine bore to just under final dimension using standard boring tool, then use special felt pads with cutting compound that is designed to "mill" away the aluminum but not the silica embedded in the block to get the final bore diameter and condition.

What this does is essentially polishes the aluminum away, exposing the silica, which the piston and rings "ride" on.

Again, the silica is cast throughout the engine block, it is NOT just a coating in the cylinders.

Jfrahm

It is way cheaper to buy a good used motor than to buy oversized pistons or have the bores sleeved, and as you have observed these engines last a long time.

As I understand it (and correct me if I am wrong) the pistons are expensive partly due to the tin coating that allows the piston to 'wear in' with the bore without scuffing. Uncoated pistons may gall in the bores and bad things happen. After boring, the bores are 'honed' with a caustic paste that etches away some of the aluminum and exposes a greater percentage of the silica in the block material. There is still a lot of aluminum exposed though. The new piston transfers a sort of lubricating metal to this composite surface, preventing the aluminum on aluminum galling. Once the surfaces have worn in a bit I do not think it matters if this material has mostly gone out with the drain oil but it is critical at first.

So a set of new oversize pistons, boring, honing, caustic lapping, and assembly is a pretty expensive process. You could probably buy a couple of used 924s or 944 cars and pick out the best motor to go forward with for the cost, and with minor refreshing the bottom ends are likely ready to go 150K farther.

Cheers,
-Joel.

V2Rocket

lindsey will do it for $600 or less depending on how out of shape they are.
http://www.lindseyracing.com/LR/Parts/944MACHINE.html