RennBod

Just contemplating buying a used tired engine as a spare for my 944 and rebuilding it... my 944 has about 160k miles on it, and I think its past its best, so want to take my time getting a spare engine together ready to go in.

In the past, I have rebuild conventional iron block engines, where if the wear isnt too bad you can hone the bores, reuse the pistons and use new rings... but I am a bit lost about the aluminium bores in the 944... I understand that they are supposed to have a special process used instead of honing, which etches away the aluminium and leaves silicone/silicon/silica(?) sitting proud, which helps the rings bed in like a hone on an iron block...

But what the hell do you do on a 944??? is it ok to just fit new rings, or should something be done to the bores to prepare them for the new rings...?

Charlie

I rebuilt my engine with 145K miles. Micrometer wise the crank was fine. I put new main and rod bearings in. Installed new rings, did nothing to the cylinders, and so far everything if fine.

nine-44

Mine has almost 324K on the clock and doing well. I've heard if the bores look clean and check out# wise then throw rings in it and go. I'd do bearings as well.

David Floyd

If bores are in good shape, new rings and go, if not then here is some info for refinishing bores

http://www.msi-motor-service.com/dow...ren/alu_en.pdf

RennBod

This is music to my ears! well... as long as whatever block I use ends up having bores that check out ok..

I was worrying about having to try and find a machine shop that could do the "special etchy aluminium" thang to the bores.

Out of interest, apart from re-sealing, does anything have to be done to the balance shafts during a rebuild?? new bearings?

nine-44

They do have replacable bearing shell on the balance housings, the nose cone housings have a full circle bearing that needs to be pressed. I've always found them to be ok and let them ride.

With the exception of one that was badly overheated and DESTROYED the #4 cyl wall and piston along with cooked oil inside, I mean nasty. On this motor, the bearings onthe upper balance shaft looked very bad, I had to hammer the front housing off it was grooved so badly.

RennBod

David Floyd,

That is one interesting document!!!

Do you know of anyone who has tried the Alusil liners for our engines?? Would they work out cheaper than getting oversize pistons? Are there other advantages to them (lower oil consumption??)

Is it possible to get felt pads for a DIY cylinder hone (like a drill mount one?) and use the paste they mention for stage 4 of the process?

This has been one hell of a learning day for me.

500

Hey, this is an interesting thread. I'm going to be taking my pistons out soon too. One question I had was in typical engine re-builds, it is necessary to ream the wear ridge on the top of the cylinders before removing the pistons, as the rings will hang up on the ridge. Is this the typical practice for the 944's cylinders? I don't have my head off yet, so I can't say how much of a ridge there is, but I am curious about the experience of others.

Thanks!

Allan

nine-44

Usually, I've found there not to be much of a ridge, I even had a badly overheated 951 engine that the wall had over 1-2mm deep gouge in the #4 wall. All the pistons came out fine.

Zero10

I plan to re-ring my 951 engine, providing the cylinders check out. So this thread has been pretty helpful so far. Keep the information flowing!

I was wondering if it would be alright to just re-ring, replace bearings, re-seal and go or whether I would need to have the block taken to a shop with a sonnen machine to be serviced before re-ringing, and apparently I don't need to.

What is the worst ridge you guys have run into, and what have you done about it? Have you just reamed it, since the piston and rings don't make it up that far?...

Would it help if the pistons came out the bottom?

joseph mitro

the advice i got was not to hone the walls. i did not (126K miles at time of rebuild) and the engine has been fine. 350 miles on it now.

David Floyd

That manual covers several different engine brands, our cars do not have liners as the V8 in the manual.

The hone is not a DIY project.

If your cylinders are ok then you just install new rings.

RennBod

David,

What I meant by liners, was that in the manual it mentioned it being possible to bore out the block and fit special liners with something like a 1mm wall thickness. I was just wondering if anyone had tried these on the 944.. I think the link above even mentioned these retrofit liners for the 944 block.

Its just they may be an option for engines with a score in the bore... perhaps not so important at the moment, but perhaps a savior for 944's in the future, perhaps when junked 944's become harder to find for an alternative block.

RennBod

Also, although the "hone" isnt a viable DIY project, would the 4th stage of polishing the bores with felt pads as outlined in the manual be possible as a DIY project, using the compound, felt pads and a standard DIY hone be possible? the manual does not make this stage out to be highly critical, even mentioning that no desernable material would be removed (or at least it would not effect the bore) kinda implying that its just minor polish to remove the aluminium between the silica/silicone particles.. which may mean that pressure or precision does not look like a real big deal..

I might be wrong, but it may be worth a bit of an experiment, if I could find somewhere that sells the compound and some felt pads, it may be a viable stage of the engine build, a bit like a hone on a conventional iron block, that will help the rings bed in??

Danno

"What I meant by liners, was that in the manual it mentioned it being possible to bore out the block and fit special liners with something like a 1mm wall thickness. I was just wondering if anyone had tried these on the 944.. I think the link above even mentioned these retrofit liners for the 944 block."

First of all, the stock bores are already Alusil. It would make no sense to over bore out the Alusil cylinders and press in a Alusil liner (unless to repair damage). The entire block and cylinders on our engines are made of Alusil, a hypereutectic aluminium alloy. This means there's more silicon than can be dissolved in aluminium in the casting process. Upon cooling the extra silicon percipitates out as crystals. Our engines are actually a MMC.

The 4th stage lapping process uses find abrasives to wear away the aluminium between these silicon crystals. This leaves a hard wear-resistant surface. This surface is porous and rough on a microscopic level, thus requiring the use of iron-coated pistons to resist galling and wear.

The only time you'd need to worry about this process is if you have cylinder-wear that's out of spec. Such as ovalized or tapered cylinders. In which case, you can bore out the cylinders and 1st oversize pistons (about U$1800 a set). Most people find it more economical to overbore and install iron or steel liners and use cheaper standard pistons (U$500-750 a set).

If you do go with this process, you have to find a shop that can do it. The press-fit of the liner to cylinder is highly critical and done at elevated temperatures. The cylinders have to be bored to a dimension just slightly smaller than the OD of the liners. The exact amount depends upon the material and thickness of the liner. I've had a block prepared with steel liners crack all the way through the #4 cylinder and liner in less than 4000 miles. This was due to them not boring out each cylinder specifically for the liner that's going in. They just bored all the cylinders to the same dimensions and pressed the liners in. The cylinder#4 happened to get the largest liner and it stressed the cylinder just a little too much. After 4000 miles, it cracked and that was it.

Also critical after installing the liners is to have it align-bored to be centered over the crank-journal. It should be bored to fit each individual piston. A taper bore works best that has minimum clearances up top that relaxes as it goes down to reduce stress and wear on the rings. Use the dimensions in the manual for piston clearance and how much material to remove in each of the 4 steps. This requires a Sunnen hone for the required precision.

Then the final step#4 you can do yourself with felt pads and a hand-held drill.