Piston to cylinder wall clearance

[guy]

I am putting a new engine together and was wondering what piston to cylinder wall clearance and what ring gap would be appropriate? I have a 340 R block and light weight Ross pistons at .040" over and 11.5 to 1 compression. We turn the motor around 7600 rpms. Any help would be greatly appreciated. Thanks.

[340king]

Ask the manufacturers. That is all that is needed. I think I saw a recent add for Ross Pistons claiming real small clearances for cyl. bores. Like less than .002" for their new series of forged pistons. You will need to check with the piston manufacturer for sure for skirt clearance.

As for Ring end gap that varies with application, carburetion, compression, fuel, etc. Most ring manufacturers will have suggestions right in the box. The only caveat to this is when the piston manufacturer has specific requirements like KB pistons do.

[guy]

The truth is that I just got the motor together yesterday and it starts fine, turns over really nice. As soon as it warms up the motor really tightens up and stalls. After a couple of minutes of cool down it starts right up and runs great until it warms up again and tightens up again and stalls again. We triple checked all clearances to the suggested manufacturer specs when we put it together. I think that the pistons are growing at a faster rate than the block and tightening up. Ross suggested .005" clearance and thats what we had it at and I was wondering what other people were setting theirs at. We will pull the motor out and tear it down in the next few nights. Any suggestions would help. Thanks.

[sanborn]

Is the block filled with "Hard Block"? I have a friend who had a problem with piston galling with the jackets 1/2 filled with "Hard Block".

[guy]

No we don't have any Hard Block in the motor. A friend of mine asked the same question and said that he had to add .006" clearance additional in his block that was filled with Hard Block.

[340king]

At what point on the piston did you measure your diameter? I varies from manufacturer to manufacturer. The forged pistons generally have quite a bit of taper to them and can be difficult to measure if you are not used to it.

[guy]

We received a fact sheet from Ross when we had them make the pistons and we measured them exactly where they said to. These piston are extremely light weight and we are wondering if they are growing at a faster rate than the our block. Is the R block thicker walled and may not be growing at the same rate? Got the engine out tonight and will take it apart tomorrow. Got a race coming up next saturday so we have to figure it out quickly.

[rjl1160]

is it a siamese bore block?, the thicker the bore, the more clearance required, you are correct about thicker bores expanding less.

[340king]

This is bringing back memories of Heat Transfer 301, with Dr. Chaing, piece of cake!! You may notice my eyeballs spinning in the smilie, I got my head to stop at least.

Cutting to the chase, you are correct in assuming that there is a differential expansion going on in the engine. This is due to several different things. First, you have dissimilar metals. These expand at different rates. Add to that the fact that the piston is much smaller and heats up/cools down much quicker. (This supports your assumption). The block being composed of cast iron and cooled by water takes a long time to heat up and expand. When it finally heats up, it takes a long time to cool down. The heat capacitance works both ways.

This is why it is important to not rev the engine until it is up to full operating temp on a low clearance engine. By this I mean an engine with less than .050" piston to cyl. head clearance. If you rev it too soon, the pistons will acquaint themselves with the heads. If you are lucky, that is all they will do.

As far as your question on what clearances I have run in the past, KB pistons, < .003". They suggest super tight P:W clearance, but we went a little wider. Never have had a problem. Ross pistons, approx. .007". The only engine I have been around that had problems with piston to wall interferance was a 451 stroker. By coincidence, it had Ross pistons in it. I was involved in the post mortem rebuild and not the original. I had to knurl several pistons, just to get the guy through the season. It finished the season.

[rjl1160]

340 king, what was kb's recommended clearance for the set you ran at .003 and how much power was that engine making? - thanks, rj

[340king]

From what I remember, this engine was built back in 1994 and I am getting older, but they recommended like .0015" cyl to wall clearance. We thought that was too tight so we went bigger. I had never ran the KB pistons and wasn't going to sacrifice all my parts and block prep on a setup on the edge. We may have given up a little HP by going bigger, but I was skeptical. Since then, I have been impressed with the performance of the pistons. I think the last engine we did with them, we set it at .002".