Machining forged Pistons

[mighty kitten]

After a spun rod bearing at oulton last Friday I've got some contact marks on the Piston which I'd like to sort out . valves are not bent and the other three have very minor marks too . 10-15 thou is probably enough to remove but is that going to create more problems ?? Tia

[mr.man]

Not worth setting a machine up for those marks. Attend to them with a 1/8" rat-tailed file or a needle file.

[DVandrews]

Looking at the crown and the depth of the dish, there is probably a lot of material there and the marks seem inboard of the ring grooves. I wouldnt worry unduly about removing some material although a picture of the side of the piston to gauge the top ring groove height and one of the underside of the piston would help.

Dave

[mighty kitten]

Over the years it's had 6 thou off the block and two light skims on the head so a little bit off the Pistons might be a good things as far as I could measure it was 8.9-1 before any machining to the surfaces and I have to run a little less ignition than friends cars with nearly identical spec to stay knock free


Thanks for the replies

[Pumaracing]

There is neither any point nor advantage to removing these nicks completely. Take off any flakes and burrs, sharp edges and leave them be. You're very lucky if the valves are still true after such contact. I'd check them out on a lathe with a dial gauge or valve refacing machine.

[stevieturbo]

Plenty of material there to grind out some clearance

[mighty kitten]

I've replaced all the inlets as they all weeped but looked ok in my slightly worn lathe . Cams have 1mm higher lift than stock so could have been my dialling in as info for the enem Saab cams is a bit varied and all four have contact marks but the machining should sort it.

[Boosted LS1]

OP, what engine is this? As for the valve imprints, a tiny bit of fetteling would remove those :-)

[mighty kitten]

Saab b234 but not where you would normally find it


Runs at 1.6 bar with an Owens 3076 and 268 cams really well but would be nice to go a bit further with the ignition as it comes in nicely above 3 k

[227bhp]

Not that there is any need to remove those eyelets, but if you did (by skimming off the top of the piston) and kept the same head gasket thickness you would ruin the squish clearance, I think this would in turn cause more (det) problems. If anything you may want to deepen them slightly to gain more clearance for if this happens again and so you can shift the cam timing round a bit without risking contact.

Out of interest, do you have a dyno graph for this engine? I can see your valve openings there at TDC, what is the exhaust and are they 268 seat to seat in and ex?

Edited by 227bhp on Sunday 2nd August 16:40

[Pumaracing]

Hang on a sec. You say there are contact marks on the other three cylinders which didn't spin their rod bearings? You need valve cutouts in the pistons. The valve to piston clearance is insufficient.

[Boosted LS1]

mighty kitten said:

Saab b234 but not where you would normally find it


Runs at 1.6 bar with an Owens 3076 and 268 cams really well but would be nice to go a bit further with the ignition as it comes in nicely above 3 k

I thought it might have been a saab. I've one of those engines myself and am waiting to tune it :-) Would like to find a later head as I've heard that they're quite good.

[mighty kitten]

I haven't run the enem cams on stock Pistons But they offer a lot more clearance than the wossners on the left .


When the engine first got built the head and block had never been machined but over the years it's had 6thou off the block and two light skims from the head so my thinking is I need to take a little from the Pistons to go back to square one . A couple of mates cars with very similar spec run 1.5 deg more advance at peak boost than I can even though the wossners are 8.5-1 compared to 9.2 std .
10thou from the edge of the piston and a 20thou relief for the inlets should be ok . The only setting data for the cams is a lot higher at tdc than I set them at as it ran like st with virtually no vac at idle set at 1.5inlet 1.2 ex . I ended up with 1.0 mm on inlet and 0.6 off closing ex at tdc best compromise .

Haven't dynoed with the bb turbo but made between 450-480 with the last three similar sized Holset turbos .

[Pumaracing]

To reiterate what was mentioned above. Taking material off the crown of dished pistons like these does very little to affect the CR but rapidly buggers up the squish clearance which should be about 1mm ideally. Deepening the dish is a better option.

