01 Cobra stroker motor problems

[Outlaw49]

What should the piston clearance be for a 2001 cobra n/a?

I recently went through my 2001 cobra making several upgrades, .020 over on the bore, 5.0 stroker rotating assembly with a 3.8 Kellog crank and 5.85 Manley rods. When I started it up it sounds like it has severe piston slap. I let it run getting up to temperature twice running for about an hour total. The sound did not change after it warmed up.

We then removed the motor and completely dis-assembled it hoping to see something wrong but there is nothing obvious other than at the bottom of the stroke the piston does seem to have lots of movement.

The block was prepped with .004 clearance on the piston per the piston manufacturers recommendations. Could this be the source of the noise?

 

[01bluesnake]

What is your actual measured bore diameter, and what are your measurements on the piston diameter on the middle of the piston. Did you by chance also measure your skirt area diameter and ring land diameter?

 

[Outlaw49]

Bore: 3.572
Half way down skirt: 3.5685 on average

One thing that seems odd is piston tapers smaller right below oil rings to 3.5655.

When the piston is in the bottom of the bore they have alot of movement.

 

[01bluesnake]

Ok. What is the piston diameter 90* to the wrist pins? Take that number and subtract from your bore diameter and you will get your total piston to wall clearance. If that is all in spec, then i would start looking for other causes of the noise. Being that the noise didnt change even after the motor warmed up, it may be something else.

 

[Outlaw49]

In that area it measures approx .566 which would put me at .006 clearance...specs called for .002 to .004. I don't currently have the pistons on hand with me. They were custom built by a company called DSS racing if anyone on here is familiar with them. I sent them back to them and they are going to review them Tuesday. We believe they may have made a forging error due to the odd taper they have in them. The stock pistons I took out have no taper at all.

 

[SlowSVT]

The problem may be associated with the 3.800" stroke and the shorter rods. Not much of the skirt will be left in the bore at BDC where the piston can rock back and forth. The shorter rods will put more side load on the pistons compounding the problem.

 

[SVR-S]

I built my engine to a similar spec,ie,Kellogs stroker crank and Manley rods,but I did fit offset wrist pin pistons to counteract any possibility of piston slap.This engine runs real sweet and quiet,having done 4k miles since build,revving to 7800 on track.
I do think your piston to bore clearances are excessive,as measuring mine prior to instal the clearance was 0.0035" max: At 0.006" you quoted, pistons will give a slap at bottom of stroke.

 

[Outlaw49]

I built my engine to a similar spec,ie,Kellogs stroker crank and Manley rods,but I did fit offset wrist pin pistons to counteract any possibility of piston slap.This engine runs real sweet and quiet,having done 4k miles since build,revving to 7800 on track.
I do think your piston to bore clearances are excessive,as measuring mine prior to instal the clearance was 0.0035" max: At 0.006" you quoted, pistons will give a slap at bottom of stroke.

Are you running a 3.8 stroke with 5.85 rods? What piston manufacture did you use? The stock 4.6 pistons do have the offset wrist pin you are referring to. My new ones had the pin centered.

 

[SVR-S]

Manley 5.85 rods with Manley pistons fully coated skirts and 1.2mm offset wristpin.
Recommended (by Manley) bore clearance 0.003" with coated skirt.

 

[Outlaw49]

Was yours a 3.75 or 3.8 stroke? Also do you have a steel or aluminum block? Thanks for the help.

 

[SVR-S]

3.75 Kellog crank running in a new standard bore Teksid Alloy block.

 

[Outlaw49]

Has anyone successfully built one with the 3.800" stroke?

Like SlowSVT mentioned with the longer stroke and the shorter rod the piston is out quite a bit at BDC.

However, after talking to both Manley and Ross Pistons today, both stated that an offset pin would be needed to prevent piston slap. The company that built mine were built with no offset, and it sounds like this may be my problem.

 

[01bluesnake]

Has anyone successfully built one with the 3.800" stroke?

Like SlowSVT mentioned with the longer stroke and the shorter rod the piston is out quite a bit at BDC.

However, after talking to both Manley and Ross Pistons today, both stated that an offset pin would be needed to prevent piston slap. The company that built mine were built with no offset, and it sounds like this may be my problem.

Too bad your not a 3.75 stroke, i have a new set of custom MMR/Manley stroker pistons in the FS section currently i wont be using cause my car is getting parted.

 

[Boss 327]

Has anyone successfully built one with the 3.800" stroke?

Like SlowSVT mentioned with the longer stroke and the shorter rod the piston is out quite a bit at BDC.

However, after talking to both Manley and Ross Pistons today, both stated that an offset pin would be needed to prevent piston slap. The company that built mine were built with no offset, and it sounds like this may be my problem.

I have a successfully built motor with a 3.800" stroke. MMR built it so Im sure they will have the answers needed.