| Author |
Message |
modok
Samba Member
Joined: October 30, 2009
Posts: 27657
Location: Colorado Springs
|
 Posted: Fri Jun 20, 2025 7:42 pm Post subject: Re: Tight wrist pin |
 |
|
| chrisflstf wrote: |
Anyone know why these wrist pins have a wide and narrow band of discoloration on them?
Its on an older motor I just tore down. Just curious what it indicates?
Edit: never mind, its just the exposed area of the wrist pin |
YEP
Bushing not centered under pistons.
Probably aftermarket rods. |
|
| Back to top |
|
 |
oprn
Samba Member
Joined: November 13, 2016
Posts: 14722
Location: Western Canada
|
| Posted: Sat Jun 21, 2025 3:18 am Post subject: Re: Tight wrist pin |
|
|
Interesting thread!
Any air cooled VW I have ever worked on had this tight wrist pin to piston fit (you have to heat the piston to put them in) and I thought that was the way it should be. Even the ones with 80K miles were like that.
When I took the parts to my machinist for balancing he told me they were far too tight and he would not let them out of his shop like that. "They should be a snug thumb press fit at room temperature." he told me "What you have here is an interference fit". So he went ahead and honed them all to fit that way.
So... was he wrong?
Looking back and thinking about all the used VW engines I have worked on I have never seen any appreciable wear on the piston pin bosses, it's always the rod bushing that gets worn out. The pistons have at least double the wear area.
_________________
Our cars get old, we get old but driving an old VW never gets old! |
|
| Back to top |
|
|
rayjay
Samba Member
Joined: March 26, 2008
Posts: 1591
Location: Buford GA
|
| Posted: Sat Jun 21, 2025 8:29 am Post subject: Re: Tight wrist pin |
|
|
| There will be plenty of clearance when the engine is running. |
|
| Back to top |
|
|
oprn
Samba Member
Joined: November 13, 2016
Posts: 14722
Location: Western Canada
|
| Posted: Sat Jun 21, 2025 11:37 am Post subject: Re: Tight wrist pin |
|
|
The problem I see with having an interference fit on the piston is that it is very hard to support it properly when driving the pin in and out. To me that is what leads to tweaked rods, piston pin clips that fail and piston pins that drift over and scored cylinder walls.
Heating the piston does not work well when disassembling an engine. When I assemble one I like to put the pistons and rings in the cylinder on the bench that put the assemble on the engine so heating the piston is awkward there too.
Assembling this last engine with the pins a thumb press fit was a real treat!
_________________
Our cars get old, we get old but driving an old VW never gets old! |
|
| Back to top |
|
|
rayjay
Samba Member
Joined: March 26, 2008
Posts: 1591
Location: Buford GA
|
|
| Back to top |
|
|
modok
Samba Member
Joined: October 30, 2009
Posts: 27657
Location: Colorado Springs
|
| Posted: Sat Jun 21, 2025 11:59 am Post subject: Re: Tight wrist pin |
|
|
| oprn wrote: |
Interesting thread!
Any air cooled VW I have ever worked on had this tight wrist pin to piston fit (you have to heat the piston to put them in) and I thought that was the way it should be. Even the ones with 80K miles were like that.
When I took the parts to my machinist for balancing he told me they were far too tight and he would not let them out of his shop like that. "They should be a snug thumb press fit at room temperature." he told me "What you have here is an interference fit". So he went ahead and honed them all to fit that way.
So... was he wrong?
Looking back and thinking about all the used VW engines I have worked on I have never seen any appreciable wear on the piston pin bosses, it's always the rod bushing that gets worn out. The pistons have at least double the wear area. |
Well.....I could talk about that at great length, but should I? 
First, consider that when the engine is working hard, the clearance of the piston to the pin will increase because the piston is hot and the piston is made of aluminum, but the clearance of the pin to the rod will decrease because the pin will be hotter then the rod.....because that's the heat path.
Real life experience has shown a full floating pin can have less friction, possibly because the pin never completely stops rotating, but I suspect this benefit is only realized if it has the right clearances. In subaru engines I have seen the pin so tight in the rod that it doesn't wear, I suspect when under power it locks in the rod, so is that a full floating pin or not?
IMO there is not a thing wrong with having a room temp zero fit of the pin in a low silicon piston, it can work today just as it did then. And as power levels increase you actually need a looser clearance of the pin in the rod.
But a tight fit in the piston is becoming rare today........becase two or three reasons.
One reason being that 2000 series aluminum (low silicon) pistons are now rarely used for anything requiring longevity, and they are almost never re-grooved or knerled anymore.....you just buy new ones, and that's ok as they are now only like 1/3 of the price they were in the old days.
And they probably would wear less anyway assuming oil and fuel and filters are much better today.
ANd the majority of pistons are now 4000 series aluminum or hypereutectic alloys which transfer heat much slower and have far better properties as a bearing material, so you would not need or want a zero fit at room temp with those anyway.
It has A LOT to do with what the piston is made of, the low silicon aluminum is just very gummy stuff, which transfers heat fast, so getting the running clearance right is just a lot more critical with that kind of piston.
