Details to convert pressed piston pin to full-floating?

-
dibbons

dibbons

Well-Known Member
Joined
Nov 6, 2009
Messages
5,742
Reaction score
3,822
Location
La Paz, B.C.S., Mexico
The 340 I am renewing has bronze bushings in a set of 340/360 connecting rods. However, there was no oil hole drilled. According to factory service manual (at room temperature) the clearance is a thumb press fit with 0" to .0005" clearance for both pin/piston and pin/rod bushing. Not knowing if these rods are converted from pressed pins or if they are rods that were originally factory full-floating rods, I have two questions:

1) Does anyone know if factory small block full-floating rods used an oil hole in the rod small end?

2) Does anyone agree/disagree with this comment I found on an internet search?
If you choose to not use this setup (no pin-oiling) make certain to have between .001" and .0015" clearance between the pins and the rods and a minimum of .001" between the pistons and the pins!

Thank you.
 
the 340 came with bushed rods from factory. the top of rod was not drilled from the factory. sunnen calls for .0005 to .0008 clearance on bushed rods. if you use looser clearance you probably will get rod knock!! what is casting number on the rod? last three numbers should be 496 or 645.
 
I have always wondered in a stock-ish setup, how any oil gets up on top of the rod to go into any oil hole there...maybe from the squirters in the sides of the rods? Or scraped off by the oil rings and through the drain slots? (Seems like it would be easier to get it onto the pins directly from that source.)
 
nm9stheham said:
I have always wondered in a stock-ish setup, how any oil gets up on top of the rod to go into any oil hole there...maybe from the squirters in the sides of the rods? Or scraped off by the oil rings and through the drain slots? (Seems like it would be easier to get it onto the pins directly from that source.)
Click to expand...

It gets up there the exact same way oil cools the underside of the piston.
(Thrown there from the rod journal oil flowing out the sides of the rod)
 
TrailBeast said:
Ha, that's what I told a friend who then proceeded to let the drill slip and put a nice hole all the way through the palm of his hand. :D
Click to expand...

Yea, some times you have to be smarter than the drill! but **** happens
I have a bad habit of getting things out of my top tool box draw and leaving it open to walk into it later. LOL
 
the holes I have seen were countersunk, like a funnel. If you want to run press fit pistons on floater pins with no circlip grooves, you can run teflon pin buttons, VWs run them often. they are just teflon pucks that sit outboard of the pins against the cylinder wall. I guess they dont wear the bores being soft high temp teflon. Any one ever use them in a Water cooled V8? I have a press pin stroker set and was contemplating running them like this on floaters, just too easy to take apart and service.
 
We ran pin buttons 40 years ago on strokers where the pin boss entered into the oil ring groove. Don't see that much anymore...
 
pishta said:
the holes I have seen were countersunk, like a funnel. If you want to run press fit pistons on floater pins with no circlip grooves, you can run teflon pin buttons, VWs run them often. they are just teflon pucks that sit outboard of the pins against the cylinder wall. I guess they dont wear the bores being soft high temp teflon. Any one ever use them in a Water cooled V8? I have a press pin stroker set and was contemplating running them like this on floaters, just too easy to take apart and service.
Click to expand...
If you read around, there is some writing about them wearing/scoring the bores on the sides. Perhaps carbon crud gets embedded in the surfaces and acts as an abrasive.
I am looking at the same thing: press fit pins on floater rods. I have been considering a small set screw from the underside with red Locktite, and perhaps a matching groove in the piston for it to lock into. The pin does not seem to take a lot to hold it in place. I've been wondering what problems (and thus disasters) might come from that approach.
 
perfacar said:
the 340 came with bushed rods from factory. the top of rod was not drilled from the factory. sunnen calls for .0005 to .0008 clearance on bushed rods. if you use looser clearance you probably will get rod knock!! what is casting number on the rod? last three numbers should be 496 or 645.
Click to expand...
Yes, I have never run enlarged pin clearances.... never had any pin issues at 7000-8000 peak RPM's or turbo or anything, floating or pressed. Maybe some really high end blown applications or constant 8000-10,000 RPM's need it.
 
1968-72 340 rods are forging 2899496 and 1973 3418645, all floating and no oil hole. clearance between bushing and pin for performance should be .001". over around 5000 rpm a 3/32" hole on top and chamfered is good.
 
Way to high!
I have a few sets, I will have to dig them up if you are interested.
 
Personally, I think having the oil hole doesn't help much. Sure, it won't hurt anything, but how much oil do you think slings off the crankshaft.......even at idle? A LOT, that's how much. Enough so the insides of the pistons and piston pins are totally saturated with oil. Trust me. The pins and bushings get all the oil they need and then some.
 
nm9stheham said:
If you read around, there is some writing about them wearing/scoring the bores on the sides. Perhaps carbon crud gets embedded in the surfaces and acts as an abrasive.
I am looking at the same thing: press fit pins on floater rods. I have been considering a small set screw from the underside with red Locktite, and perhaps a matching groove in the piston for it to lock into. The pin does not seem to take a lot to hold it in place. I've been wondering what problems (and thus disasters) might come from that approach.
Click to expand...
I had hemi pins w/ the right end play knock the old style pin locks out and drag the cyl. wall.Doesn`t happen w/ spiro locks tho. A dragraceing friend that was darn successful tried the buttons on a couple of his race motors, he went w/ spirolocks when the first came out. He said the buttons took too much attention to keep using them. I ran steel , stock chevy rods that were honed out to make full floaters, w/ an 1/8" , chamfered oil hole in the top of those rods, on the street in a 383 sbc, worked fine !
 
yes, I have ran steel rod bores no bushing, they work just fine. the oil ring floods inside the piston top with lots of oil.
 
nm9stheham said:
If you read around, there is some writing about them wearing/scoring the bores on the sides. Perhaps carbon crud gets embedded in the surfaces and acts as an abrasive.
I am looking at the same thing: press fit pins on floater rods. I have been considering a small set screw from the underside with red Locktite, and perhaps a matching groove in the piston for it to lock into. The pin does not seem to take a lot to hold it in place. I've been wondering what problems (and thus disasters) might come from that approach.
Click to expand...
Sounds like a disaster waiting to happen... don't try to reinvent the wheel.
 
Krooser said:
Sounds like a disaster waiting to happen... don't try to reinvent the wheel.
Click to expand...
If the set screws held, why not? Is it not done due to weakening the small end of the rod? Or just harder to install in a production environment? Or just prone to fall out? (Perhaps some expansion problem waiting to happen?)

Not trying to be obnoxious at all... but grooving piston pin bores is not free or doable in a home shop, and if the grooves are done wrong, the lock rings are liable to come loose.
 
You must log in or register to reply here.
-

Back
Top