What is the max safe overbore on a 3.8?

[jjvites]

Was wodering what is commonly the largest overbore on a 3.8 motor. What are common size pistons that are available? I have a spare block I picked up but it's at .030 now.What is the safe overbore size for 3.8?

 

[polin2424]

Bore

I think its .030 but maybe you can go .040.

 

[Rhinospeed84]

I've seen quite a few at .040 that ran great.

 

[REDS HOT AIR]

every block is different .. theres probly plenty that would take .100 over

but without sonic testing .030 to .040 is plenty safe

 

[Blown&Injected]

I'm at 40 over but due to leaking fuel, I have not yet had a chance to run the motor yet - I do not believe there will be any trouble

 

[jjvites]

Wall thickness ... how much

If sonic tested what can one expect to see on a production3.8 block for wall thickness. Anyone know what it is for stage 2?

 

[MAX PSI]

I ran one at .040 over. It ran many low 10's without any problems. A stock rod broke and killed that block. I remember an artical in a GS Extra that said they havn't seen a 109 block that wouldn't except a .040 overbore. But if it is for a race motor I would have it sonic checked.

 

[jjvites]

Phil,
Thanks, just wondering when sonic testing what is good wall thickness on these motors. Need a reference point.

Thanks,
Marty

 

[MAX PSI]

I found the GS X-TRA that had the article (Sept-Oct 2003). The recommended wall thickness is .250" on a major thrust side and .150" on the minor thrust side. Also in a seperate article " Mid To Low 11-Sec. T-R Buildup" they say " All the 231's that we've sonic checked have been able to take a .040" overbore."
With all that said, I'll throw this in, on my last motor I had 2 cylinders crack and it was bored .020" over. Good luck!!!

 

[pfredric]

Here is the sonic check I got for an 85 hot air block that had 100K miles:

http://home.comcast.net/~fredricp1/Paul/GNProject/4140SonicMap.JPG

 

[jjvites]

Here is the sonic check I got for an 85 hot air block that had 100K miles:

http://home.comcast.net/~fredricp1/Paul/GNProject/4140SonicMap.JPG

Thanks Paul,
That helps , I would think that the 86 and 87 blocks would be similiar.

 

[chevyII]

aftermarket blocks are only .250. I had a stage1 sonic checked and most thrusts were .170-.190. Between the cylinders is were things got tight .077,.090,.120. Production blocks including stage1's were mass machined were stage2's were done by buickmotortsports. 3/16 is a good number for stage2 blocks,doubtfull to find a stocker that thick.

 

[jjvites]

aftermarket blocks are only .250. I had a stage1 sonic checked and most thrusts were .170-.190. Between the cylinders is were things got tight .077,.090,.120. Production blocks including stage1's were mass machined were stage2's were done by buickmotortsports. 3/16 is a good number for stage2 blocks,doubtfull to find a stocker that thick.

Thanks, good reference point.