Hello all, it’s been a while. Anyways rather than swapping my VG30DETT that had 70 psi compression cylinder 1, I decided to pull it apart and try to find out why and fix it. Anyways I had pulled the heads and didn’t see anything visibly wrong with the valves other than being old and dirty. So I pulled the #1 piston and rod out and inspected for cracked ringland or broken rings and couldn’t find anything wrong with that either. The rings still had a healthy spring to them, nothing was cracked or broken, and the cylinder walls looked fine. Which I suspected since adding oil for the compression test only went from 70 to like 88 psi. So my assumption is once again valve issues. So I reinstalled the piston and sent the heads off to the machine shop which I am still waiting on.

My main question since I’ve never done this before though is this. Since this is an interference engine, what is the best way to reinstall the heads without bending any valves? Since I will have no idea which way to face the cams until the back timing cover and gears are on. My current idea is to bring cylinder 1 to TDC, then back it off just a little bit so that all of the pistons are sunk down into the bores, then install the heads and put everything back together. Then before the timing belt is on, I will line the cam gears up with the timing marks BEFORE the crank. That way I know the pistons are all sunk down and I know nothing will hit while I am rotating the cams around. Then I will finish up by bringing the crank that little bit back over to bring #1 to TDC once the cams are all lined up and then put the belt on and all that stuff. Is this method correct? Or should I do it differently than that?

A leak down test would have shown you exactly where the compression was lost. Be sure to ask how much is milled off (if any) from the head work, and make up for with thicker head gasket if required. Your machine shop will know what you need.
You can rotate each cam with the gears attached loosely to match the approximate position of the rear plate marking before mounting on the block. This way you do not need to rotate all the cams a lot to match the timing marks.

itsa300zx wrote: ↑

Sat Sep 24, 2022 2:04 pm

A leak down test would have shown you exactly where the compression was lost. Be sure to ask how much is milled off (if any) from the head work, and make up for with thicker head gasket if required. Your machine shop will know what you need.
You can rotate each cam with the gears attached loosely to match the approximate position of the rear plate marking before mounting on the block. This way you do not need to rotate all the cams a lot to match the timing marks.

Yes, unfortunately I never got to do one though. The engine didn’t have any smoke in the exhaust or any blow by coming out of the oil fill hole with the cap off and engine running either though. From what I’ve heard, most people who hurt pistons on these engines push their dipsticks out under boost after the fact and mine never did anything like that. Like I said though, I pulled and inspected the offending cylinder’s piston just to be sure anyways because I would hate to put it back together and still have low compression.

Thank you for the advice, I guess I will attach the top rear timing belt plates to the heads and loosely attach the cam gears and align them as close as possible to the marks before installing the heads. I will probably still keep the crank just slightly backed off of TDC on #1 as well just to keep all the pistons sunk down a little so that way if I do end up having to rotate the cams I don’t have to worry about any contact being made.

Also I am slightly concerned about the head milling, I had actually bought a set of ported 30p heads that were supposedly freshly done in hopes of getting my engine together faster. Upon receiving the heads though I thought they seemed questionable because there was lots of fine metal dust from porting still in them so I decided to take them to the machine shop anyways so ultimately I saved zero time or money on that one, but at least I gained a set of ported heads though. They have been milled before so I am worried about that because I cleaned the block deck by hand with a scraper and scotchbrite pads and as far as I can tell, the only company that makes thicker head gaskets in stock bore size is cometic. I do not want to run an MLS gasket since I am not having the block machined. What would happen if I ran a set of milled heads with a stock thickness gasket? Just slightly higher compression? Or would there be piston and valve contact?

I don’t think you will have any major issues (unless they hog a lot off the heads). Here is a good read on what happens with 0.010” milled off the heads. I just commented so you were aware of what can happen. If you wanted to squeeze every single hp out of the car, then adjustable gears can compensate.

http://www.twinturbo.net/net/viewmsg.as ... id=2260884

itsa300zx wrote: ↑

Sun Sep 25, 2022 4:17 pm

I don’t think you will have any major issues (unless they hog a lot off the heads). Here is a good read on what happens with 0.010” milled off the heads. I just commented so you were aware of what can happen. If you wanted to squeeze every single hp out of the car, then adjustable gears can compensate.

http://www.twinturbo.net/net/viewmsg.as ... id=2260884

Wow, that was a great read. Thank you. I only had considered compression being raised slightly, the thought of the idler pulley on the lower intake being dropped as well hadn’t even occurred to me. Overall, I am not really concerned about running thicker head gaskets unless the machine shop informs me that too a lot has been taken off. They told me they would measure them since they’ve been milled before, so hopefully it’s not too much where it will affect anything. I’m basically just trying to get my motor running good and healthy again with the stock turbos still, not set any dyno or track records although I do plan on taking it to the local drag strip when it runs again. Not competitively though, just to have fun.

Hoping my heads come back soon though so I can get cranking on getting this thing back together so I can enjoy boost season. My poor Z has been sitting in the garage down since January of last year due to life and finances repeatedly getting in the way.