Broken Piston (1991 Auto Carat) 2.3ish (with 82mm crank)

[Dave Harrison]

I've finally had enough time away from work to drop my engine and remove the heads. The compression test I did a few weeks back showed:
cyl 1 - 185
cyl 2 - 186
cyl 3 - 185
cyl 4 - 0

and a leakdown test looked good for the first 3 cylinders as well...

so...

After a whole lot of reading... (Bentley bible, numerous posts on the samba, as well as much of the info on Ben's site, and a few youtube videos), I've got the engine in the stand and the heads off. The #1 piston is cracked.

I've done a little searching re: cracked pistons, but at the moment, it's late, I'm tired, and I haven't found anything helpful. I'll get back to research tomorrow, (bentley is also at the shop with the van)

In the meantime, i'll post a few pics (above), and if anyone who's dealt with this wants to make any suggestion as to what I need to address, or if I've left out some important information, and whether it's likely I'll get a few thousand more miles out of this engine if necessary parts/rebuilding happens... Is this a problem with the top end only? ie, would metal from the piston be able to make it past the rings into the case? (if the rings are intact)

Thanks to all who've posted information on the samba. It's been a valuable resource to me. Without it, I'd and having a lot less fun.

Dave

[D Clymer]

Who built that engine and what piston set did they use? If that is a standard set of pistons that had the crowns machined off by 3mm to correct for the increased deck height of the 82mm crank, it may be that the area between the top ring groove and the top of the piston became too weak due to the material that was removed.

The only piston set out there that was really designed for an 82mm stroke is the GoWesty JE Piston set. Expensive, but installing a set of those may be your best option for getting that engine back in business.

David

[boof1306]

You may have a detonation problem. What pistons and rods are you running? Unless you have Gowesty or custom pistons your combustion pressures might be to high. Then again if you have something from gowestys early offerings that might be the problem. Lets start by finding out what pistons, rods and cam you have.

[AtlasShrugged]

Big time detonation problem on that one cylinder. The fact that the other three cylinders are fine indicate that the problem was exclusive to the particular bore. The other three are fine. It was not an overheating problem, engine build parts quality or ignition timing issue or you would see the damage on all four pots.

Could be an air leak on that one cylinder or the injector was not spraying well or was partially plugged. That pot ran lean and caused the detonation damage.

My bet would be the injector. Your engine probably idled fine when the fuel requirements were low..at speed the engine was at the edge and that cylinder ran lean.

Were the injectors replaced (always best) or cleaned and serviced?

I would replace that cylinder and piston, replace the head and buy a set of new injectors.





http://www.contactmagazine.com/Issue54/EngineBasics.html

[Dave Harrison]

Some specs for this engine installed by a PO are in the following document. I assume that this means the pistons are made by AA?
I think I've seen these pistons advertised in the classifieds.

I'm lucky to have all the PO's receipts for the past 7 years...

Would it be likely that this detonation damaged anything beyond the obvious (bottom end)?

The injectors were replaced by a PO back in '06 (less than 60k miles ago)

If the deck height on these pistons tops out one mm short (like the QCS PISTONS do), could that also be a factor in causing this piston destruction?

[D Clymer]

Those AA pistons were made for a stock 76mm stroke. The 82mm crank would push them 3mm higher than stock deck height (2mm in this case since the old AA pistons like the QSCs were 1mm too low), so they would have had to machine the crowns to get the deck height back to where it should be.

If this was caused by detonation, then yes, the rod bearing on that cylinder can be damaged by the detonation. But it may be okay. You can change rods without splitting he case if you are patient and have good dexterity. It would be worth it in my mind to replace the rod bearings and then attend to the pistons and cylinders.

D

[boof1306]

They would have had to machine the face of those AA's to fit them. What you don't know is what compression ratio they left you with. I would cc a good piston and work out what your static and dynamic where to at least know the cause of the melt down. Did the po tell you to run it on premium fuel? The issue you have is that you can't buy an AA piston off the shelf and bolt it into that engine. The other issue is I believe they have corrected there pin height now? You could buy a qsc 96mm p&c set and you will need to have about 1.8mm machined off. This alone will leave you with a very high ratio, but it is doable. You can increase the bowl size of the piston if you want to lower the ratio. Then there is gowesty. I don't know what length rod they would use on a 82mm stroke, you may have to buy new rods for this option. If you want to buy something and bolt it in gowesty are probably your only option. If you want to hand the engine over to someone who can sort it for you Rocky Jennings is your man.

[Dave Harrison]

Thanks to everyone who posted advice so far.

It looks like a bad fuel injector is at fault and detonation killed/melted the #4 piston. I had a local race engine builder/guru take a look at what I've got and he thinks the rest of the engine looks in good shape, and I should be fine if I replace the pistons/jugs and he can do a valve job/clean up the heads. (I'll obviously be replacing the fuel injectors)

I looked into buying the go westy set up for new pistons and cylinders, but they use an 83mm crank for the 2.4 now and not 82, and I'd need to buy rods as well due to the 22mm wrist pins. The cost is quite a biut more than my other options that could work. I've got a few more things to figure out.

I might try going with either the AA big bore pistons and liners (95.5mm) from cip1 (on sale) if the deck height can be made to work out. (someone there is in the process of measuring one for me) I've also looked at the qsc sets that have been used in the past by tencent and some others on the samba and Shoptalkforums (with a little machining for proper zero deck). It's been a little difficult finding any info on either of these P/C sets that isn't a few years old. If anyone out there in samba land has some experience with either of these piston sets, i'm all ears.

I need to get back to the engine parts, and remeasure everything I've got, but at present, with the info I've read from samba posts, this is what I've come up with.

