aaddpp

I have an 87 with a flip up wing, so I believe its an early model year car. Pistons are out, and I'm in the process of cleaning them with Berryman's.

I was under the impression that part of the rebuild / reassembly process was going to involve drilling oil drain / oil relief holes into the bottom of the pistons. The Technical Bulletin which covers the oil holes says cars from Jan 1988 onward should have their pistons drilled. Can someone confirm that a car from early model year 1987 can / should skip this oil holes, or was it at sometime extended to the 87's as well?

Thanks!

GregBBRD

The easiest way to tell is by looking. A picture from the bottom side would allow an easy answer to your question.

aaddpp

Greg, thanks for taking a look. I've posted several shots I took of the bottom side of the #4 piston and one from #8 - these are the pistons I've cleaned so far. I hope these capture what you need to see, if not I can take some more photos and post.

http://www.flickr.com/photos/51967142@N00/6944644154/
http://www.flickr.com/photos/51967142@N00/6944642358/
http://www.flickr.com/photos/51967142@N00/7090713327/
http://www.flickr.com/photos/51967142@N00/6944644266/

GregBBRD

Yes, add the holes.

The "problem" that Porsche was trying to cure was that the oil that got pushed down the outside of the piston was being trapped between that lower full circle skirt and the oil control ring. Drilling the
holes give the oil a place to escape.

If you look at your pistons, before they are cleaned, the "problem" should be evident.

There are instructions in the "technical book" about how and where to drill the holes.

aaddpp

Thanks Greg. As you say, you can see the need for the oil holes...its just a bit odd that Porsche chose to address the bulletin toward 88's onward.

What about weight / balance in this process? Drilling the oil holes will change the weight of the pistons, perhaps unevenly. I'm familiar with the process of matching the weights of the rods and pistons, but its my understanding that the piston / rod assembly weight also has a relationship with the crank-shaft and overall system balance - this I know less about. If the weight of the piston / rod assemblies are altered, is it sufficient to match all the piston / rod assemblies at a similar lower weight and reassemble, or would the weight difference be significant enough that the crank-shaft balancer counter weights would also need adjustment?

BTW, the copy of the Service Bulletin I have is not the best scan...its missing page two, and the left side of page 1 is not really legible. Wondering if anyone has a better / more complete copy of this bulletin that they could share. On the bulletin is says...Part Identifier: 1310, and Number: 8806.

Thanks!

BerNard94

Cheers.

GregBBRD

The amount of material you are removing is very small, in relationship to the total weight of the piston, but if you are really interested in making everything exactly the same, remove all the carbon from each piston, modify the pistons, and then remove material from the heaviest pistons, until they all weigh within one gram. Be sure to include the piston pin for each piston, when you are doing this. Don't "mix" the pins...keep them with the original piston...unless the pistons and pins are new.

Stock, I believe the tolerance for the pistons is 5 grams. I believe that this is the case for the rods, also. I'm not aware of the factory matching rods with pistons.

Rod are different. although you can also bring them all within one gram, it is critical of where the weight is located. You need to not only have the weights with-in one gram, but all the different ends should also be with-in one gram. You need a special "low friction" rod balancing fixture to do this. Summit or Jegs will have one of these. Before you start removing material from a rod, make sure that you can "duplicate" your finding from rod to rod. Remove and measure the same rod several times and adjust your process until you can remove the rod from the fixture, re-install it, and get the same results.

As long as you don't get "crazy" about removing material, even if the crankshaft was perfectly balanced for the piston/rod conbination (which it was not) you will end up with a slight "overbalance" of the crankshaft, by removing a bit of weight from the pistons/rods. "Overbalance" is a good thing. Many people prefer to slightly "overbalance" crankshafts originally (the factory certainly did).

aaddpp

Thanks for posting the oil hole technical service bulletin, thats much clearer than the one I have. As far as years go, I agree, 88 onward, but its not clear why since it looks like the 87 can benefit from the oil holes just as much as the 88+ model years.

