I have an R50 with 168k miles on it, and just blew out my #2 plug. I am looking at the options of the "head on" repair methods, or pulling the head for the repair. One of the questions is how long does the timing chain/tensioner last (This question was inspired by a recent experience with my Ford Explorer with the 4.0 SOHC). I figured if I am pulling the head I may as well take the opportunity to do some other things like that. Any other ideas if I pull the head?

 

According to the dealer there is no specified service interval for the timing chain. I rebuilt my cylinder head at 95k miles due to a broken intake valve spring, so I replaced the guide rails and the tensioner, and left the chain in place.
Changing the chain is quite a bit more work on top of pulling the head, but it is the right time to do it if you decide to go that way. When pulling the head, make sure the chain is pulled tight at all times and no slack is allowed to go down into the motor or you will have a hard time resetting the timing according to the marks.
At TDC the timing mark on the cam gear is located at 90 degrees to the surface of the head (valve cover mating surface). I also used a sharpie to make a mark on the chain link that matched the timing mark and a scratch on the block and crank to make sure all 4 points were aligned when I put it back together.

 

I am not very concerned about the timing chain itself. I am somewhat though of the tensioner (That's what failed on the Explorer). Just trying to get my thoughts together before I start.

 

The guide rail came apart on mine went down thru crank gear and chain, skipped time, and bent valves, it's in my garage torn apart working on in spare time. Changing supercharger oil, since I have to take supercharger off to get the belt tensioner out of the way to get the timing chain cover off. Doing a lot of preventive maintenance, since every thing is torn apart.

 

I have decided to pull the head due to the a missing electrode tab on the spark plug, and also I am not comfortable with doing it with the head on and not getting junk in the cylinder. A question is are the head bolts reusable, or are they one shot deals like some other cars I know of?

 

The guide rails and tensioner are relatively cheap, and do fail pretty commonly. It'd be good insurance to replace them while your head is off (or even if you repair it in place).

 

Head bolts are not reusable. Get the ARP bolts as they are cheaper and better then the OEM bolts.

 

I looked at the ARP bolts. They are much more expensive than the OEM. Pelican parts has the head bolts at $4.25 each for $42.5 total for the ten. The ARP bolts/stud kits are $175.00+ for the ones I was able to find. Am I missing something?

I will say in general I want to get my hands on the idiot that came up with the one use torque to stretch bolt idea. As a mechanical engineer I find it questionable to tighten a fastener into the yield zone of the material and thus closer to the failure point than make the fastener to stay in the elastic zone where it can be reused indefinitely.

 

When I bolt is in the elastic zone the seal between the head and head gasket could be compromised because the bolts at that point have the ability to "stretch" a certain amount therefore causing slight lift under compression. I would agree that larger bolts would probably remedy that. You are correct about the ARP bolts being more expensive. I was going to go that route but decided to go OEM. I have never had an issue with an OEM BMW or MINI head bolt.

 

Coming from a mechanical engineer:
Yes, it seems counter intuitive to design a bolt to specifically to yield. But I assure you it's for a good reason. There is a correlation between the torque on a particular bolt and the tension in the bolt. As you tighten with a wrench, you put the bolt in tension. There are equations to calculate the tension, but they are just equations. In fact, they don't accurately predict the tension much at all. There are too many uncontrolled variables such as the cleanliness of the mating surfaces, the surface finishes and the presence (or lack thereof) of a lubricant, just to name a few.

When you have a bolted joint that requires a precise amount of tension in the bolt (like a cylinder head), the most cost effective way of accurate controlling the tension is to yield the bolt. The diameter and material of the bolt are easy to control during the manufacturing process. Therefore the tension required to yield the bolt is accurately known.
If you look at a stress/strain curve, it nearly flattens out at the yield point. Therefore, a small increase in stress give a large increase in strain. You end up with a fairly large window of strain in the bolt that will result in very small variation in stress. Consequently, all the head bolts will have about the same amount of stress in them, which correlates to the same amount of tension. The end result is a head gasket that is compressed precisely and evenly across all the head bolts which makes a much more reliable seal.

 

Okay... So are you saying there is, or is not an advantage to using ARP bolts?

 

The advantage is they are reusable.. and this cause less "wear" on the block.. thus, if your car is setup for race use and the head comes off every other week.. it makes sense to run APR studs.. otherwise... go with the OEM bolts if you don't plan on taking the head off any time soon.