Another reason being that people are just lazier and stupider, so the piston makers are better off making even the 2000 series forged pistons loose out of the box because the end user does not have the knowledge or tools to adjust the fit appropriately and spend the extra time to warm the pistons for assembly.
I've got examples of each every week. I put together a jeep with some cheap pistons the customer bought, they sure don't look any different than 40 years ago tech LOL
But you use hyper pistons in an old engine, have to check your ring gaps!
Like the 351 ford I just put together.
And I've got a big block with forged pistons and pressfit rods where all the pins seized-up, maybe they were too tight, maybe they just weren't lubed.....hard to tell. Didn't build it but I'll be rebuilding it.
And I did all the work for a pontiac with icon forged pistons and the pins were kinda loose, and I'm not mad about that, but I refused to use press fit rods, hmmm why? well because if this relic only gets driven seasonally I am not risking it seizing a pin on start up, maybe the pins being extra loose would prevent it or maybe not, I'm playing it safe.
Last edited by modok on Sat Jun 21, 2025 12:35 pm; edited 2 times in total |
|
| Back to top |
|
|
Zed999
Samba Member
Joined: March 04, 2018
Posts: 1387
Location: UK
|
| Posted: Sat Jun 21, 2025 12:22 pm Post subject: Re: Tight wrist pin |
|
|
| My Haynes manual suggests heating the piston with a 100w light bulb for a few minutes.... if such things still exist. Works for me removing and fitting, I was surprised. So much better than using force. Whether they should be like that I leave to the experts. |
|
| Back to top |
|
|
modok
Samba Member
Joined: October 30, 2009
Posts: 27657
Location: Colorado Springs
|
| Posted: Sat Jun 21, 2025 12:31 pm Post subject: Re: Tight wrist pin |
|
|
The main reason the pin is so hard to remove in used engines is due to a burr at the clip, and the gunk on the pin.
If I carefully deburr where the clip was and heat the piston it's usually possible to push the pin out without using any drastic means, but, still a chore.
I used to think the reason bent rods were so common in old VWs was people hammering these things apart, but now I think it was just absolutely terrible quality rebuilt rods is actually the more common reason, and IF the rod is slightly bent then that will hammer the clip, making the burr much worse, so that's situation normal, all fkd up.
I guess new rods are a good idea if nobody can rebuild them properly, for now.
But in reality checking the rod alignment was one the key things that should always be done when rebuilding an engine, always was and always will be, the manuals used to all say it, and they still should. |
|
| Back to top |
|
|
Rome
Samba Member
Joined: June 02, 2004
Posts: 10524
Location: Pearl River, NY
|
| Posted: Sat Jun 21, 2025 1:36 pm Post subject: Re: Tight wrist pin |
|
|
| Quote: |
| checking the rod alignment was one the key things that should always be done when rebuilding an engine |
I remember either reading, or seeing a video (as in VCR) decades ago for a very simple check for rod alignment on all 4 rods. I tried that on an engine I was building in my basement and it worked... This only works if the person is assembling the rods onto the crankshaft; if you are receiving a pre-built shortblock, then it'll only work on the two rods on one side of the engine.
With the bare crank and all 4 rods, you can rotate all rods so that their small ends are all directly lined up in a row. Then you can insert a wrist pin into each small end bushing and try to push/insert the pin into the adjacent conrod. If the rods are not bent, then the wrist pin will travel into the adjacent rod. If any of the rods are slightly bent, you won't be able to push the pin into the adjacent rod.
To deburr the outer edges of a used piston's wrist pin hole, I use a 1" flapper drum for a drill. My local HW store carried these regularly. The individual sanding flaps bend enough to fit into the pin hole, and I give the hole just a quick shot with the flapper; clean off the hole, and try the pin so that it slides in easily at room temperature. Then wipe any sanding dust out the wrist pin retaining groove in the piston with a Q-tip.
|
|
| Back to top |
|
|
SGKent 
Samba Member
Joined: October 30, 2007
Posts: 42420
Location: at the beach
|
| Posted: Sat Jun 21, 2025 2:28 pm Post subject: Re: Tight wrist pin |
|
|
I built 1000's of racing engines for one of the best racing engine builders of his time. He taught me when fitting wrist pins, they should be a lite fit and not loose. The way I would show someone if they asked was to oil them and then let gravity work. If in the piston they moved really slowly or barely with gravity or a light push they were perfect. Within the rod, gravity would slowly move them.
The heat and capture type wrist pins say in a chevy are an exception to thatrule. The pin is captured and they float in the piston, so on those the pin should move under gravity when lightly oiled.
Guaranteed that if the pin is loose with wear, the ring lands will be way worn out. One does not want to rely on expansion to get clearance in a piston to pin or pin to rod.
_________________
"Most people don't know what they're doing, and a lot of them are really good at it." - George Carlin |
|
| Back to top |
|
|
oprn
Samba Member
Joined: November 13, 2016
Posts: 14722
Location: Western Canada
|
| Posted: Sat Jun 21, 2025 10:43 pm Post subject: Re: Tight wrist pin |
|
|
^^^This!^^^ is nearly word for word what my machinist said!
