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piston skirt to crank clearance, help engine guru's

Posted By: dave571

piston skirt to crank clearance, help engine guru's - 06/15/09 04:36 AM

OK

My buddy is putting his 451 together.

When we roll it over, the comp height is good, etc, PROBLEM is that the piston skirt just touches the crank counter weghts almost dead on at BDC.

It seems to be about .004" interferance. The skirt is quite thin there, so we were going to have a little machined off.

How much clearance should there be there?

.020? ,030?

Thanks!
Posted By: Anonymous

Re: piston skirt to crank clearance, help engine guru's - 06/15/09 07:47 AM

minimum .120 at any forced angle. (simulate piston rock at/near BDC)
Posted By: 440Jim

Re: piston skirt to crank clearance, help engine guru's - 06/15/09 12:37 PM

Are you talking about the piston pin area, or the very bottom of the skirt?

What counterweight diameter did your crank have? Many builders cut them down to 7.25" dia, and this helps clear the pistons too. But some leave them larger and check the block to counterweight and piston clearance.
Posted By: dave571

Re: piston skirt to crank clearance, help engine guru's - 06/16/09 04:01 AM

counterweights are still stock 440

Block is clearanced for this already, and the crank turns fine without a rod/piston on it.

When the rod and piston is on, I believe it is on the skirt, near the pin boss.(I haven't seen the marks myself) but I do know that the piston was almost a BDC. It just touches enough to stop the rotation.
Posted By: Anonymous

Re: piston skirt to crank clearance, help engine guru's - 06/16/09 05:20 AM

if it's not balanced yet, i'd cam cut the counterweights. that leaves the skirt alone and reduces rotating mass.
Posted By: BILLYJAY

Re: piston skirt to crank clearance, help engine guru's - 06/16/09 01:23 PM

in My Big stroker Motor(4.75Stroke) i had the block clearanced for a 5" crankshaft and still had the counter weights cut .180 off the OD of them..it was close but they did put some mallory metal back in to balance..like dram said ,if it hasnt been balanced yet..cut the crank not the pistons...

Attached picture 5295914-INDYMAXXBUILD018.jpg
Posted By: dave571

Re: piston skirt to crank clearance, help engine guru's - 06/18/09 04:49 AM

Quote:

if it's not balanced yet, i'd cam cut the counterweights. that leaves the skirt alone and reduces rotating mass.



I agree, but it has been balanced.

That's why we are thinking the skirt. It is very thin where it contacts, and the amount of material would be very little.
Posted By: Anonymous

Re: piston skirt to crank clearance, help engine guru's - 06/18/09 05:42 AM

The guy acroos the street did that with his BBC in his boat.just ground enought off the piston under the pin so the crank didnt hit.Somehow when it ran it hit.It only pulled the pins out of 4 or 5 pistons.On its first day on the water.
So yeah by all means dont cut the crank and rebalance it save your money for your next motor
Posted By: Ron Silva

Re: piston skirt to crank clearance, help engine guru's - 06/18/09 07:27 AM

Most Crank builders do not cam cut the counterweights any more. I think Moldex still does. Of course they don't care because they can charge you more to balance (mallory metal)

If you think about the shape of the piston you might be able to just Chamfer the counterweights and get the clearance you need and even that could be done on an as needed basis similar to a cam cut.
Posted By: 440Jim

Re: piston skirt to crank clearance, help engine guru's - 06/19/09 01:24 AM

Even if you cut the piston, you should rebalance the crank to match; but it will be "small"

I bet you are hitting at the piston pin boss area. In this pic, these pistons are 0.150" thick in that spot. Strengh and thickness vary with design. But I wouldn't take 0.060" off of these (ie .090").

Are you using a 6.76" rod length or somthing shorter?

Attached picture 5301658-Ross_99496_dim.jpg
Posted By: 440Jim

Re: piston skirt to crank clearance, help engine guru's - 06/19/09 01:27 AM

This is from a post a few years ago:

As far as counterweights hitting the piston at the bottom of the stroke, the typical problem is the piston pin boss, ie metal below the piston pin. This thickness varies a lot from one piston design to the next. My Ross 99496 pistons have 0.150", Wiseco K296A35 have 0.220", my Venolias have 0.180" (see attachment). Everything must be checked at assembly for clearance, but this is the math:

CWmax = Counter Weight max dia
Pthick= Piston thickness below the pin
Pin = Piston pin dia
Rod = Rod Length

CWmax = 2(Rod - Stroke/2 - Pin/2 - Pthick - Clearance)
For example:

CWmax = 2(6.535 - 4.25/2 - 0.990/2 - 0.180 - 0.075)
CWmax = 2(6.535 - 2.125 - 0.495 - 0.180 - 0.075 )
CWmax = 7.32" dia

That happens to be the dia of the "cam cut" on a Callies 4.25" stroke crank counterweight for a BB Mopar.

With a 6.535" long rod, 4.500" stroke and assuming the Ross 0.150" and 0.060 clearance:
CWmax = 2(6.535 - 4.500/2 - 0.990/2 - 0.150 - 0.060)
CWmax = 2(6.535 - 2.250 - 0.495 - 0.150 - 0.060)
CWmax = 7.16" dia

That should work depending on the bobweight. I cut my Callies 4.250" down to 7.14" with a 2185 gram bobweight.

But with a 6.385" long rod and assuming the Ross 0.150" and 0.060 clearance:
CWmax = 2(6.385 - 4.25/2 - 0.990/2 - 0.150 - 0.060)
CWmax = 2(6.385 - 2.125 - 0.495 - 0.150 - 0.060)
CWmax = 7.11" dia

The rest of the counterweight can be larger since it isn't at BDC. Balancing will need to be addressed with those counterweights. It might work, but I like the 6.535" rod for that.
My Callies was cam cut from the factory at 7.55"/7.32", but I cut it down to 7.14" dia (round) and balanced at 2185 gram bobweight. The balance shop went between the lathe and the balancer to get this close, don't just use my dia with other cranks and bobweights.
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