Oil Fed Tensioner Modification Tech Article

by George Aigel

Oil fed tensioners in the flat six engine are generally considered to be the safest setup for keeping the timing chains properly stretched at all times. These tensioners were introduced in 1984 in the 911 Carrera and are therefore often called "Carrera chain tensioners". Most older flat six engines, 1983 and earlier are upgraded to these tensioners. While these tensioners are considered safe, they are not 100% bulletproof.

This article describes how to modify Carrera chain tensioners to be 100% fail safe. For this, while I had taken the Woods / Anderson engine rebuild class, I looked over the shoulder of Jerry Woods, one of the countries most reputable Porsche racing engine builders. According to Jerry, oil fed tensioners can see very rare failures caused by oscillations of the chains if an engine runs at very high rpm and then abruptly drops to low rpm:The tensioner may collapse, the chain can jump and this loss of valve to piston timing can lead to a collision between the piston tops and the valves - terminal engine failure will be the result.

For this modification, disassemble the chain tensioners by taking out the grenade pin and then the c clip retainer. The grenade pin comes with new tensioners. If you remove used tensioners, you will not have this pin. The c clip retainer may be hard to remove. Use a small screw driver or a pick to pry the c clip out and while you do this,

compress the piston down, to take the load off the clip. Then remove the tensioner spring. (Figure 1) 

Put the piston back into the bore with the spring removed and install the tensioner in the engine. Measure the total travel between the collapsed tensioner and the fully tensioned idler arm with a vernier caliper as indicated in   (Figure 2 )

Now remove the tensioner again and limit the total available travel to 1/8" by installing a piece of tubing into the bottom of the tensioner that is 1/8" shorter than the measured total travel. The tubing has to be 5/8" outer diameter and about 1.5 mm wall thickness. This has to be a hollow cylinder, because oil is flowing through it and it has to clear the spring also, to keep the spring force the same. Hydraulic tubing is a good choice but I had found bronze bushings at the hardware store that had the perfect dimensions. 

They were 5/8" outer and 1/2" inner diameter. (Figure 3) shows this limiting tube cut to length, together with the piston and spring, before it is installed back into the tensioner housing.  

Discard the grenade pin and the C clip retainer because they will not go back on with the limited travel, and are not needed once the tensioner is installed. You can now re-install the tensioners.

In the unlikely case that you ever have to take apart a Jerry Woods assembled engine, you will notice that the tensioners have the c-clip missing and the tensioner piston travel is limited. According to Jerry, all his engines (even the stock rebuilds) get this final tweak to bullet proof oil fed chain tensioners.