mark kibort

maybe, im still learning. However, if there is one offset for all pistons, porsche did it wrong. this means the offset is ALWAYS on the valve relief side of the pistons, or if its a early 32 valver, always on the exhaust side of the pistons. this is key. so, if one side is a benifit, then the other side is just the opposite of a benefit. keep in mind, the 2 valve pistons and early 32 valve pistons always had arrows pointing toward the exhuast side. ONLY the S4 with its equal valve cuts, could have an arrow pointed to the front of the engine, taking advantage of the offset.

Now, the reason for this is that the crank is going CW, right? so, on the passenger bank, on the power stroke, the rod grows ever closer to the right side of the piston (intake valve side) however, on the driver bank, the rod grows closer to the exhaust side of the piston on power stroke. moving the rod to the right on both banks makes the angle better to reduce piston slap. (wasnt this called the "jacoby effect" or something?)

Now, you are saying to reverse the offset, so if there was an arrrow on the piston pointing to the front, now swap banks to have the pistons face the rear?
I can do that very easliy, but im not convinced the offset should be on the left side of the piston as you face them looking at the front of the engine.

Porsche defiinitely had it wrong, as the 2 valve cuts are on the same side as the smaller offset side, which would be correct on the driver side and the exact opposite on the passenger side, right???? the rod always will grow closer to the right side of the piston on the powerstroke. if the offsets are the same on all pistons,(which they are on all 2 valve and early 32 valves) then the offset on the passenger side pistons would be on the left of the piston as looking at the front of the engine. (opposite of the other side)

mk

C.F.

Mark
With Mopars ,from the front of the engine the pin is offset to the exhaust port on the pass. side & to the intake port on the drivers side , you seem to be saying that the 928 has the pins offset towards the exhaust ports on both pass. & drivers side, which would seem to be wrong as you say.
I would install them with the pin offset towards the intake port on the pass. side & exhaust port on the drivers side.

mark kibort

wheew, sounds like im finally starting to understand this. Ill have to make up a model so i can visualize it.

so, yes, if the mopars had it the other way, then it was to do something different, right? so you moved the mopars to put the offset to the right (toward intake on pass and toward exhaust on driver side)
porsche had one offset, so it was corrrect on one bank and like the mopar on the other. again, porsche fixed this on the S4 where the valve reliefs are equal size and there is an arrow pointing toward the front of the engine. this means the offset is correct on both sides but the same piston is used on both sides.

now, im trying to understand why the offset helps additional torque or reduced friction, or ive even heard, longer transient time at TDC area..

just to be clear, the offset is the narrower side, right?

I wonder if the austrailian engines have the opposite offset. does the engine spin counter clockwise down there? hey, maybe the mopar engines are counter-clockwise!

thanks,

MK

danglerb

The crank is exactly centered on the bores?

But the piston pin has a slight offset to the intake or exhaust side?

If so that offset would change the point in the crank rotation where the rod is parallel to the bore, either BTDC or ATDC.

Sounds like ultra techy stuff, where a static model might not make clear the benefits.

C.F.

Yes

Now I am trying to understand Porsches logic in having the offset different bank to bank.

mark kibort

easier to stock one piston for 2 valve engines? they finally caught on when it came to the 4 valve engines, in that they could have the same cuts top and bottome and the pistons would be reversable. (only with the S4s)

MK

GUMBALL

Mark, these problems come from the block not being machined properly. We had a shop bore one of our blocks for us, and it came back to us with the bore diameter being different at the top than at the bottom. In addition to that, there were 1 or 2 cylinders that were not prefectly round. I am not a machinist, so I can not answer your question of how it was done, I just measured the results.

I have been told by a few machine shops that they have found blocks with inconsistant casting - it seems the silicon content in the aluminum is not uniform through the complete block.