_________________
Our cars get old, we get old but driving an old VW never gets old! |
|
| Back to top |
|
|
modok
Samba Member
Joined: October 30, 2009
Posts: 27657
Location: Colorado Springs
|
| Posted: Sat Jun 21, 2025 11:09 pm Post subject: Re: Tight wrist pin |
|
|
The difference between a pin moving freely and stuck is only about .00015"-.0003" difference, depending on the smoothness and accuracy of the pin, OR,
one tiny burr
So (as if you couldn't guess what i was gonna say)
measure the hole!
I just bought another mitutoyo bore gauge, the new one that can measure blind holes all the way down to .1 from the end, it was like 300$, shipped from japan
AND IT'S SWEET |
|
| Back to top |
|
|
VWporscheGT3
Samba Member
Joined: September 01, 2006
Posts: 2173
Location: Gardnerville, NV
|
| Posted: Mon Jun 23, 2025 8:41 am Post subject: Re: Tight wrist pin |
|
|
| modok wrote: |
| oprn wrote: |
Interesting thread!
Any air cooled VW I have ever worked on had this tight wrist pin to piston fit (you have to heat the piston to put them in) and I thought that was the way it should be. Even the ones with 80K miles were like that.
When I took the parts to my machinist for balancing he told me they were far too tight and he would not let them out of his shop like that. "They should be a snug thumb press fit at room temperature." he told me "What you have here is an interference fit". So he went ahead and honed them all to fit that way.
So... was he wrong?
Looking back and thinking about all the used VW engines I have worked on I have never seen any appreciable wear on the piston pin bosses, it's always the rod bushing that gets worn out. The pistons have at least double the wear area. |
Well.....I could talk about that at great length, but should I?
First, consider that when the engine is working hard, the clearance of the piston to the pin will increase because the piston is hot and the piston is made of aluminum, but the clearance of the pin to the rod will decrease because the pin will be hotter then the rod.....because that's the heat path.
Real life experience has shown a full floating pin can have less friction, possibly because the pin never completely stops rotating, but I suspect this benefit is only realized if it has the right clearances. In subaru engines I have seen the pin so tight in the rod that it doesn't wear, I suspect when under power it locks in the rod, so is that a full floating pin or not?
IMO there is not a thing wrong with having a room temp zero fit of the pin in a low silicon piston, it can work today just as it did then. And as power levels increase you actually need a looser clearance of the pin in the rod.
But a tight fit in the piston is becoming rare today........becase two or three reasons.
One reason being that 2000 series aluminum (low silicon) pistons are now rarely used for anything requiring longevity, and they are almost never re-grooved or knerled anymore.....you just buy new ones, and that's ok as they are now only like 1/3 of the price they were in the old days.
And they probably would wear less anyway assuming oil and fuel and filters are much better today.
ANd the majority of pistons are now 4000 series aluminum or hypereutectic alloys which transfer heat much slower and have far better properties as a bearing material, so you would not need or want a zero fit at room temp with those anyway.
It has A LOT to do with what the piston is made of, the low silicon aluminum is just very gummy stuff, which transfers heat fast, so getting the running clearance right is just a lot more critical with that kind of piston.
Another reason being that people are just lazier and stupider, so the piston makers are better off making even the 2000 series forged pistons loose out of the box because the end user does not have the knowledge or tools to adjust the fit appropriately and spend the extra time to warm the pistons for assembly.
I've got examples of each every week. I put together a jeep with some cheap pistons the customer bought, they sure don't look any different than 40 years ago tech LOL
But you use hyper pistons in an old engine, have to check your ring gaps!
Like the 351 ford I just put together.
And I've got a big block with forged pistons and pressfit rods where all the pins seized-up, maybe they were too tight, maybe they just weren't lubed.....hard to tell. Didn't build it but I'll be rebuilding it.
And I did all the work for a pontiac with icon forged pistons and the pins were kinda loose, and I'm not mad about that, but I refused to use press fit rods, hmmm why? well because if this relic only gets driven seasonally I am not risking it seizing a pin on start up, maybe the pins being extra loose would prevent it or maybe not, I'm playing it safe. |
As a rule of thumb on the Icon side... we tend to go at a minimum of .0008" clearance on the ICON wrist pins...with a top limit of about .001" . with forced oilers and broaches you really get more of a float on that pin during operation. even on the VW's in 4032 we give about the same... its why i loaded these guys with the broaches , forced oilers and grooves... those pins are very well oiled now.
but i have talked to folks that thought we were on the loose side like your saying. and i dont blame anyone for feeling that way. your typical cast or AA is only about .0005" pin to bore clearance... and i have always taken AA's in the past and sat and just worked the pin back and forth in the bores until it fells "right" alot of that will happen in the motor anyway , but i would rather start off looser and not run the risk of the pin seizing in the piston.
Top fuel, blown alky, really strenuous applications ive seens as much as .0015" to .0018"
_________________
If you have any questions about Forged ICON 4032 VW pistons just shoot me a line. |
|
| Back to top |
|
|
|
