Piston pin bosses distance question

[Acd_ex]

Is there anyone that has experience in piston design?

I need to know how tight a tolerance could be between connecting rod small end and piston pin bosses before any issues could arise ( and what issues would those be).

Out of piston and rod specs, I've found out that most engine setups have a minimum of 1.5mm on each side of the rod.

I need to know if a tolerance as tight as 0.5mm on each side is safe to be ran, for an experimental piston upgrade I'm working on.

Any info would be very appreciated, thanks in advance ?

[William1]

I've run 0.020" which is about .5mm No tighter though. That is risky as you have to take into account piston rock and is the crank centered on the bore as well as skirt length and bore length..

[Acd_ex]

6 minutes ago, William1 said:

I've run 0.020" which is about .5mm No tighter though. That is risky as you have to take into account piston rock and is the crank centered on the bore as well as skirt length and bore length..

On each side I suppose?

All aspects mentioned will be tested.

Honestly, I'd never expect a rod not to be 100% centered to the bore, but I'll check to make sure.

[turbo dan]

Tight as you want to go as long as you're not tighter than the side play at the other end of the rod.  Most engines have the thrust washers down in the big end.

[William1]

3 hours ago, Acd_ex said:

On each side I suppose?

All aspects mentioned will be tested.

Honestly, I'd never expect a rod not to be 100% centered to the bore, but I'll check to make sure.

The more offset the crank center line is the more the piston will tend to rock at BDC. You can take a crank, fit a piston and get that 0.020" clearance, but that assumes the piston remains square to the crank. For an extreme example, imagine if you had 0.1", plenty right? But what if the piston to bore was real loose and the piston tilted?

[Acd_ex]

7 hours ago, William1 said:

The more offset the crank center line is the more the piston will tend to rock at BDC. You can take a crank, fit a piston and get that 0.020" clearance, but that assumes the piston remains square to the crank. For an extreme example, imagine if you had 0.1", plenty right? But what if the piston to bore was real loose and the piston tilted?

I'd expect the crank centerline possible offset (regarding fore or aft I understand) would translate in rocking motion in BDC but I can't see how that would affect side movement or clearance.

If there's something I'm not getting I'd appreciate it if you could explain again.

 

10 hours ago, turbo dan said:

Tight as you want to go as long as you're not tighter than the side play at the other end of the rod.  Most engines have the thrust washers down in the big end.

That makes total sense.

My rod is spec'd for less than 1mm total side clearance IIRC, so I'm good here.

Thanks ?

[Acd_ex]

So, I had the chance to get info from a local company that designs and manufactures racing pistons and rods.

Answer I got to the question is this.

Minimum total clearance between rod small end and piston pin bosses should be at least 0.2mm larger than the sum of bottom end clearance between rod and crank webs plus crank to cases clearance.

Accurately measurable and totally understandable.

Thanks a lot for everyone's help ??

[EnglertRacing]

On 2/17/2022 at 2:41 PM, Acd_ex said:

So, I had the chance to get info from a local company that designs and manufactures racing pistons and rods.

Answer I got to the question is this.

Minimum total clearance between rod small end and piston pin bosses should be at least 0.2mm larger than the sum of bottom end clearance between rod and crank webs plus crank to cases clearance.

Accurately measurable and totally understandable.

Thanks a lot for everyone's help ??

That's actually not correct.

Crank end play - rod end play

If we have 0.020" crank end play and 0.012" rod side clearance...

That means your crank centerline can move 0.010" off center, say left....   and the rod can move 0.006" off center right meaning if that you would only need 0.002" clearance per side.

Piston guided engines do exist for various reasons. On journal rod bearing engines the crank float may be 0.006" but the rod clearance may be 0.040" the crank is unable to thrust the piston and the piston rod side clearance is run at very low numbers like 0.002"

The scenario you described is thrusting the crank left and the rod left and still having clearance on the small end

However what I'm describing is thrusting the crank left and rod right and still having clearance.

In other words, the rod thrusting the piston or crank is OK,

However the crank thrusting the piston via the rod is NOT ok 

[Acd_ex]

11 hours ago, EnglertRacing said:

That's actually not correct.

Crank end play - rod end play

If we have 0.020" crank end play and 0.012" rod side clearance...

That means your crank centerline can move 0.010" off center, say left....   and the rod can move 0.006" off center right meaning if that you would only need 0.002" clearance per side.

Piston guided engines do exist for various reasons. On journal rod bearing engines the crank float may be 0.006" but the rod clearance may be 0.040" the crank is unable to thrust the piston and the piston rod side clearance is run at very low numbers like 0.002"

The scenario you described is thrusting the crank left and the rod left and still having clearance on the small end

However what I'm describing is thrusting the crank left and rod right and still having clearance.

In other words, the rod thrusting the piston or crank is OK,

However the crank thrusting the piston via the rod is NOT ok 

I believe that the example I used contains the scenario you describe as ok, and the one described as NOT ok, dimensionally speaking, so it's safe in any way.

Thank you very much for the detailed response, I'm glad I get the chance to understand more over the topic.

Edited February 20, 2022 by Acd_ex