cofab pistons? again...

[joop]

I like the stock KS and mahle pistons, but I bought a set cofab DG pistons wich look cheap (they are cheap lol) and the measurements are off too! But the cylinders are OK

Top picture is a picture of a stock DG KS piston and on the right a DG cofab piston.

IMGP4712.JPG

Bottom picture is a DJ piston on the left and the same DG cofab on the right

IMGP4711.JPG

The distance difference is @ 2.5mm.

Is 2.5mm too much to remove?

[Joe vw]

Those are 1.9 pistons
do the math-
76mm-69mm= 7mm divided by 2 (half the stroke) = 3.5mm and all Cofab pistons are 1mm short and you come up with 2.5mm

[Piledriver]

If you can deal with a higher CR using 1.9 slugs machining 2.5mm off might be a much better plan vs. running 2mm+ deck.

To get the deck right with off the shelf aftermarket pistons, folks have used longer rods and/or stroker cranks.

The top ring is WAY down on these pistons so 2.5mm likely won't weaken them, but the CR could become an issue, or it could be a good thing depending on cam/usage etc.

The trick is finding or making a good piston vise for machining... It's not something you pick up at NAPA.

[joop]

Those are 1.9 pistons
do the math-
76mm-69mm= 7mm divided by 2 (half the stroke) = 3.5mm and all Cofab pistons are 1mm short and you come up with 2.5mm

I did that too, but I like toying around with my camera to make it visual (pics tells more then 1000... and my first leguage is friesian then dutch) . A DG engine is a 1.9 but I'm not sure you guys have DG's also on the other side of the big pond. I ordered the set for the cylinders and like the idea to reuse the pistons also.... (I'm dutch)
The dish in the cofab's is bigger also, so maybe the CR won't be much higher and running on LPG it won't hurt

IMGP4716.JPG

DJ bottom cofab top.

Or can put them in a stroked aircooled lowbudget WBX with water heads..... some day

[Joe vw]

Top picture is a picture of a stock DG KS piston and on the right a DG cofab piston.

I read this quote and thought you had bought the wrong pistons. I now realize you have 3 different pistons in the photos, the first pic is blurry.
2.5mm would be the max I would take off. Or take off 1.5mm and 1mm offset wrist pin bushings
at the rod.
I would run the new DG pistons with no head sealing ring like I have done on my turbo MV, the profile gasket surface must be cut 1mm.

[buildabiggerboxer]

also make sure the lower dg piston skirt doesnt hit the 76 crank.
Joe, i like the no sealing ring idea, another option for those struggling for more compression, thanks for sharing.. you could shave the head seal surface 1mm, might save some corroded heads, although the geometry /p, rods would need modding.

[Piledriver]

also make sure the lower dg piston skirt doesnt hit the 76 crank.
Joe, i like the no sealing ring idea, another option for those struggling for more compression, thanks for sharing.. you could shave the head seal surface 1mm, might save some corroded heads, although the geometry /p, rods would need modding.

If you are using a non-stock cam all valve geometry bets are off anyway.

[Joe vw]

With hydraulics the pushrod just runs deeper in the lifter.

[Piledriver]

With hydraulics the pushrod just runs deeper in the lifter.

That has nothing to do with rocker arm geometry.
Just because it "fits" does not mean it works properly.

Increase your lift 30% and I guarantee if it was right before, it's jacked up now.

[Joe vw]

Most hydraulic grinds are lift-limited and we were talking about pushrod length due to the engine shrinking 1mm per side, there is some leway because of the hydraulic compensator, unlike solids. Geometry is affected any time you adjust valves with a hydraulic lifter, it should always be rechecked.
My response was more aimed at buildabiggerboxer and a stock motor that wants its squish back.

[Piledriver]

Gotcha.

[Joe vw]

Ok so, I want to bump this thread for similar reasons.
I'm putting together a stock displacement 2.1 on a budget. I want to run a K8 with type1 lifters but I want to bump my compresion. I would use a good set of used DG pistons if they were available in the US. My Idea is to use a set of MV pistons with modified 5.5 length rods and deck the pistons. The problem is the valve notches and my big valve heads. The intake notches need to be opened and probably deepened. The stock chambers are so shallow I think there will be valve interference with the pistons. And notching them deeper will cause natural piston porting . under close examination you will notice that the stock MV valve notches end just at the same depth as the top ring groove.
How deep are the valve notches on the DG piston, since I have never seen one up close?
I am wondering how this engine will perform with 9 to 1 as opposed to 10.5 to 1 with the K8 cam and 42x36 valves and 48 dellortos .

[buildabiggerboxer]

hey Jo, for some odd reason which i cant get my head round, the valve notches on the MV piston are shallower than on the DJ piston, its as tho VW may have wanted a bigger cam in the DJ at some point during its lifetime, i'll try and get a piccy from the 'shop tomorrow.
what i did, after a pointer from fastback to run a 5.5 rod in my 2.5, was to clearance the pockets by hand with a die grinder, i had a web 86c and 44x 38 to accomodate with 1.4 rockers, i also sunk the valves to gain installed height. i did a quick calc; with a chamber at
55cc, (cut MV at 37cc and an 18 cc chamber) and a .035" deck, thats with an old used pre compressed ring, i have kept all the thin ones for exactly this, and came up with 9.7 scr. the same with a stock DJ piston on 76 stroke gives 11.7 on a stock 5.4 rod, but using the 'no ring' trick. ive run .030" deck with no problems before at 7500 rpm,shame you cant find a set of o/e DJ pistons and keep the 5.4 rod with T1 bolts, i recon they would clear the valves easy, short rod? good suck on the cylinder with the piston speed.. ill do the pic in the am...
regards.bbb

[buildabiggerboxer]

I found gold..
they went in here...

[Joe vw]

So those are DG (edit) 1.9 pistons? according to my calculations using stock rods I would have to take 3.5mm off the top of those. Add to that the fact that I built these heads using cores that I skimed 1mm for no headrings adds to the problem.
Would I still have any valve clearance?