BerNard94

Cheers.

aaddpp

Much appreciated Greg, thats very helpful. Sounds like I should be able to get away with just cleaning and drilling, though I'll take some before / after weight measurements of the pistons / pins to be sure nothing is grossly out of range when I'm done. Thanks!

aaddpp

Vilhuer

Engine numbers listed in Service Bulletin are first engines which used factory drilled pistons. Numbers are late '88 MY made after 861 M28/41 5sp and 2200 M28/42 automatic engines were made. Which means most of the '88 MY cars still had 1st style pistons which need drilling same way as all '87's.

If one wants to remove even only one gram from piston by machining away material it takes afful lot of work with grinder. Usually factory set installed into one engine is much closer to same weight than 765 +/-4g. I have seen few engines where all were well under one gram.

aaddpp

Before tackling the oil holes, I decided to weigh out the pistons / pins / rings to establish a baseline weight. The tech specs book calls for a piston weight (piston with parts - piston pin, piston rings, flap rings) of 765g +/-4g. If that is the correct number, piston assemblies #3, #7 and #8 exceed the upper limit slightly with the average weight of the new Goetze rings. I don't have weights for the old rings at this time.

http://www.flickr.com/photos/51967142@N00/7216588080/
Since I was at it, I decided to weight the piston rings individually - the numbers assigned, 1.00, 2.00...8.00 are just identifiers, not necessarily piston numbers for now. Oil rings #1.00 and #3.00 are outliers compared to the other rings. If you normalize the numbers to say 14.1g, the DIF falls to just .1 or .2g. It will be interesting to see if the difference in the numbers has any relationship to gap. Bumping up the outliers could potentially push pistons #2 and #6 over the 4g limit as well.

(all values in grams, expect ring numbers)
http://www.flickr.com/photos/51967142@N00/7216587998/

I'd always prefer to avoid grinding things, so I was wondering if I could bring the pistons closer together just by moving the pins around - easy enough to do virtually w/ Excel. FWIW, its seems its not recommended to swap pins in general, but the Tech Spec section does state: "If piston pins have been interchanged by mistake, reallocation must be carried out by checking the total weight." Thats seems to suggest that you can do it if you measure everything carefully... though I am wondering if the pin / bearing have worn together, and thats a factor to consider as well. Also, worth noting is that the oil holes will drop the piston assembly weight so its not worth speculating about swapping pins to deal with the 4g threshold until after the holes are drilled. It might however be worth it to bring the range of the pistons closer together without any grinding.

Below are the figures I came up with by moving the heaviest pin to the lightest piston and visa versa. Now all pistons are below the 4g threshold, just barely in most cases.

(all values in grams, expect piston / pin numbers)
http://www.flickr.com/photos/51967142@N00/7216588158/(Pin # corresponds to the piston it is originally associated with)

Enough analysis...its oil holes before all else, but, I'm a numbers guy, and thought it was an interesting exercise....

aaddpp

Just to follow up incase this helps someone out along the line, I weighed a set of old rings and came up with the following figures:

Top Ring: 8.9g
Mid Ring: 12.8g
Oil Rings:14.6g
Total Wt.: 36.3g

This makes the average Goetz set 5.34g heavier than the old stock rings...of which I had a one sample. From past posts on RL, that is apparently a lot when it comes to weight / balance.

If I plug the 36.3g value into my Excel sheet - instead of the 41.64g average Goetze value - I get the following results where all piston are now within the +/- 4g range of the 765g recommended weight. Note: red text shows the pistons that were out of range with the Goetze rings.

http://www.flickr.com/photos/51967142@N00/7221151328/
Again, once I have oil drain holes, I would expect the weight with Goetze rings to be a non-issue since I'm estimating that each 4mm hole will remove less than 1g of material from the piston.

jeff spahn

When I put a new piston in my engine last summer I strove to get the weight the same as the old piston. Figured that since I got it to within .1g of the original after drilling the holes I am ok.
Fun isn't it!