Looking for some advice on a street build. 20+ yrs. ago I was working on a '70 350 block for my minor knocking 76 Olds (213K) and got all machining, balancing, etc. done, but unassembled. That same year an asphalt dump truck lost brakes and tipped over sideways on my car -totaled. 2 weeks later found the same model 442. Now 20 years later it threw a rod (200K).

I just sent the block to have all tolerances checked since I measured about .004 on piston wall clearance. I used feeler gauges .003 and .004 (very snug fit vs. the 003). Piston upside down in bore measuring skirt. Shop says I have .005 and they recommend .0025. These are +.030 Speed Pro forged flat tops with teflon coating. Everything else checked out within tolerance.

Do I spend the extra (+40 pistons/rings, bore/hone, press pistons, balance) or just build as is ?
Thanks.

Neil

 

2.5 thou on a speed pro forged piston? That would be really really really tight and probably tear off the teflon coating real quick.
IIRC 3 to 4 is normal. 5 is loose but not crazy - it'll rattle when cold but it'll run just fine. That kind of clearance becomes recommended to necessary for high power stuff, forced induction, etc. The lesson to learn is you need to add more power to justify the clearance. Gap the rings, add nitrous, build for 7,000 RPM, etc.

 

What part number piston is it. Some of the Speed Pro / TRW pistons were set up tight and some not so tight. You cant just go by the fact that it is a Speed Pro forged piston. Need the part number.

 

004 on piston to bore, or it will run hot. There are 10s, if not hundreds, of posts involving TRW/SpeedPro slugs.

 

My understanding as well, should be just fine. Run them, any off the shelf pistons are big dish cast or much lighter and definitely needs rebalanced with modern forged pistons. The Speedpro forged aren't available above .030" oversize.

 

If the bore was done correctly the clearances should be correct because they are built into the piston. Also the clearance might be measured somewhere other than the bottom. If you still have the piston box it will tell you where to measure.

Ask them what the actual bore size is
.005" sounds loose to me but it probably wont hurt anything. The only 350 Forged Piston I see is the L2321F30 and it calls for a .002" minimum clearance.

 

Seems to me the box says .001" and made in India. Quite a few had issues running hot with them at .002".

 

Thanks for all the replies guys. Box marked Speed Pro and was part of a Summit rebuild kit with piston # TRW-2320-F30. As I recall the spec sheet given to machinist (20+ yrs ago - 2005?) said .001-.002 clearance. These pistons have the teflon and horizontal slot cut just below the oil ring.

I had a 73 Z-28 engine I rebuilt with my brother with TRW forged pistons and I am pretty sure I remember the clearance was .004 or 5. That's why I am wondering if this is too excessive, but maybe also why the machinist did 0.005. I couldn't find any Speed Pro +40s either. Mahle and DSS make 4.100 but a lot lighter at 525 grams, uses 1/1.2mm rings, and a good bit more cost. More race than street piston.

Machinist said he would check his sources for Speed Pro +40s, but if he can't find them then maybe 'as is' wins the toss. I just hope it doesn't slap itself silly with all that clearance. I have been prone to bad luck my whole life so got a bad feeling about this one. Day after machinist called about .005 my water heater started leaking - day before Fri 13

Thanks again guys.

 

Speed Pro never made those pistons in 40, at least not according to the catalogs I have. The catalogs I have say .0025" minimum clearance. I would probably put it together and be done with it.

 

Not true. You’d be surprised what you’ll find if you were to pull apart a modern day engine, even a 100hp four cylinder. Ring and piston tech has come a long way.
I think the next size for a Speed pro would be 4.125, nos 425 pistons. Probably the only .040 over speed pro you’ll find today will be cast.

 

I have those Mahle 4.100" pistons on Molnar rods on top a 330 crank. Very street friendly .0025" piston to wall clearance and a 4032 material. Hasn't used any oil but don't have many miles on it.