Bad Head (.020 in. off. Disaster?)

[Madness]

Dropped my 210 head off at the machine shop yesterday and got a call today. The head has already had .015 taken off of it from the previous builder and now they need to take another .005 off.... After doing a little research i've found that only after .030 do i have to worry about valves hitting the pistons and would have to find cam shims. Compression ratio is a concern but I'm moving to a .030 over piston so that should keep me on pump gas. But Valve timing is what I'm really stressing about, I'm pretty sure its going to retard my timing quite a bit, enough to make it run horribly. Considering .008 is all that nissan recommends. Is there such a thing as an adjustable cam gear for these engines anymore? That should solve my problem and would be fun to mess with :D. I know I know, its a 210 head and it isn't worth the aluminum its cast from but I want to see if i can make it happen... Also the guides, valves, cam, lash pads, seals are all brand new from the previous builder.

 

Im starting to realize why this thing was parked after it was rebuilt. Smashed piston ring journals (Dropped piston i think) + Cam timing way out of whack = smokes and runs like crap.

 

btw its a L16...

 

What do you think?

[Charlie69]

check with Hainz "L Motor God"

[datzenmike]

Cam shims only lift the cam back to it's stock height to keep the chain tight and at the correct timing. It won't lift the valves so you will need thicker lash pads to take up the slack introduced by the shims.

 

If compression is a worry, going to oversize pistons won't help. It will make the cylinder volume more. If you squeeze more volume into the came combustion chamber the compression will go up. I did a quick comparison:

L16 with stock B&S, 7.01cc dished piston and 38.5cc 210 head = 8.75 While the same motor as above with only a 1mm (0.040") oversize piston would have a compression of 8.9

 

If worried about cam timing use the procedure for checking the chain stretch. On the L16 you have two more positions of possible cam advance. Each position moves the cam 4 degrees advance. If you move to the 3rd position (8 degrees advance) and the cam timing mark still is still not correct you can move the cam sprocket ahead one tooth. Each tooth represents 9 degrees of cam timing. (40 teeth/360 degrees = 9) Set the cam on the number one setting and you have two more 4 degree settings to work with. I doubt you will need to go to this extreme.

 

For a few hundred dollars you can buy an infinitely adjustable cam sprocket probably capable of fractions of a degree adjustment. Not worth it for a few degrees on the street buy a 7,800 RPM it sure does.

 

 

....and yes I know what you mean about making it work with less.

 

 

Smashed piston lands could be from not checking the ring end gap clearance. There has to be room for the ring to expand as the piston heats up. Top ring gets the hottest.

Edited November 26, 2009 by datzenmike

[Madness]

Here is the old piston

it for sure doesn't look like it came out of my block. The cylinder walls are almost perfect. I have no idea what could have done this.

 

 

so what I'm getting is ill have plenty of adjustment from the stock cam gear?

As for compression, what will .020 take me to? Could it put me into the 100+ octane range?

 

Edit... i need to slow down when i read.

Edited November 26, 2009 by Madness

[datzenmike]

 

The ring lands look more or less ok but the area above the top compression ring is badly eroded. Extreme blow-by would do this. Running a stock very used L24 flat top in a very worn L16 cylinder could be the cause. (would also explain the oil burning too) Measure the piston diameter at the skirt and then the top of the L16 bore and compare. The difference between the two should be between 0.002 and 0.0036" or 0.001" to 0.0018" between piston and cylinder wall.