Flat top Pistons

[my_mad_z]

After stripping my L28, it came to my attention that piston # 6 was severely pitted. The piston has been like this since I rebuilt the engine after picking it up in a "patrs" car I bought. I didn't bother replacing it due to $$$. This time, I decided to sand the pitting out of the piston and took off approximately 0.5mm..... I'm wondering whether I should get the rest of the pistons machined in order to suit. The reason I did this was to prevent detonation with the turbo. I realise that shaving the piston will have an affect on the compression, but it doesn't really concern me if I'm going to put the turbo back on. The thing that concerns me is if I decide to go back to the stock SU carbs or tripple carbs. Should I not worry about it, should I shave the rest, or should I replace all pistons? They are all within tolerance but I haven't measured the bores yet, but I'm assuming they are still within tolerance. I've got another 4 L28 motors lying around, but I'm not sure if they have flat top pistons......if they do, can I mix n match??? Any suggestions???

Merry Christmas to all and a safe and happy new year.

Marc.

[2ManyZs]

First, milling the top of the piston is going to have to be done very precisely or you'll end up with hot spots that will eventually melt the piston crown. I would not do this unless you have access to some pretty precise machinery to do the job properly. If you do it, you'll need to do all the pistons the same or you will throw the whole assembly out of balance and cause even more problems.

Secondly, if you are going to run a turbo with flat top pistons, I think the compression ratio is going to be too high for that to be feasible without special pistons(read, very costly). The compression ratio needs to be lowered not only for the cylinder pressures the turbo will build, but also to control detonation.

I think you are going to build your engine to be either turbo or non-turbo. You can run a turbo spec engine without a turbo with less problems than you can a n/a engine with a turbo. So, ultimately your piston choice will depend on whether or not you are going to install the turbo.

[sjcurtis]

Marc, 2Manyz's has hit the nail on the head, you have to work out where you want to go with this engine. If you continue with the same combination and set up you will loose either an engine or a piston crown. The pitting in this engine is the first sign of inpending doom. With boosted engines the process is sped up at an alarming rate it appears that the fuel or boost management of your engine combo has gone astray at some time. this is the area you should concentrate on once you get the hardware sorted out.

Did any other pistons show signs of pitting, scouring or bubbling durrng inspection or even possible edge burning. If so consider replacing the whole set.

Good Luck:classic:

Steve

[my_mad_z]

Sorry guys, I should have been a little more specific on the engine combo. The L28 is an F54 block, P90 head with the flat tops. Initially I changed the head to the N42 to increase compression for a n/a motor. It was then that I noticed the pitted piston which looked as though it had been the victim of corrosion; the motor had been at idle for some time and all I did was change the rings, bearings, timing chain and a few other bits and pieces. I left the piston as is. I noticed there was a bit of detonation with the n/a motor; this was due to several factors:

1. The higher than normal compression (I was running approx. 10.5:1 after having the head shaved)

2. Poor fuel grades

3. Mechanical advance on distributor

4. Pitting on piston 6

I then got my hands on a T03/T04 hybrid turbo setup (running twin stromberg carbs, 42mm wastegate, 3 inch mandrell bent exhaust and water/methanol injection), so I used the P90 head to drop compression so I could run the turbo, I think it was 8.0:1 or there abouts. I cleaned the carbon deposits from the pistons and completed the head swap. I still had detonation problems until I reco'd the dizzy and added the water/methanol injection (well it was only water cauz I couldn't get my hands on methanol).

Keep in mind I was running 18psi boost and the motor handled it quite well! After the tragic car accident, I stripped the motor down and it looked fine, no other pistons were damaged! This time however, curiosity got the better of me and I decided to sand the pitted piston by hand to reduce hot spots. The only reason I did this was because I remembered "Stormin Norman" from Zdriver sanded his pistons by hand....... and I have plenty of time on my hands before I finish the 240z.

Deciding on the n/a or turbo setup is not a big thing for me, all I have to do is swap the heads and a few other fiddly bits. I do all of the work myself, so it's only going to cost me my time. I want to stick with the flat tops if they are still within spec, but if not, I will pull down the other motors I have and see whats inside. 2ManyZs and Steve, thankyou for your advice, I agree with what you both say; I think I will check the pistons in the other motors or get a new set of pistons. Thanks a lot for your feedback......

Sorry the post is so long, I've tried to keep it short....

Marc.