[enkeivette]

Originally Posted by 94cobra69ss396

You and I are talking about two different things. I'm talking about the piston to cylinder wall clearance and you're talking about the piston ring gaps.



Total Seal rings I bought for the Cobra showed to set the top ring at .0055 x bore, the second ring at .0035 x bore and the oil ring at a minimum of .015 for boosted applications. I set the top rings at .023, the second rings at .016 and the oil rings were .016 out of the box.

The Mahle rings that came with my engine kit were not file fit rings but the end gaps were all over the place. They ranged from .008 up to .016.

No, Im talking about cylinder wall to piston clearance. I just mentioned ring gaps as a second example of erring on the side of caution.

.012 piston to wall clearance. Or .024 difference total. Meaning, if I had .003 clearance, and I opened up the bore .01/ or .005 on each side, I now have .008 piston to wall clearance, and it's well within spec for a forged motor. So Im not worried.

[enkeivette]

Originally Posted by BRUTAL64

Because I used Hypereutectic pistons on my current 400 engine, the end gap had to be big..026....but it works with the hotter Hypereutectic pistons.

The .004 per bore inch is general rule and is different for forced induction and Hypereutectic pistons........

I run .024 top ring, .022? second.

[94cobra69ss396]

If you measure the piston and then measure the bore, that's the clearance. It's not half, what makes you think that the piston stays centered in the bore?

[enkeivette]

And then you divide by two... ?

[94cobra69ss396]

Why are you dividing by 2?

[enkeivette]

Im not, I havent even checked it. I thought you would divide by two, makes sense no?

Either way, if .012 is good enough for a forged car engine. It's good enough for my pressure washer engine. Ha

The collective input from engine experts in this thread is now worth twice what the engine is.

[94cobra69ss396]

Dividing by 2 would only make sense if the piston stayed centered in the bore but it doesn't. The piston moves around in the bore as it travels up and down and with load changes.

.012 is too much piston to wall clearance. A forged piston needs more clearance than a hypereutectic piston does due to the greater heat expansion. Probe recommends .002 - .0075 for their forged SRS pistons depending on bore and application where as they recommend .001 - .0055 for their high silicon FPS pistons.

[enkeivette]

You may be right about the way its measured, but Im telling you, Ive seen a .012 spec for a forged piston.

[enkeivette]

.011

Originally Posted by MaxFlow

I agree that 1/16, 3/16 would make the day a little easier at that clearance. This is a drysump, lateral gasport. CP piston measure .5 on the skirt. Never measured but I think that would be .011 or so at the pin boss. CP recommends .004 and I usally go .005. Wouldn't pull vaccuum very well either if any.

I'm sure the ringpack works in this combination. But if you don't have the bores right your in trouble.

.012

Originally Posted by Ed-vancedEngines

Manley and JE used to use the old TRW Forging made from if I remember correct it is 4032 low heat expansion and very heavy alloy. Those could be run at suggested .0045 to suggested .006 and I have run them at as much as .008.

The older alloys used by Forge True, Arias, and others way back in the buggy days, was a 2018 alloy and it really expanded a lot. My Aries 2018 were recommended to be at a .012. There was a saying with the circle track racers that they would work ok if the race was not loger than 250 miles. Anything longer the pistons would just keep growing as the engine got hotter. I do not know if that is true or not.

Most custom pistons now are made with blanks of 2618 and the different piston mfg do recommend different clearances becuase the finished product is not all the same. I have seen .0065 to a .012 in different engines and applications.

You need to contact Manley with the work order number if one is on the bottom of the wrist pin boss and find out from them.

Ed

.01

Originally Posted by Racerrick

Agreed I see that as a dry sump or vaccum pump ring pack. I feel 3mm oil ring are overwelmed if clearences and finish aren't just right. The trw/manley forging that is the focus of this thread has a 1/16 3/16 setup. It will live quite nicely at .008 and IMHO trw race pistons require .0065 min clearence measure at the pin so .008 is where it will be in short order due to wear.
Here a lower budget (under 10,000) real life example
(the customer provided the heads, cam, intake and carb)

A 418 drag sbc with 175hp nitrous that I machined and built 7 years ago, started with .006 clearence. 2 years ago I replaced the arias pistons due to ring land failure with 1.345 c.h. 13-1 ross pistons, the clearence is .010 at pin centerline.
This stock 2 bolt 400 block with spacer bearings has 3.845 stroke, 4.160 bore, 5.7 rod, 850 carb, comp 300 street roller and rpm heads is in a completely original but fully caged 3800 lb 69 nova with no t brake, shifted at 6000 rpm it runs 6.55 in the 1/8 at 4000 ft elevation year after year. According to et thats a +650hp motor.
I'am currently trying to get the owner to ugrade his block, install 18 deg heads and a race cam. He says no way the combination is staying the same but he will change out the block cause the caps are walking big time.

[BRUTAL64]