• Welcome to The Truck Stop! We see you haven't REGISTERED yet.

    Your truck knowledge is missing!
    • Registration is FREE , all we need is your birthday and email. (We don't share ANY data with ANYONE)
    • We have tons of knowledge here for your diesel truck!
    • Post your own topics and reply to existing threads to help others out!
    • NO ADS! The site is fully functional and ad free!
    CLICK HERE TO REGISTER!

    Problems registering? Click here to contact us!

    Already registered, but need a PASSWORD RESET? CLICK HERE TO RESET YOUR PASSWORD!

Full floating wrist pin clearance and lube

Will L.

Will L.

Well-Known Member
Messages
18,546
Reaction score
35,065
Location
Boulder City Nv
Too tight clearance and the full floater becomes partial floater and if it is rod to pin and if offset she is a gonner. If the pin locks in the piston - gone as well. Too loose is rod knock followed by seizing.

We know oil squirters were added for the turbo heat issue, normal for turbo engine gas or diesel until all the manufactures started coating their pistons to deal with the heat. But something I picked up on was the massive amount of added oiling getting to the top of the rod and into the bearings surely didn’t hurt the pin getting lubed better. Saw the difference in tear downs.

One thing I normally do is open the oil hole in the bearing on the small end to match the hole in the top of the rod. My theory is better oil flowing in just like you do everywhere else. Pic of new bearing port seen through the rod hole where the splashed oil gets in. (I held white cloth inside to help the picture be seen.)

So what do you all do there?
What clearance spec do you all run for rod to pin & pin to piston?
Anything learned here you can share?

I know there is stuff online but not everything translates to this engine.
Any take on what matches us and why?


@Twisted Steel Performance
iirc you said you don’t coat/treat the pin because of our low rpm range? I know the pin hardening processes used aren’t normally done until 8,000 rpm and up.

I know there is stuff online but not everything translates to this engine.
Any take on what matches us and why?

C341E1B4-3AB3-45E7-93CD-FC7DAB8D462F.jpeg
 
I coat the pin & rod bushing on my personal motors.. Their is such a tight cold clearance a lot of burnishing is required after coating to get them in the piston again, I also coat the rod bushing.

Normally it's not needed, ( pins ) the pin locks in the piston due to higher heat in aluminum than the bronze rod bushing as well as the tighter cold clearance in the piston/pin. I coat the pin when customer asks for it and when coating rods I do both rod & bushing...

Our motors do have floating pins in the rods by design..
 
Twisted Steel Performance said:
I will add, I have NEVER seen signs of pin movement in used pistons the piston locks the pin nearly instantly so the only movement is the rod..
Click to expand...

I have found most pistons from Factory are WAY out of GM’s spec. Most pins a one thou in full floating, but GM spec on this is looser. I pulled apart probably 40 gm engines and 6 or 7 gep - ALL of them had too tight piston pin bores according to their own specs.
I think that’s why they lock up.

When I put in my optimizer i had to clearance 7 of them and 1 was barely in the tight side of spec. when I pulled it apart after 70k ish
The 7 I resized still pivoted but the one that I left barely at minimum spec had to “pop” free.
So all new pins and clips for this build.

So I am thinking going to the open side of spec this time.
 
it’s like sweating copper pipes. The oil that splashes there gets drawn into the gap.

It is a full floating pin design, thats how come there are clips on the piston pin ends- the pin is supposed to float. Semi floater piston pins like sbc most often lock onto the rod but still float in the piston so clips on both ends. The other semi floating pins that float in the rod but lock into the piston are 1-2 thousand oversized pins compared to piston and you have to heat the piston to install the pin instead of heating the rod.

Any normally full floating pin they don’t lock up into the piston. I think the mass production error of piston pin bore sizing is a difference of why these engines sometimes die soon and why we see so many bent rods.

When I did the nitro propane- I had to open up ALL the tolerances a lot because of the extreme heat. iirc the pin bore clearance was doubled and never seized, but not like the pistons ever got much use to see how bad they would open up.
 
That is my understanding of the difference in full float pistons.
The pin should be free of the piston just like a small block chevy, but also free of the the rod end.
 
Well 99% of pistons designed as "full floating" have oil passages in the piston pin bores, I'm guessing ours is defective...

I know the new Mahlie's I packed up today I could hold the pin for a minute and get it warm and it wouldn't enter the cooler piston till it cooled down...

Post up a link to the pin bore clearance your speaking of please...
 
From the manual WarWagon added the pdf to.
I know I have it elsewhere too.. i will post that in a minute
DED1DBBD-901E-45B9-9C81-3E7CDDE587C4.jpeg7D0F4E3F-EA5B-491B-AFDD-4335271B91D8.png
 
Here is a random video I found showing how tight floating pins should be to remove. He already removed one clip before starting video.
the fact that he is not fully removing the pins and checking clearances to the new rods he is going to use and not checking piston pin bore means they are probably a little tighter than they should be. Most pistons are made tight- err on the side of caution. A pin locking up in the piston means lost power, slightly lower life. A little to loose adds to the hallow slap that gm fought customer complaints on since day one.

Keep in mind how bad out of balance how far out factor ring gap, etc is on new 6.5s. Most piston pin bores are way tight. The pin to piston is tighter because you want a drag not a pin spinning around in the piston. But if you can not drive the pins out like the in video - it was too tight at install.
 
Well I discussed this with my machine shop today, they build all sorts of big cube race motors, not our motors at all, I won't get into his statements because they mirrored mine..

So exactly what wrist pin bores are you planing to enlarge and what will be your final clearance, piston bore or rod bore both?
 
