Just wanted to inform you fellows to be aware of a piston issue I just discovered with the new Lunati 4.125" bore 4" stroke kits mainly used with Darton MID blocks.

I did a trial assembly of a customer block to find the reluctor wheel hit the machined piston for cylinder 8. Lunati had two such pistons in this set and neither would clear. I blued a piston to see where the interference is. I'm going to set this piston up and machine the side a bit more and match the weights of the other seven pistons. I checked the balance on the crank yesterday after seeing a problem with their bob weight calculation. I doubt the piston machining will warrant rebalancing the crank another time.

I sent Lunati a photo of the piston to make them aware of the problem. Several sets have been shipped already so heads up!

Rod bolts clear with no problem by the way.

Second problem is the piston is up out of the block .018". This on a new LS6 block I machined .005" off of after installing the sleeves. Some of these blocks are a bit short of the 9.240" deck height and used blocks need to be machined more than .005" in some cases because of warpage. The new Cometic MID specific gasket as well as the 1041 Fel Pro gaskets are about .040" thick compressed. With a .040" thick gasket that would leave only .022" piston to head. Not nearly enough. I made Cometic aware of this and they are making me a set of .054" gaskets for this job which brings the piston to head to .036". I recommended they produce gaskets for stocking in .054" as I am certain deck height problems will be a continuing issue with any brand of stroker kits. Compression height is very limited with 4" stroke and 6.125" rods without running a support rail in the oil ring groove.

Steve Demirjian
Race Engine Development

I just got a call from Cometic regarding a gasket change. Instead of the older three layer gasket .054" thick they are coming out with a better 4 layer .052" thick gasket for these applications. Still leaves me .034" on this engine which will be sufficient piston to head clearance.

 

Great post!Thanks for keeping us informed and doing everyone's R&D!

 

thanks sir...good info...
We machined a Diamond piston for reluctor wheel clearance (3.905 bore, 4" stroke) and callies said it did not warrant a rebalance of their rotating assembly.

 

I guess the Eagle kits aren't the only ones out there with problems.

 

I machined the side of the piston for reluctor wheel clearance and figured I would post a picture in case anyone else has this problem. I ended up taking .020" off the surface of the piston. Weight removed was only half a gram. I'm going to e-mail the piston photo to Lunati so they can correct the sets they have on the shelf.

I also have a picture of the rod bolt clearance which is adequate - no notching required.

Photo of block on engine stand for those not familiar with a MID sleeved block.

Steve Demirjian
Race Engine Development

 

thanks for the info

sent you a PM

 

Steve, once again, your info is very succint and helpful. Keep up the good work.

That block looks purty on that stand.

 

good info.. pics look sweet..