[mighty kitten]

So it's less of an issue to have squish under sized than over ?

[227bhp]

Set it to 1mm and have it correct. Less than that and you risk collision, more and it's not really a squish band any more.
The merits of tight squish clearance are easily found by using Google, it's no secret.
I'm not sure why you want to try and run similar ign advance to someone else's engine?
Regarding your cam timings, as you correctly state; it will be lumpy and have less vac on idle with more overlap, but have you considered the pros? For a 500bhp track car I can't see those slight disadvantages being much of a problem.

[Boosted LS1]

mighty kitten said:

So it's less of an issue to have squish under sized than over ?

Squish forces fuel towards the centre of the chamber, it'll get burnt faster that way because it's meeting an advancing flame front. To little squish clearance and you run the risk of mechanical contact. To much squish and it simply won't work which means your engine could be more prone to detonation, wasted fuel blah blah.

[227bhp]

mighty kitten said:

I've replaced all the inlets as they all weeped but looked ok in my slightly worn lathe . Cams have 1mm higher lift than stock so could have been my dialling in as info for the enem Saab cams is a bit varied and all four have contact marks but the machining should sort it.

If your valve seats are good and newly cut then to check for a good seal you use engineers blue. Too many people use the unbroken grey line from the lapping process as an indicator which is incorrect. What they don't realise is that the stuff is so thick it takes up any unevenness and makes it look like good contact has been made.
Put a very thin smear of blue all around the seat and spin the valve round in it, the witness marks will tell you all you need to know. You can test it afterwards with a vac tester or liquid, but it's pretty pointless if you've already got a good unbroken blue line.

There is an old engineers saying about applying thick amounts of blue to make a job look good, I can't just bring it to mind, anyone know?

[mighty kitten]

227bhp said:

Set it to 1mm and have it correct. Less than that and you risk collision, more and it's not really a squish band any more.
The merits of tight squish clearance are easily found by using Google, it's no secret.
I'm not sure why you want to try and run similar ign advance to someone else's engine?
Regarding your cam timings, as you correctly state; it will be lumpy and have less vac on idle with more overlap, but have you considered the pros? For a 500bhp track car I can't see those slight disadvantages being much of a problem.

The extra power above 6k is fine but costs me at lower rpm so the best setting I've found means I never need to dip into 2nd gear to stay in boost which is a lot kinder to my gearbox . I'm going to put the reliefs in for the valves and do some measuring to see what the squish height is . The enem cams and going up a grade on the plugs has got rid of the knock I was seeing so I will try a little more once it's bedded in .

[227bhp]

mighty kitten said:

227bhp said:

Set it to 1mm and have it correct. Less than that and you risk collision, more and it's not really a squish band any more.
The merits of tight squish clearance are easily found by using Google, it's no secret.
I'm not sure why you want to try and run similar ign advance to someone else's engine?
Regarding your cam timings, as you correctly state; it will be lumpy and have less vac on idle with more overlap, but have you considered the pros? For a 500bhp track car I can't see those slight disadvantages being much of a problem.

The extra power above 6k is fine but costs me at lower rpm so the best setting I've found means I never need to dip into 2nd gear to stay in boost which is a lot kinder to my gearbox . I'm going to put the reliefs in for the valves and do some measuring to see what the squish height is . The enem cams and going up a grade on the plugs has got rid of the knock I was seeing so I will try a little more once it's bedded in .

That's why I was asking if you had any dyno charts, you'll find with more overlap (up to a point) you'll get earlier spool and more mid-range.
The recommended lift at TDC for those cams sounds ok to me, on an engine with similar architecture to yours we're running wilder cams; 272/272 11.3/9 so use a 2/1mm lift at TDC and it's running pretty good, it's still maxing out the GT3076 at just over 500, torque makes its initial peak at around 4500.




;);)