Full floating is as already been said, the gudgeon pin rotates in both the piston and little end. This is to spread the load across 3 bearings rather than one, in this case of course aluminium being a fine material for a bushing. I also recall discussion at some point in my past regarding the redundancy of the bearings in that a seizure in one of the 3 does not become a show stopper.

Using the clearance quotes above for the piston either:
0.0003 - 0.0006 this translates to Hard Drive Fit - Thumb Push Fit
0.0004 - 0.0006 this translates to Light Drive Fit - Thumb Push Fit

In both cases we can see that at room temperature there is specification for a fit such as shown in the video tight to get out. Note though that is all that video is good for hammering out the pins IMHO is not to be done (you can risk distorting the piston) when simply heating the piston with a heat gun has them effectively push out easily. Some manuals have you heating the piston in oil or water. In the winter time I just plop em on the stove for a bit.

Essentially because the aluminium has a greater expansion than steel with heat the tighter fit at room temperature for the piston itself is to account for that, when in the engine running heated up it will fully float with enough clearance for oil to get in and lubricate. If you want to be super anal you can check fit in the piston by heating both the piston and pin and then seeing how freely it slides in.

In my experience on different engine makes for aluminium pistons with full floating pins the fit is tight and requires a bit of heat to remove them cleanly. Its uncommon for me to find one that is a thumb push fit at room temp.

Cheers
Nobby
 
What got me started on this was when I was explaining to my son tab A goes in slot B.
the pin did not slide in as it should in one piston so I grabbed the next piston and pin beside it and it fit. So after “class” I went to slide each pin into each piston and could feel a slight Variation in fitment, one was mighty snug. The pins are not gm pins, (Manley iirc). The machinist balanced piston and pins as sets. Then balanced all rods individual. Then bob weighted to balance crank.

I called the machinist and asked bout the pin fitment. He said that a finish hone wouldn’t even take of 1/10 of a gram, but he didn’t hone them because he didn’t know which of the 4 clearance specs I wanted. He actually had me on paper from doing the last 6.5. When I asked which 4, he had the two that were in the book, and 2 much looser fitting ones- one of which is where most sbc run on aluminum pistons and one not quite that large. We realized there was a missed call from him to me months back and when I called him back he couldn’t remember why he called. The lightbulb went on for us both. SMH

So appears when I did the stupid fuel engines I ran them like a sbc but full float instead of the partial sbc that has a locked rod/pin probably was looking for more free hp.

Chris mentioning heat up the piston and see if the pin moves kinda makes it obvious looking at the spec GM didn’t want much movement in the pin:piston interface. Hotter

I went out and grabbed my telescoping gauges and reached for my apparently gone outside mic. UGGH!
So using a basic caliper that only reads 3 decimals (which of coarse you cant get perfect feel from T-gauge to caliper like a mic) all the pins read 1.220 diameter.
But the pistons had 1.219 to 1.220. So apparently I need to buy another tool before I continue.

And Yes heating and or cooling parts for removal is always better than a hammer. I just grabbed a random youtube video to show they should be able to come apart without damage.

I have a thing about new pistons, new rod bushings, and old pins- so mine got new pins and I had him find me some quality ones that use upgraded retainers. Thinking back, I have never seen a 6.2/6.5 pin thrown loose. It just scares me so I rather the extra insurance.

Is checking the pin to piston fitment just something most don't do?
Or does everyone just make sure they are in the factory range?
Am I the only one that agonizes over which end of the range to be in? Like piston rings- I worry more about overheating so I dont go for the absolute tightest tolerance. I want more room for movement and ‘sacrifice’ the last 5,000 miles of engine life and sacrificing 2-3 hp for the extra long term safety factor.

On the wrist pins to rod, I never really saw pins seized in rods that didn’t first have an oiling issue. And only on crazy high mileage engines saw sloppy rod to pin fitment. So I am quite happy with keeping lower spec numbers of that fit.

So it’s really pin to piston that I wonder about. Has anyone ever saw a 6.5 piston hammered out by the pin?

@Twisted Steel Performance Do you think they wanted a locked in pin but kept clearance open for better assembly line?
 
I would think it's a 'ease of assembly" thing..

Like I said I have never seen evidence of a pin rotating in a piston, I have seen blueing on the pin under the rod...
 
Just realised that in my old school way my fit translations I gave in the last post were more for reamed bushings than honed.
With a honed bushing one can expect a free fit in aluminium at .0002 thou that I would say translates to a light thumb fit. One older book that I have when discussing honing calls for a fit of '0.0001 - 0.0003 free' in aluminium pistons with 3/4 - 1" OD pins. So comparing back to the prior specs posted if using those clearances as a guide if honed i would say that one would expect the pin to be free at room temp.

Regarding measuring etc as Twisted Steel said its an ease of assembly as in feel thing rather than measuring. We are talking less than a thou clearances here a PIA to measure which is why we start to discuss fit feel at different clearances, you know palm push, thumb push, free fit etc. In my world of everyday marine and industrial diesels I have simply looked for a Thumb Push Fit in the con-rod and and for Al pistons the expectation of warming the piston somewhat to remove/install the pin.

As to the semantics of what is rotating/free where if its a fully floating pin I have the expectation as per the book explanation that in the engine at temp it will do just that, my earlier choice of words in the first sentence of my prior post I used the term rotate, oscillate would of been a better word to use perhaps for the indication of being free to move. What actually happens damned if I know but I do know that what ever engine I have torn down with free floaters usually I do not find issues between the pin and the piston. Does this mean that essentially the pin is not free in the piston at running temp, that the path of least resistance is to oscillate on the con rod still when both bushings are free, or in fact because you have as by the book spread the oscillating load over 3 bushings rather than one reducing impact by a 2/3 at each point? I've never really given it that much thought!?

Cheers
Nobby
 
You must log in or register to reply here.
Back
Top