Printable Version of Topic
Click here to view this topic in its original format
914World.com _ 914World Garage _ Cracked piston skirt for a big bore kit: machine or replace?
Posted by: porkchopexpress Apr 2 2014, 06:12 PM
I recently picked up a 73 1.7 (was originally told it was a 1.8.... didn't think to check the engine stamp to verify. 
but I got it for a song).
I also and looking at picking up a big bore kit (94mm by Kolbenschmidt) for trade. The original plan was they should bolt onto the 1.8 and be a pretty easy setup.
With the 1.7, I understand I will need to have some head work done to accept the larger cylinders and pistons.
Here is my dilemma: one of the pistons has a chipped skirt. The cause of the broken piece is unknown.
I realize it MIGHT work to just use it as is, but as the person who is offering these in trade so rightly stated: " Emotional side of me says I don't like putting an engine together with something I know is not quite perfect"
So I figure my options are to
a) get all the piston skirts machined off to match
b) inquire on the classified section if anyone has a single 94 mm Kolbenschmidt they would be willing to sell (hopefully cheap since its only one!)
Anyone have input on the first option to machine off all the skirts to match?
Thanks as always!
Posted by: Dr Evil Apr 2 2014, 06:15 PM
Toss it and get another.
Posted by: r_towle Apr 2 2014, 06:17 PM
A perfect situation where JB Weld might come in useful.
Posted by: yeahmag Apr 2 2014, 06:33 PM
JB won't hold up in an environment like this. You need to user Crazy Glue or maybe Gorilla Glue if you have lot's of water in the oil (it's activated by water).
Posted by: messix Apr 2 2014, 06:37 PM
it's a paper weight!
the weight of the piston is now different to the others and will throw of the balance of the rotating assembly. that is the really big problem !
the other issues would be the stability of the piston would be off also and it would rock in the cylinder enough to gall the walls.
if this guy thinks it would run ok with it.... I question the rest of the parts.
Posted by: stugray Apr 2 2014, 06:47 PM
I am pretty sure I have a set with one bad piston.
The wrist pin would not go in on just one.
Let me check.
Posted by: Vacca Rabite Apr 2 2014, 06:51 PM
Big bore is usually a 96mm piston (or larger), not a 94. Sure about the piston size? Or are you just getting stock 2.0 pistons and the seller is calling them "big bore."
Zach
Posted by: wndsnd Apr 2 2014, 07:14 PM
A perfect situation where JB Weld might come in useful.
Ya think?
Why not just bench grind it flat and then do the other side?
Posted by: Jake Raby Apr 2 2014, 07:21 PM
I thought this was an April fool's joke.. Seriously?
Posted by: Johny Blackstain Apr 2 2014, 07:26 PM
I'm gonna go out on limb here & be politically incorrect & say screw the paperweight, make it an ashtray.
Posted by: Java2570 Apr 2 2014, 07:29 PM
or a cupholder!
Posted by: cary Apr 2 2014, 07:31 PM
I thought this was an April fool's joke.. Seriously?
You and I are on the same page ......................
Posted by: wndsnd Apr 2 2014, 07:37 PM
A perfect situation where JB Weld might come in useful.
Ya think?
Why not just bench grind it flat and then do the other side?
Reply to my own reply.
It's called "blueprinting an engine".
I read it in another thread....
Posted by: timothy_nd28 Apr 2 2014, 08:26 PM
Maybe if I was stranded on a deserted island, I'd consider the glue 
Posted by: Allan Apr 2 2014, 08:41 PM
Butt plug... 
Posted by: 396 Apr 2 2014, 09:14 PM
A perfect situation where JB Weld might come in useful.
Ya think?
Why not just bench grind it flat and then do the other side?
Reply to my own reply.
It's called "blueprinting an engine".
I read it in another thread....
Actually, it is blue printing...you will need to reduce the other three and keep it under 1500 rpm...oh just short shift.
Posted by: porkchopexpress Apr 2 2014, 10:09 PM
Seems as though most people didn't read the full post. My bottom line question was can I machine off all the slipper skirts so all the pistons match. If that wasn't clear, then that's on me and my bad. I have seen several new big bore kits that have pistons that do not have those slipper skirts so it seemed like a valid question.
Consensus seems to be, "bad idea"
Additionally, I am doing all of this on a budget. Would love to have the cash to send an engine out to one of the guru's on here and have them do it for me, but for now, that's just not in the budget. And in case you didn't notice the "Newbie" designation, I still have a lot to learn. And I enjoy learning so its great to ask about these things to try to figure out what I can, or am willing, to do myself.
I am all for being sarcastic, and can take and dish with anyone.... just wish some of you might have added something valuable to say along with it. Even if its "no clue"
Next time I will be less likely to ask, which sucks.
Posted by: Mike Bellis Apr 2 2014, 10:22 PM
Bad idea.
How many times do you want to rebuild it?
