Just a little curious whether this setup is going to work. I'm working on a better bullet for next year, but needed something to get through this year so I screwed together a piece for the interim.

It's a 4.004" sbc with 6" eagle rods, SRP pistons, and a BRC 3.550" crank. Whoever machined this setup has the pistons .005 out of the hole, and I've put a set of 1003 Fel Pros on it with .041 compressed thickness. I have always been told that .035 was the minumum clearance, but have never built one that was this close. I've fired the engine, and it seems fine, but before I get it hot or run it I want to know it's not going to grenade This engine is expected to see 7,500 rpm and will run 30 minutes at a time in a 2,700lb GT1 car.

TIA

 

"4.004" ..... assuming you meant 4.04 right?

Good forged crank, forged rods and a good bore fit without too much piston rock... it will be fine. I've run tighter and know quite a few gents who have. Can't get away with it running flexy-flier components and/or poor machine work though.

-Mindgame

 

Whew, that's cutting it very close, isn't it?

Of course you are using forged pistons, but what's the piston to wall clearance?

At 7500 working rpm I am assumning very light pistons. How long is the piston skirt and what's the compression height, about 1.230? I'm concerned about how much piston rocking you'll get. With .006 clearance at the fat part of the skirt, rocking the piston could give .010 to .012 rock which takes away about half of that in piston/head clearance.

Must be a 358 cube limit: you're very close (357.6), so I'm assuming you are pushing most things in the engine to their limits. If it is 358, what do you do to freshen the engine? It sounds like you started with a fresh block. Bowtie?
edit
Oops, Mindgame picked up on 4.040 probable bore size, and your class adjusts weight to displacement, I think. If that's the case, forget the 358 cube thing.

I'd measure every piston to see how far it's out of the hole and double check the compressed gasket height. If all that was spot on, I'd break it in and run a few laps. Maybe pulling a head then to look for any signs of contact would be reassuring. If you don't see any, go for it!

My $.02

 

Depends on the taper of the barrel. Easy enough to check and I agree, he should.
I've run as tight as .032, 4340 crank, 4340 rods, JE claimer pistons (~4.028), shifting at 7300 rpm. Never had a contact problem but I agree, this is getting really close. Sometimes you just work with what you've got as in my example but I'd like to see a bit more... I personally wouldn't sweat .037-.040 though.

-Mindgame

 

I'm the conservative one here, I guess. Are you sure about the compressed gasket height? Fel Pro catalog lists .038" for the 1004 gasket. If that's the case, with 0.005 out of the hole your clearance is only .033". Of course, that's "only" two thousanths of an inch less than .035", you could always let the pistons clearance themselves

Most of these numbers have a little fudge factor, and Mindgame says he's done it, so I guess it might work. But racing it that tight for 30min means eveything shoud get well and truly up to temp and it would make me nervous.

Rich Krause

 

Frankly, I don't understand. The reason some engines are ASSEMBLED with .001" clearance, and others at .008" clearance is cuz the clearance is based on piston design and material composition. The combination of the two will determine the expansion rate for a given operating temperature. Well, ok, and how much the wall moves (in or out) is a factor also. At operating (or anticipated max) temp the clearance should be about a .001" (or less). So, unless you anticipate reving the engine while still cold, (in which case any piston/head contact is deserved) piston rock should not be a concern.

As far as .030" with STEEL rods, a 'piece a cake'.

 

That is definately close.

I have ran as tight as .028 and shifted at 8,700 on a factory forged crank with carrelo rods and ross pistons. make sure you check your piston to valve clearance as they tend to get close when you are that tight. write them down and remember when you change cams or rockers it will get much closer

Good luck!

 

Phwew! Lots of replies and I appreciate them all.

I'll try to remember all the questions I read..

1) Bore is 4.004"

2) I don't exactly recall the piston clearance, but will check my notes tomorrow. I was thinking it was .005, but I don't know that SRP makes odd sized pistons. I believe SRP designs about .0045 into them @ size, so this might be .0085

3) We're running this bore/stroke combination because it was inexpensive and the budget is TIGHT right now. This shortblock was actually intended to run in a sprint car, but I disassembled it to have the front tapped for oil. The previous setup was in the filter housing, and I'm not a big fan. So far, this project has run into the mid $30k range this year and we picked up this short for $1,800 brand new. If this combination can hang on until early June we'll be stepping up to a far more serious combination

4)The gasket number is 1003 and Fel Pro lists compressed thickness as .041.... unless my vision is failing also.

5) The pistons are .005" out measured across the pin, and I was unable to rock them sufficiently by hand to change that reading... this all according to a fixture I built some time back that holds a dial indicator.

6) Piston/valve clearance will not be a problem under any circumstances with this engine. Clearance with 1.6/1.6 rockers came in at >.100 intake (forgot exactly) and >.150 exhaust. At this point I'm running 1.5/1.5 rockers, but will experiment with 1.6 on exhaust since the lobe lift on the cam is the same on both sides but the exhaust has slightly more duration.....I don't have the card here right now.

7) These rods are steel, albeit cheap steel( Eagle ESP), crank is BRC steel, and pistons are forged srp. It's obviously not a serious setup, and not unusually light.

8) The block is new, but is a goodwrench, not a bowtie.

9) We are limited to 360" with the car in it's current configuration. When the budget allows, I will remove ~200lbs ballast and build a serious 304" combination.

Does anybody make a gasket that's .045" compressed? I'm strongly considering pulling this thing apart tomorrow to check the bottom end, and changing the gaskets.

 

Trust me, I'm plenty nervous Even though this isn't an expensive piece, I'd like to get through a few weekends with it. I have about a third of the parts for our new setup, but in the meantime, if this one goes, I'll have to use the backup motor- which isn't set up for a dry sump yet, which means complete teardown. Worse yet, the bastard has terrible piston/plug clearance.

 

As for measuring piston/wall clearance, I just follow mfr recommendations as to the height. If none is available, I measure at the bottom of the pin perpendicular to it's axis. It's been a few weeks since I did this one, and I have forgotten exactly where SRP recommends, but that's where I measured it.

 

Every piston company is different. JE/SRP is one place, Wiesco, Diamond, Kieth Black they all have their place and P to Wall clearance for each setup. Gotta make sure you measure the bore and the piston off the same guage. That helps a ton.

Bret

 

I'm surprised that piston skirt to bore clearance is less than .001 under load. That seems like a recipe for scuffing to me. I guess roundness and taper of a cylinder bore of .0005 TIR probably wouldn't be good enough for that kind of clearance, would it?

 

Only ~200 lb. penalty for 56 or so cubes? That's about 18% more displacement for 8% more weight. Sure, unloading the tires a little should help some on lateral g, but unless there are other restrictions on the exta cubes like head port size, restrictor plate/carb size, intake manifold size, etc. I would consider a large bore, shorter stroke 358, like a 4.155 bore and a 3.30 stroke to keep the piston speed down, and hp up.

How about a link to the rules. I'm curious.

 

Check with the gasket manufacturers..... detroit, corteco, rol, mr gasket, I'm sure you can find something a bit thicker. If the deck faces are good enough, you can use a copper gasket as they come in a real wide variety of thicknesses.
Important to mic the piston where (along the skirt) the manufacturer specifies.

-Mindgame

 

The rules are property of SCCA and IMSA and you won't find a link out there. Unfortunately I don't have the book here.

Whatever combination we decide on will be a big bore setup with a set of SB2 heads if they are allowed, and a trick set of 18* heads if not ( 14* heads I don't believe to be legal right now).