chilibluepepper

Following Jeremy's lead, does anybody else have input regarding clearances. Could this be the culprit?

Something I did, or didn't do precipitated the problem, so something has to be done differently. Although I would like to farm the repair and reassembly of the short block out to a reliable entity, and be done with it, reality and budget may force me to get a little more involved. Therefore, its time to get re-educated.

special tool

Do you have a pic of the rods?

chilibluepepper

I don't have any pictures of the rods in their current condition, whatever that may be. I have not taken the engine out yet, and it will probably take a week before that happens. But here are pictures of them before assembly.

special tool

Oh, yeah I'd love to see them whenever you remove them if you had a chance.

Jeremy Himsel

Is there cylinder wall scuffing (discoloration) or is there scoring that you can feel that may need to be repaired. I'm wondering if what you're seeing is residual ring material from the break-in. Was there a light coat of oil on the rings/cylinder wall or were they installed dry?

Ski

Myself and Jeremy were discussing clearances a couple of weeks ago. I had pretty much decided to go with .0254mm/.001" or maybe just a tad more, like .032mm/.0012".

The new forgings (slipper skirt design) have a bit more expansion rate is the reason Mahle suggested the new clearance and at first I went like, that's double. .038mm/.0015" is what the Tom C Arias run at over 101mm, under 101mm, it's .001"

evil 944t

Ski, I run mine at .0012"

chilibluepepper

Wrong choice of words on my part - it is actual scoring that can be felt with your fingers and nails. It is towards the bottom of the cylinder bores, perpendicular the the center line of the engine. I was not in picture taking mode at the time, but I'll take and post pictures of the carnage when I get the engine out. The rings had been soaked in oil, and the cylinder walls had also been coated with oil.

Well, the good news is I fouled the bores with .020 mm clearance, so I may still have some wiggle room to repair, while retaining an acceptable, or perhaps preferable clearance.

gpr8er

Wondering how this turned out?

My motor is at the machine shop right now with the Mahle/Andial 104.5mm pistons. I'm sweatin piston/cylinder clearances. Mahle told me via phone to measure .5" up from the skirt bottom and finish size is +.0005". My machinist is telling me don't go less than .001" After reading this thread I'm spooked.

The piston skirts are now coated with Mahle's FerroTec coating, not sure if that was the case back then.

Opinions please

Jeremy Himsel

Well I don't think you'll find a solid answer on this because there are a lot of variables. The original pistons on our cars used Ferrostan (SP?) coating which is no longer in use and I believe has been outlawed for environmental concerns. The clearances were set by Porsche using this coating about 30 years ago. I’m assuming you have the German Mahle pistons as opposed to the slipper skirt pistons made by Mahle NA. I’m not sure if there is a difference in the coatings that are used between the German and US manufacturing plants but I know the coating being used on the original pistons are NLA.

I do know that Andial, who were very good many…many years ago, are somewhat out of the R/D aspect business for our cars and are simply regurgitating a clearance for what they’ve always used on our cars. I’m not saying they’re wrong but I also bought my 104.5 pistons from them and when I questioned the clearances, thier best answer was “that’s what the Porsche spec has been for 25 years”. They had no idea about what aluminum or coating were being used on the current pistons.

When I bought my pistons in 2006 there were 4 sets available. Two of those sets were installed in motors that burned up very quickly (not mine). While I don’t think it was proven that it was a clearance issue it was very suspect. The clearances were .020 mm’s and .016mm’s. Andial is telling you to set total clearance at .0127mm’s which is really on the tight end of the spec. JME will tell you to triple that number.

Since my motor is sitting on the stand fully assembled, I have not run it yet to give you any real world durability results and can only give you my rationale for choosing the clearances I did. Based on my measurements of existing “good condition” motors during rebuilds, piston to wall clearance wear has always been very minimal and I’ve never seen one measure higher then the .08mm wear limit (thus require newer pistons) unless there was actual damage or it was a severely neglected motor (rod &main bearing wear, excessive oil build up / coking in cylinder head, etc.). I’ve pulled apart a half dozen 100K+ mile motors that were within 50% of the wear limit which tells me that the coatings and walls are very durable if oil changes are routing using a quality oil. I also like to build a high boost/heat turbo motor on the looser end of the scale to deal with expansion and they typically make better power. Again this is just personal preference and I’m sure for every argument for this method, there is a knowledgeable person that can argue against it.

So, based on the burnt up motors, Andial’s lack of recent development on the 944’s, the change in piston coatings (and potentially aluminum), the factory wear limit of .080, JME and Mahle NA’s clearance recommendations on the upper end of the new spec, previous observations on cylinder wear….I set my clearance @ .034 to .036mm’s. I can get you my exact notes and clearances when I get home (it’s been about 2 years since I had the block honed) but you get the idea. While I do not know who is doing the work on your
block, I would start at the .001” recommendation and open it up from there.

You may also want to PM Ski and ask him how his motor has held up. It was a Mahle NA piston motor but I believe he’s had good results. Good luck and feel free to PM me of you want to.

Ski

Mahle NA being North America. Clearance on our new motor is at .0012", going to the dyno in two weeks if all goes right, hoping for 370/360. Motor has run well so far, we just have a lower than wanted oil pressure at idle but we figured out why and has nothing to do with the build or piston or OPRV. Machine shop error but we've managed. Above 1500 oil is fine

gpr8er

Thanks for the input guys. Talked with my machinist yesterday. We decided to go .00015" Better safe than sorry.

Chris White

I think you have an extra '0' in there......

gpr8er

I was thinking about the extra 0 today..LOL
yeah, I meant .0015000000000.....