Datsun Roadster Forum-311s.org

I'm overhauling my '68 U20 and have some dopy questions.

First, how much slack should I have on both timing chains? Ive got a machined head that measures 4.449" and have heard some talk about an extra thick head gasket or cam tower shims. Any advice one way or the other? I'm using H20 pistons with a slight dish so I don't think valve clearance is an issue.

Second, where to send rockers to get reground. Some are pretty worn but I've got extras.

Third, I need the non-adjustable lower chain tensioner or whatever you call it. The metal bracket on mine was broke. Anyone got one? I probably could replace all the plastic for that matter.

Thanks in advance,

The lower guide, get rid of it. It will just break again. There should be very little slack in the chains, the way I did it was to get some spaces that fit over the shaft of the tensioners. When I assembled the timing chains, I put the tensioners on last. I added spacers till the chain was tight (you will have a tiny bit of deflection, but hardly any.) I would get some shims for the cam towers to get them back to as close to stock as possible. (Basicly take the stock thickness, subtract your mesured amount and get the ones that will get you closest.) My head is not cut at all.
Why H20 pistons? You will loose quite a bit of Hp with them. I just found out that VG30 pistons might be useable, they have been used in a VERY hot U20. While you have the head off, remove any burrs or imperfections in the combustion chambers. (My head had strange veins in each of the chambers.) I used a dremmel tool and went very slowly and was extreamly careful around the valves. The reward for this seemingly small task was an engine that doesn't ping even with close to 10.75:1 compression, and 20 degress timing. You can go one step further and clean everything with 1000 grit sand paper. Carefull work in this area will lead to fewer carbon deposits in the head as well.

Will

In January this year I sprung a leak between 2 and 3 cyl. Took the head off and it measured 4.45". I decided to replace the head as I intend to keep the car (owned since 1979) and did not want to shim the towers that much. This can create some wipe problems for the cam. Brian Hollands has posted some useful information on this subject (in the archives). You may be able to find a good used head (at least 4.50" and flat) and replace the valve seals, guides, and seats for a reasonable price. I would check with the vendors. I think it will save you money in the long run as the engine will be a lot more reliable. I would replace the lower tensioner as it reduces stress on the jackshaft gear, although others would disagree. I have about 100,000 miles on my engine (rebuilt at 75,000 with chains,gears,pistons, tensioners, etc )and have not had a failure of the lower tensioner. I agree with Will on the H20 pistons. If a shop is going to do the work, try to get your hands on a manual. They will need it. Check Gordon's site on the chain test. Phil Herring SRL311 07036

You DO need the tensioner, the lower guide is up to you. I don't have one on my engine, and if you get the tensioner shimmed enough, the chain won't go anyplace either.

Will