Do you think you would hear this?

[brighton911]

Teaser So, as part this years engine build project, I have checked my small end rod clearance and found it is out of spec at .0015 thou on all six. I've always thought my engine a bit on the noisy side, do you think that that amount of small end clearance would be audible or contribute to overall engine clatter? Thanks

[Schmierung]

Oh no. Valve clatter is greater than that audible addition. Consistent on all six, that is good news. Worry about your outdoor faucets.

[Trackrash]

0.0015" is within spec. The wear limit is 0.0021".

[brighton911]

Thanks for your replies.
Gordon, yes, my mistake on being out of tolerance. I used the upper end of the small end range (23.033 mm) as the out of spec value. No matter, I'm in there already and I would rather have the clearance on the lower end of the tolerance range, everything else is new. Thanks

[lvporschepilot]

.0015 out of the high end of spec will not cause noise, but maybe oil pressure would be a bit low.

[356RS]

Originally Posted by lvporschepilot

.0015 out of the high end of spec will not cause noise, but maybe oil pressure would be a bit low.

Small end rod bushings have nothing to do with oil pressure.

[Trackrash]

Originally Posted by brighton911

Thanks for your replies.
Gordon, yes, my mistake on being out of tolerance. I used the upper end of the small end range (23.033 mm) as the out of spec value. No matter, I'm in there already and I would rather have the clearance on the lower end of the tolerance range, everything else is new. Thanks

Maybe a little background would help.

Have you had the rods rebuilt and are actually measuring them at 0.0015" with your new wrist pins? Usually the new bushings in the rods will be reamed to size matching the new wrist pins.

[brighton911]

Hi Gordon No, I just tore the engine down last week for the purpose of going to 3.4 P&C During the inspection of the top end parts, I noticed wear on the top part of the bushings. I measured them with a telescopic gauge and got 1.5 thou Had my trusted machine shop guy (Bob) measure them and he came up the same. So new bushings on the way (from Pelican) and in they go and Bob will ream them to size on his Sunnen machine. Should be a happy ending.

[KTL]

Dave,

If I may ask, to what clearance are you having Bob ream them? I've gathered from some of the pros that a street car build is good at 0.0004 to 0.0005 and a performance/race is good at 0.0006 to 0.0008

Thanks,
Kev

[lvporschepilot]

Originally Posted by 356RS

Small end rod bushings have nothing to do with oil pressure.

you're exactly right. my fault!

[KTL]

Doesn't everybody have Pauter rods with direct pin oiling?

That'll affect oil pressure. Rod Info - Pauter

I also use 30 wt ball bearings. C'mon guys.......... it's all ball bearings these days!

https://www.youtube.com/watch?v=pbWWxGmbS9s

[m42racer]

Originally Posted by KTL

Dave,

If I may ask, to what clearance are you having Bob ream them? I've gathered from some of the pros that a street car build is good at 0.0004 to 0.0005 and a performance/race is good at 0.0006 to 0.0008

Thanks,
Kev

Are you suggesting these clearances in tenths? If so that is too tight. You are adding an oil clearance.

Street engines generally run a tighter clearance but its not uncommon for this to go out to 0.0020" - 0.0024". The pin diameter, pin boss spacing and engine speed all come into play here.

If the vertical clearance (generally the worst) is at 0.0015" and can be cleaned made round) up at 0.002", you have no reason to change the bushings. The extra 0.0005" will not hurt at all.

But if you want the engine to be assembled at factory spec's and the spec's say 0.0015" then change the bushings. I'm just suggesting that the added clearance will not hurt. Your machine shop should be able to tell you this.

[KTL]

I'm just asking the question. I'm not saying "do as I say/post." Yes those numbers are tenthou that I said.

The Porsche specs for the rods with the 23mm pins are a bore tolerance of 23.020mm to 23.033mm which is 0.013mm or 0.0005"

The specified allowable clearance is 0.020mm to 0.037mm which is 0.0008" to 0.0015"

That's why I asked because those numbers I heard had seemed quite a bit lower than what the handy little book called for

[m42racer]

I think you will find that most places that recondition rods will try to hit the middle of the two numbers. If they see some wear, they may even add some. You are adding clearance for oil, so 0.002' is nothing and will never hurt you.

You may be experiencing some piston boss flexing which can cause the pin to lose it clearance and the bushing shows contact. I cannot image in these engines the pins are bending. A 23.0mm pin is overkill for these engines. These early piston designs have the pin boss spacing's wide apart and are unsupported.

[brighton911]

Thanks to everyone for their responses, my knowledge level on the topic is going in the right direction.

Kevin, going by the specs in the Bentley manual (same numbers as your post) I was going to have Bob ream/hone them just a tad more than minimum of .0008" so .001"

M42 racer Yes, the stock pin wall thickness is quite stout (.200"), I can't see that pin ever flexing. I note the pins (that came with the Carrillo pistons I will be using)(edit: not JE as previously stated) have a thinner wall thickness at .162". I guess they felt they could reduce the thickness/weight without loss of strength.

Dave

[Tippy]

When the machinist did my little ends, he honed to just fit the wrist pin. After doing a lot of fits of 0.0005" for things (been machining for a couple of years now), I'd bet it was that or tighter since it was honed.


CAVEAT: Although I didn't actually measure it, you could not wiggle the wrist pin at all in the little end. In my experience, when you can't wiggle something that is concentric (smaller diameter stuff of course), you are probably < 0.001" slip fit - hence my belief of stated numbers.

[KTL]

Understood that we're adding clearance for oil lubrication with the pin fitting. Since it's splash oiled, it would seem like we need to have some decent clearance. How much is decent I really don't know. That's why I asked. Too much clearance would seem to pound the pin in the bore as the piston is stopped and then accelerated in the opposite direction cycle after cycle after cycle.

I think the overkill 23mm pin in these 3.2 engines is just a parts sharing move by Porsche. The 23mm pin would seem to be more important in the 3.3L turbo engine and that's where the 3.2 Carrera engine originates, since they share the same exact crank and rods. So when they switched from the 3.0L non-turbo, which has completely different rods in C-C length, pin size, big end bore and rod bolts, to the 3.2L which has same exact rods as the 3.3L turbo, all they had to do was change the compression height, pin diameter and dome volume of the 3.0L piston to make the 3.2L work.

That said, I do recall JE having some issues early on with use of tapered wall thickness pins in these Porsche applications. So pin flex can indeed be an issue if we cheat too much on the wall thickness?

Thanks for sharing your intended clearances Dave!

When we say the pin bosses are unsupported, are we saying that because the pin bosses are not connected to each other across the piston parallel to the pin? Because they are pretty beefy bosses in terms of their thickness. Here's a 3.2L Mahle piston



However I get what you're saying m42racer if we compare the above Mahle design to something like a JE FSR piston that has bracing all over the place.



Funny how that FSR picture mentions oil squirters needed for "extreme horsepower applications." Yeah, extreme HP or air cooled engines that need squirters or else the piston is on the verge of meltdown? Crazy how Porsche kept these engines alive from '65 all the way to '98.

[prschmn]

The narrow skirt has been used for years in drag and other race apps. It is not well suited for lower RPM usage-skirt loads are too high and the film thickness too low.. Clearance is also extremely important.