-the cylinder length from the center line of the case should be 211.86mm.
-half of the 82mm stroke is 41mm
-my 5.4" rods should be 137.16mm (i need to remeasure to be sure)
-in order to get a zero deck, the piston height should be 33.7mm...
41+137.16+33.7=211.86

so I guess as long as the pistons I get are within 1mm from the 33.7, I can either get the rods rebushed (see Rocky Jennings' technique), or have the tops of the pistons shaved down (see tencentlife's technique) and I'll end up with a much better deck height than the engine has now. I still need to do all the calculations to make sure the compression is going to work out, but as far as the pistons go, if I can get the right height, then I can avoid having to replace the rods, and in the end, spend less than the go westy route

For now I'm learning a bit, and that was half the reason I bought this van in the first place.

[garryv84]

Does a 2.1 case have to be clearanced if using a 82mm crank?

[boof1306]

Garry, yes, mostly the back of the stud bosses, sometimes where the halfs meet. Also the cam will need clearancing.
Dave. This is your problem - buying pistons with a 33.7mm height. If they where available with a decent compression ratio we would all be buying them. Have you rung Gowesty and asked them the comp height of ALL of their pistons? If you go qsc you will have to take 1.8mm off and then decomp by increasing squish.
Remember : Genuine piston comp height = 36.7mm
Qsc = 35.7

Greg

[Dave Harrison]

I have decided that at the moment, it looks like my best bet is going to be a set of qsc P&C. I need to contact Rocky Jennings to see if he's got any P&C sets. If I can get my rods rebushed to be 1mm shorter, then the pistons will require only a small haircut, and My static CR won't be too different.

I looked into the idea of going with the AA P&C set and then having some shorter rods rebored for a 24mm wrist pin. that would also bring the deck height within less than a mm too heigh, but then I have to think about all of the clearancing that my rods have received (probably as a result of the 82mm crank)

I'll try to take some pics along the way and possibly I can post a top end rebuild thread that could be useful to someone. (hopefully not as a 'what not to do')

[mbwesty]

Dave:

Did you find any qsc set? I am interested in a set too but it seems like Rocky had stopped offering it and his offset rods in the classified.

I also live in CA very close to QSC and from talking to the woman there it seems like they are out of stock and I doubt they are gonna bring them back in any time soon. She just kept giving me the yeah two more months answer.

So it seems like the GoWesty set is the only way to go without going to ebay...please let me know and thanks for your help!

Tim

[MarkWard]

If there is a demand for these pistons, why not go to a company like CP Piston and have a set made? They would be forged and would hold up nicely.

[boof1306]

Its a good question. The States is definitely the place to make them for quality and economy. What I would suggest is needed is a DJ piston with a 3mm lower comp height. I guess the profit margin must not be enough to get a piston maker to produce them and then on sell. I would buy two sets now if someone made them. If QSC do stop production I wonder what guys like Chris/Vanistan and Rocky will do. Backyard guys like us could mill down used MV's for higher comp and stroke.

[Dave Harrison]

PostPosted: Yesterday 11:34 am
Dave:

Did you find any qsc set? I am interested in a set too but it seems like Rocky had stopped offering it and his offset rods in the classified.


mbwesty,
I searched all over for a set of the qsc pistons. I actually ordered 3 sets from places that 'had them in stock'... They did not.

I have been too busy to put any more time into this until this week, but I am probably going to settle on buying a cofap P&C set (that should have the 35.7mm piston height, and have them machined to proper deck.

The advantage of the Cofaps over the QSCs (from what I've been told) is that the rings are good and the cylinders are round (QSCs being somewhat notorious for low quality control)

They're 94mm instead of 96, but my plan here is to hopefully have a running/ reliable engine, and not to maximize power. I am purposely looking for pistons that are too short, to minimize the amount of material that needs to be removed in order to get proper deck height.


Thanks again to everyone who's helped me with info. I'll try to document things better as I get into putting this thing back together (with pics) so that someone else can either learn from my success, or learn from my mistakes...

[silverbulletuk]

Pardon me for saying, but judging by the OP's photo those pistons have been turned off to reduce the compression height. There's very little valve cut-out left, compared to any standard VW design.

It's very hard to say what has caused all the erosion and pitting; it could be detonation, piston debris or steam erosion. I have had non-VW engines that have had slight water ingress (when hot) due to cracks between the valve seats and they leave a very similar pattern of destruction. Is the top of the barrel pitted above the top ring witness?

Either way, the pistons didn't stand a fair chance. The machining had compromised them before it was ever fired up.

Shorter rods is what this wbx needed!
_________________
1986 syncro twin-slider rhd, 3.2S Oettinger wbx6
1990 SA Microbus - Supercharged wbx going in with UN-1 trans.
www.025motorsport.com

[MarkWard]

You can contact CP Pistons directly and they do one off custom runs. Ship them a decent piston and tell them what changes you'd like. We do this for our race cars when we need pistons. You can also download their custom piston form and design your own from scratch. www.cppistons.com 949 567 9010. PS on our 4 cylinders, I always order 5 pistons. It is easier to have a spare from the same batch than to get a replacement later.

[Steve Arndt]

It is worth it to go through Rocky Jennings.

He hand selects the cylinders from large batches. Some of them don't pass quality control so he rejects those. He also hones the cylinders.

I run Rocky Jennings 96mm pistons coupled to an 80mm crank and 9.5:1 compression. Rocky build the engine. I'm sure he can set you up with a combo that will work. I would also have him service the heads.

Steve

edit; And run premium fuel only.

[jberger]

[boof1306]

In my experience the reason we see a lot of out of round qsc liners is that at 96mm the lower skirt thins out and becomes malleable. All the discrepancies I have seen are in that lower section. So no surprise the cofap liners are straight at the original 94mm. I wonder how many qsc liners are bent when they leave the factory and how many are damaged in transit.