Dolmar 64xx/73xx/79xx series, smurfs and solo 665/675/681 thread

[mettee]

I don't work for the same company anymore, but they had a flow bench, I would love to have tried each filter set up on that.

 

[mettee]

I took some of the advice I got a little further and looked at the piston in the case as well as the cylinder. You can see where and how the fuel mixture is stuffed, flow is almost shoved outward and then up the transfers.

 

[huskyboy]

I took some of the advice I got a little further and looked at the piston in the case as well as the cylinder. You can see where and how the fuel mixture is stuffed, flow is almost shoved outward and then up the transfers.

Makes me ? why they bothered using a windows piston in early 7900s. I’m thinking the slab side has to have more case compression than the windows 7900 piston?

 

[mdavlee]

I’m thinking the slab side has to have more case compression than the windows 7900 piston?

Yes

 

[huskyboy]

Seems like a window piston is better for wall fed cylinders?

 

[mdavlee]

Seems like a window piston is better for wall fed cylinders?

I think so.

 

[Deets066]

Makes me ? why they bothered using a windows piston in early 7900s. I’m thinking the slab side has to have more case compression than the windows 7900 piston?

7900 has plenty of case comp as it is.
Some of the strongest 7900’s had windowed pistons in em

 

[huskihl]

Makes me ? why they bothered using a windows piston in early 7900s. I’m thinking the slab side has to have more case compression than the windows 7900 piston?

The one that weighs more should have more case compression

 

[mdavlee]

I think where it’s at can be more beneficial than just more of it.

 

[Deets066]

The one that weighs more should have more case compression

The one that has more mass below the rings is the one with more case comp. not necessarily overall weight

 

[Deets066]

But case comp isn’t really sought after in a 7900. Don’t need more than they have really

 

[mettee]

Makes me ? why they bothered using a windows piston in early 7900s. I’m thinking the slab side has to have more case compression than the windows 7900 piston?

Absolutely. The next thing it made me start thinking about is a way to plug both ends of the piston pin without adding weight.

That would probably put back everything porting took out.

But seems the concensus is that there is plenty.

 

[huskihl]

The one that has more mass below the rings is the one with more case comp. not necessarily overall weight

Yes. I figured they both had the same compression height and overall height.
Provided they're made from the same alloy, the heavier one has more mass below the ring. Unless there's a hollow cavity in the lighter one

 

[huskyboy]

The main reason I liked the idea of the slab side in the 7900 was that its quite a bit lighter = less reciprocating weight. In theory... also less stress on crank.

 

[huskyboy]

The slab side and windows were within .05 of the same height. At least the ones I had here.

 

[huskyboy]

One thing I did noticed was the ring pin on the windows piston was better position if one wanted to widen the uppers more if they were to port it. Or maybe they don’t really need to be widened that much?

 

[huskihl]

One thing I did noticed was the ring pin on the windows piston was better position if one wanted to widen the uppers more if they were to port it. Or maybe they don’t really need to be widened that much?

Not sure why they did that. The early slabs had the ring in the middle of the intake

 

[mettee]

Maybe the lessor of two evils?

 

[NCFarmboy]

Both of my slab pistons had ring pin to one side not in the middle like the window piston. They were in 7900 P&C kits. I didn't know a slab piston ever had a center pin.
Shep

 

[huskyboy]

I wonder if they moved it to the side to improve wear characteristics?