TheSamba.com :: View topic

Thu Jul 18, 2013 9:42 pm

While checking measurements, and triple checking all of my rod to case and cylinder clearance, I came up with two different deck heights for cylinders one and two. I ended up swapping pistons and cylinders back and forth trying to figure out, was it the case deck, cylinder, or piston that was different. At one point I put the Mahle pistons on the bench, grabbed an AA piston, and got a very different measurement. Positive deck, not negative, the AA sticks up proud of the Mahle by .040". I'm not sure if it's true of all Mahle pistons, these are older 90.5 "B" pistons with the 2mm top ring. Or is it the AA that's different. The AA piston has a 35mm pin height, the Mahle has 34mm. I searched and the only mention of a 35mm pin height was 94mm "B" pistons.
I did figure out the cause of the deck height difference, one rod is longer than the others by .020. :( Looks like the whole shebang is going back to the shop that rebuilt the rods to get fixed. I really hope they don't piss and moan about having to rebalance everything..
Just goes to show that you can't really measure everything too much.

Thu Jul 18, 2013 10:07 pm

Mahler and AAs have different pin heights according to Modok. He brought it up during some discussion about a year ago.

Thu Jul 18, 2013 10:13 pm

I found it. Aa - 34.9mm vs Mahle - 34mm.

Fri Jul 19, 2013 2:22 am

just another reason to buy new rods..... I have listed 3 diferent pin hights for mahle pistons(strokers)& 2 diferent for non strokers. and the aa I have I added to the list and it is.003" less than the mahle 94 stroker but it is a 92 aa and the mahle 92 is .015 less than the aa 92.
befor you go off the handle and do something to give you a rash, get your rod situation corected then go from there.( I would get a new set of rods,china rods)

Sun Jul 21, 2013 10:14 am

mark tucker wrote: just another reason to buy new rods.....
I would if it was that simple, this is a set of porsche journal carrillo's so I will see if there is a way to salvage them. First try is to see if there is enough bushing thickness to essentially stretching the shorter rod by installing an offset bushing. The bushings are pretty thin though. Second would be to take the needed few thousandths off of the cap mating surface and re-hone the big end. The machine shop said that may be possible, but didn't sound crazy about it. We'll see, it's all getting dropped off on monday morning.

About the pistons, I hadn't seen Modok's post. I was mentioning it, as the .9" difference might make a big difference to someone setting up an engine and looking at a ridiculous amount of shims.

Sun Jul 21, 2013 2:28 pm

I don't understand how one rod would be .020 longer.

The pin bushings can probably be offset bored lets say .008 either way, but to see what is going on you need to measure the lengths. They should be what they are supposed to be, and rods do not grow longer!

Sun Jul 21, 2013 3:45 pm

Offset grind the crank for chevy journal and you will get more stroke and you can get a inexpensive set of high quality rods!

Sun Jul 21, 2013 3:46 pm

check to see where the issue is at, if the bush's are right you can cut the rod&the cap &hone your ass off use a stock rod to keep it straight. I reckon you could make another cap&just cut rod&hone withanother cap then swap to the good cap when close. or get another set of rods, somebody should have them or 1. good luck

Sun Jul 21, 2013 5:43 pm

Modok, I'm not sure exactly how much shorter (not longer as I said) it is. The tools I have at my disposal aren't reliable for measuring rod length, but it is shorter for sure. Mark, there are some other rods available, but not singles. I know this set is made of spares, which is how I ended up with this mess. I'd do it differently if I had to do things over. Oh well, just see how it goes with what I have for now

Mon Jul 22, 2013 1:03 am

Maybe it is not so bad, but enough to make checking deck a pain.

Whatever the cause I bet it can be fixed, and those should be a joy to work with too.

Mon Jul 22, 2013 8:27 am

you could always shorten the cylinder and then bore the heads to match...like you said spairs. .020 short,then shorten the cylinder .020, then bore the ajacent cylinder head hole .020 deeper. now you have the same deck hight(quench) and the head will sit flush on both cylinders.cc the head and get them the same,it should not take out much on the .020 deeper one. but remember to mark the head so it always go,s on that side.now stop fretting and go have some fun with it.

Mon Jul 22, 2013 9:35 pm

Bad news from the machine shop, the pin bushing is already bored offset, so I can't get the needed few thousandths to make it longer. I am pursuing some other sets of rods, in the hopes of actually building this thing some time soon.
About the original reason for posting about the difference in pin height.
With the 5.4" rod and mahle pistons, deck is at an even .010"
If I went to a porsche length rod 5.352", with the mahle piston my deck height grows to .058", I was shooting for .05
If I use the AA piston instead of the Mahle, my deck only increases to .022" and I can shim it back to where I want it to be :)
A 5.5 rod nets me an unwanted thick stack of shims, and tins to be modified.
So, if I can't get the 5.4", then Porsche length will do as well.