Again, this note was just an FYI.

slate blue

Hey Mark just my engine back from the machinists, looks nice, my wall thickness is generally 0.220" or 5.65 mm, I have deburred the block and polished the edges of the liners both inside and out to try and avoid splitting. The pistons are a real snug fit. Unfortunately they decked the block, maybe to do with the material mushrooming around the top of the liners given they had to remove 1.5 mm of the walls. I hope my piston deck height is going to be O.K.

The job including rebalancing the rotating assembly was about $1000, this included all parts balanced to 0.00 grams! So the rods and pistons, crank, balancer, flywheel, pressure plate balanced to that fine tolerance should be interesting to experience. They gave me the Sunnen print out to prove their work, it shows the starting points and then the finishing point. All parts needed balancing BTW.

Cheers Greg

mark kibort

Thats pretty cheap. did that include the decking the block too?

i measured 5 to 5.5mm on scots block, but as soon as you go down an inch or two, the thicknes goes way up to over 6mm.

our bill was $1600 for just the boring and the flycutting of the pistons, but the pistons were REAL dirty, so that cost extra too.

I dont know why it cost another $1000 for balancing and putting the rings and rods on the pistons, but thats the cost of not doing things yourself.

mk

danglerb

Mark do you have a run down on what the total will be, ignoring detours?

My own back of an envelope estimate for a hybrid was $3k to $4k labor plus parts, and maybe wishful thinking including the swap into the car. No boring planned, just something about lapping, but that was in the parts, part of my budget.

mark kibort

If we are talking a 5 liter short bock and just cutting the pistons (best way)

costs are :
$1300 for the block including shipping
$500 for cutting the pistons
$500 -700for bearings and seals and rings and pilot bearing
$200 for the head gaskets
so that is near $3k and you just have to clean the pistons and block yourself (not that hard) put it all back together. (not that hard, but have someone help you with the pistons, as that can be a HUGE problem if you do this wrong)

then, put all your old stuff on this engine.

now, since we are doing this as often as some change their underwear, we dont need many of the gaskets or the pilot bearing. But, if you are in there, you may want to put a new clutch kit in there. ($500). new waterpump? ($200), timing belt? etc etc.

If you are having someone help you, thats $2000 to a shop to put the engine in, at least.

scot and I got his engine out of his car and on a stand in 3 hours. race cars are a little easier and ive also done this a few times before too.

MK

slate blue

Hi Mark if you include the cost of fly cutting the pistons, the total bill would be $1150 including postage etc. They decked the block and did not mention it or charge for it. The cost of balancing including the mallory was $440 and as I mentioned that was crank, balancer, flywheel, pressure plate.

As far as thickness goes mine is pretty uniform, never less than 5.3 mm and that is only is very small areas and occurs maybe 3 times all up. As I mentioned I hope my polishing of the lip of the liner both internally and externally does the trick.

I also must thank Dozman, without his help my car wouldn't be where it is today. He donated the pistons to me and I have to say they were in superb condition. He has also organized numerous packages for me and many of those weren't even parts that I bought from him. I also must say that of the all the stuff I have bought from John, that it always has been in absolutely spectacular condition and anybody that knows me, I'm fussy, as fussy as they come, my parents reckon it was a good thing that I wasn't an obstetrician.

Thanks Johno!

danglerb

I guesstimated 12 hrs to R&R the motor, 2 people x3 hrs each way.

Greg has this giant green automotive version of a dishwasher for parts, and the block and pistons BrendenC has are clean and include a WP.

Seems like a lot for head gaskets, but I guess thats Porsche, little surprise I'm insanely expensive bits.

Clutch is done, ditto TT, soon tranny.

I'm not sure there is a point in holding back on the WYIT in the front, anything with 145k miles on it that comes out anyway should be rebuilt before going back in.

Thinking about having Sonny Bryant do some custom camshafts, wonder if that would be pricey?

IcemanG17

MK
I thinks it safe to say Scott would have saved some huge $$$$ if he just bought DK's track beast!