The Stroker build up saga continues.....

When we last left our would-be stroker builder, he was left with a dilemna. Having over-honed the cylinders .0007" to make room for the thickness of coating the stock pistons, he found himself with out of spec ring end gaps. (.58mm/.50mm to the factory's recommended .20mm to .40mm) "No problem" thought the stroker builder, "I'll just get over-size rings, they do it all the time in Chevy-land".

Well, just because the rings get longer, doesn't cure the fact that they may need to get thicker. So the question is, anyone have any idea what the factory ring back spacing tolerances are? (space between the inside edge of the ring and outer diameter of the base of the ring land)

Smokey says he likes to .010", and at .040"-.050" you are SOL. Haven't pulled out the dial bore gauge and micrometers yet, but a quick go with a digital caliper has me hovering at around .030".

Any comments are welcomed.

 

Don't want to hijack the thread but what are you doing about head gaskets when you've bored out the block? Obviously this needs custom gaskets - do you make them?

 

Sounds to me like the pistons need to have the coating milled off and the cyllinders need to be coated or sleeved.

 

I'm not so sure there is a problem here Adam. I'd try and get some sort of measurement before you do anything drastic.

 

Adam C wrote:

Motorsport in UT can custom order Cometic's. Mine will need to be a custom thickness due to my negative piston/deck clearance. The good thing about getting the custom gaskets is that they can make a 928 gasket with bigger fire rings. Using a pair of 968 gaskets requires one of the gaskets to be flopped over and cut for the coolant pathways.

Ernest wrote:
Sterling wrote:
Pistons haven't been coated yet. If I coat or plate the cylinders, I'd go with domestic pistons. Always an option, just not my first choice.

BC wrote:
That's what I'm trying to determine. I was told by everyone I asked regarding the ring end gap situation that the car would run, and "probably" wouldn't be a big deal. Even my machinest concurred with this, but said he'd like to see it brought into spec. From what I've read, ring end gap is a pretty critical tolerance.

I'm assuming that back spacing is also pretty critical, since too much apparently causes too small a contact surface on the top and bottom of the ring (depending on which stroke) for the ring to properly seal the charge (or crankcase pressure).

In an ideal world, someone out there would have a bare block that they could put on OEM ring into, measure the internal diamater of the ring in the cylinder, and then measure the diameter of the piston in each ring groove. With a digital caliper, I got a 95.4mm I.D. measurement for the ring in the cylinder. The ring channels in the piston are too narrow for the calipers to fit in, so I measure just above the ring land, and subtracted the depth of the groove (4.96mm x 2 = 9.92mm) from the O.D. of the piston crown of 103.60 for a piston diameter in the ring groove of 93.68mm. 95.4mm - 93.68mm = 1.72mm difference in diameter between the two measurements. Divide by 2 leaves a back space measurement of .86mm = .033".

So, if .010" is golden, and .040" is a do-over, does that mean that .025" is an acceptable tolerance? My measuring methods so far are far from precise, but even when I do have the time to do them right, I'll still need to know what would be considered an acceptable tolerance.

Hmmm. Maybe time to give Mahle motorsports a call on some Ferro-stan units. 968 group 1's would be just the ticket.

 

Listen - maybe I can help.

I have my 89 block in my garage waiting for when I can remember to clean the ring grooves and send them to swain finally.

I haven't ordered my rings yet, but EBS engine supply said that they would be about 200 dollars, and its time for me to order them anyway. They have been superceded to a 944 turbo ring for the stock cylinders, and that is what I will be using.

Can I help with the pieces that I have and will buy? What measurements do you need other then the ID of the rings?

 

I see that you need the OD of the piston IN the ring groove.

 

Adam, doesn't your caliper have a depth gauge that you can use for the ring groove? usually this is a rod that comes out the bottom of the caliper and can get into 1mm or narrower recesses...

 

Dave wrote:

Nice graphic. Actually, that is what I used to get the 4.96mm depth. The only potential problem is that the base of the caliper handle doesn't straddle across the ring lands with any amount of precision. It would be ideal to actually measure the piston diameter inside the ring groove.

Something else I noticed in one of the rebuild books, it talks about ring back spacing being measured when the ring is flush with the side of the piston. Meaning you push the ring flush with the side of the piston, and then see how much further it will go into the channel. This particular book doesn't give a spec, it just says "as close to zero as possible". Doesn't sound quite right to me.

 

"Close to zero" sounds reasonable to me. You want maximum engagement between the ring and the ring land. Since the aluminum expands faster than the steel ring, this is safe. Due to the piston-to-bore clearance there will be some additional space between the ring and the bottom of the groove. This is a tough dimension to measure, and 4.96 is probably close enough. If you can find a ring where this dimension is 4.95, and it sits flush or just an RCH below flush, then you should be OK.

As for the graphic... thank Google & V. Ryan!

 

Ring ...back
Ring ... back
OH Ring bank my Bonnie to me .. to me ..

Ring .. Back
Ring .... back
Oh ring back my Bonnie to me.

Speaking of which ... where has our little buddy MPEsik been lately??!!!

 

Heinrich, put down the beer and step away from the keyboard!

 

 

Sharkskin wrote:

After a little more investigation it turns out that the ring back spacing measurement is taken starting with the ring flush with the O.D. of the piston. This being the case, my back space spec is fine. What I don't understand is how this correlates to operating temp tolerances. For example, a domestic V8 may have .010" ring back spacing, but when the piston sits in the cylinder, the ring isn't flush with the piston, the tension of the ring will hold it open against the cylinder. In this case, the cylinder bore would typically be about .004" bigger than the piston. That would create way more ring back spacing than what I'm dealing with in my motor. Once the engine gets up to operating temps, the diameter of the piston should catch up to the cylinder, but at that point I'm assuming that there would still be more than a nominal piston/cylinder wall clearance, meaning again that the ring back spacing would be more than .010". (Bad assumption?)

I think the only measurement that is going to give me a definitive answer will be a cold OEM measurement like I asked for above. If anyone has a bare block that they could put a ring in and measure the inner diameter of the ring in the bore, I'd be much obliged. I have an S4 piston that I can take a O.D. measurement of the base of the ring channel.


Heinrich wrote:
Next time a little forewarning would be in order, I barely had time to grab my stein.

 

Adam, I think the 928 Forum magazine would love to do an expose on your travels through stroker land .... how 'bout it?