Just install it like it is. It will make a cool slapping sound. People may even think it's a rod knock. But you'll know better, laughing all the way with your self machining pistons and bent crank... 
Posted by: Valy Apr 2 2014, 10:30 PM
To your questions:
$200 will get you a new set of 94mm AA pistons, cylinders and rings.
$230 for a new single piston and cylinder from Mahle (German). I think KS that you have are NLA.
Either choice is better and cheaper than what you try to do.
Posted by: cary Apr 2 2014, 10:53 PM
http://www.europeanmotorworks.com/vw/type-4-pistons-cylinders.html
Posted by: Java2570 Apr 2 2014, 11:06 PM
You could put a WTB ad here in the classifieds, I'd bet someone might have one
KS piston or a full set of Mahle 94mm pistons. I see used sets all the time on Samba or ebay but typically people try to sell them for too much. You can certainly get
a new set for a good price.....AA has 94mm sets for budget builds and the afore-mentioned European Motorworks has 96mm pistons and cylinders for not much more.
I would never put a broken part back in an engine, too much risk.....and since you are budget minded, why spend more on another rebuild when that broken part fails.
Posted by: stugray Apr 2 2014, 11:07 PM
I checked my stash and I have:
94mm piston & cyl set - all good
93mm piston & cyl set - one piston with very tight wrist pin
90mm pistons only - all good
So I dont have just one oddball 94mm
Posted by: jaxdream Apr 3 2014, 08:16 AM
I'm gonna go out on limb here & be politically incorrect & say screw the paperweight, make it an ashtray.
I got a couple of Harley pistons as ashtrays ......
1 900 Sportster , 1 61cui Panhead
Jack
p.s. yeah make an ashtray out of it , or paperweight....YMMV
Posted by: The Cabinetmaker Apr 3 2014, 08:41 AM
Heck man, I've got a used one I'll send you for the price of postage, or a set of 4 for 50 bucks.
Posted by: HalfMoon Apr 3 2014, 05:22 PM
I agree. And I'm sorry if you got a browbeating over what was/is a valid question. Don't let the naysayers rule your curiosity friend.
Actually, back in the day when we were making go fast Triumph dragster bikes, it was fairly common to chop the skirts thus reducing piston mass to gain some piston speed. If you chopped off all the skirts so that each piston weighed the same this would work. Durability and or longevity? Well that's quite hard to say.
And remember this....there are no dumb questions, just dumb answers.
Seems as though most people didn't read the full post. My bottom line question was can I machine off all the slipper skirts so all the pistons match. If that wasn't clear, then that's on me and my bad. I have seen several new big bore kits that have pistons that do not have those slipper skirts so it seemed like a valid question.
Consensus seems to be, "bad idea"
Additionally, I am doing all of this on a budget. Would love to have the cash to send an engine out to one of the guru's on here and have them do it for me, but for now, that's just not in the budget. And in case you didn't notice the "Newbie" designation, I still have a lot to learn. And I enjoy learning so its great to ask about these things to try to figure out what I can, or am willing, to do myself.
I am all for being sarcastic, and can take and dish with anyone.... just wish some of you might have added something valuable to say along with it. Even if its "no clue"
Next time I will be less likely to ask, which sucks.
Posted by: malcolm2 Apr 3 2014, 07:13 PM
So you say the piston diameter is 94mm? It started life as a 1.7?
Check this out:
https://www.cbperformance.com/v/enginecalc.html
plug in 94 and 66 for the stroke and hit the displacement button.... 1832cc? you know 1.7 was a 1679 cc with 90mm pistons and the 66 mm crank.
2.0 liter has the 71mm crank... plug that in with the 96mm pistons....
You can go to 1911 with the 66mm crank like you said with a little head work. But they are off anyway, so touch them up.
Posted by: Johny Blackstain Apr 3 2014, 07:26 PM
I agree. And I'm sorry if you got a browbeating over what was/is a valid question. Don't let the naysayers rule your curiosity friend.
Actually, back in the day when we were making go fast Triumph dragster bikes, it was fairly common to chop the skirts thus reducing piston mass to gain some piston speed. If you chopped off all the skirts so that each piston weighed the same this would work. Durability and or longevity? Well that's quite hard to say.
And remember this....there are no dumb questions, just dumb answers.
& if you machine 1, you have to machine them all (balance). How this will hold up I have no clue. I'd take Curt "the cabinetmaker" up on his offer & pay for postage. Seems to be the easiest/cheapest solution by far.
Posted by: Dr Evil Apr 4 2014, 08:24 AM
Hey man,
Dont take people too seriously around here. If there are two ways to take a comment, and one is joking and the other is mean, 99.99% of the time we all mean the former not the latter.
Your question will become humerus to you once you stick around and learn a few things. Dont be put off.
Powered by Invision Power Board (http://www.invisionboard.com)
© Invision Power Services (http://www.invisionpower.com)