Thread: 300tdi Helicoil failure and fix

So if you've been watching my posts then you'll know that in the most recent in a long line of engine related issues, I recently removed and resealed the sump in my quest to be the owner of the only 300tdi globally which does not have an oil leak.

After cranking the engine for a minute of so to build up oil pressure (the trick is to remove the right fuse so it won't fire) the old diesel was purring nicely with a new filter and belly fully of Penrite. Or so I thought. On our first outing anything above 2000rpm saw the introduction of a loud ticking noise which was in-keeping with the camshaft. Plus a little oil leaking from the rocker cover.

Not having time to delve deeper I nipped up the three rocker cover bolts, the middle being more than a little lose. I scratched my head trying to figure out why taking the sump off would (a) cause the loud tapping and (b) cause all three rocker cover bolts to come lose. It was about now I posted for help via the AULRO FB page for hints as to what could cause the ticking noise.

After another 8km round trip I pulled up at home and [insert significant swearing here] there was oil spray up the back of the D'Fer, all over the bottom of my long range tank, the rear diff and under the front end of the truck. Popping the bonnet I discovered the middle rocker cover bolt sitting proudly 30mm above the rocker cover.

It's about now I should add some sort of disclaimer. I am not a qualified mechanic. I have a workshop manual, quite a few tools and worse case scenario a nephew I can call on who is a qualified Diesel mechanic when I really get stuck. If you don't know what a dial gauge is, or what a torque wrench is then it is probably best that you laugh at my multiple thumbs and find a professional should the same happen to you. I however am an obsessive and compulsive tinkerer who decided to have a go myself.

When I tried to lose the rocker cover bolts, all three were spinning in their holes. In fact, once I removed the breather and oil return hoses the rocker cover lifted off, complete with the bolts and pedestals in place. (Note: The pedestal is the threaded rod that goes through the rocker shaft and into the head. There is a nut that secures the rocker shaft to the pedestal. There are two bolts and three pedestals which hold down the rocker shaft).

So I'm pretty lucky that I got to this before those two bolts let go or the rocker shaft broke.

If you've been reading my posts you'll know that I recently had all this apart to replace a failed head gasket. My first thought was maybe I'd over-torqued the three pedestal bolts and two other bolts when reattaching the rocker shaft. But I've decided to put that down to Murphy's Law because said Nephew was watching like a Hawk when I did that job. Pulling it all apart again, I made sure to keep everything in the order in which it was in place. The rationale for this is that assuming you back everything back into the same place you might not need to reset the tappets.

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Taking advice from the AULRO FB page, I checked the threads in each hole using a M8 x 1.25 tap. Holes 1 and 5 brought up oil and gunk but otherwise appeared sound. Holes 2 and 4 were clean as a proverbial but hole three was clearly gone. A visual check plus a check of the endless receipts handed over to me by the former owner told me that Helicoils had been put in during the engine rebuild now about 90,000km ago. But no clue as to which threads in the head had been repaired.

Based on a comparison of the size of the hole to the size of the bolt or pedestal, I suspect that threads 2, 4 and 5 are original and 1 and 3 have been repaired. 3 had clearly failed.

So then came the research on what to do following the failure of a Helicoil. A new high-flow head from Turner Engineering is the preferred and long term solution. But I couldn't immediately wait the time for the part to arrive with the D'Fer off the road. A helpful hint via the AULRO FB page suggested I try some goop from SuperCheap or Repco to effectively 'glue' in the center pedestal. This fix lasted about as long as it took to write this sentence.

So back to Internet with Cross Tools at Kotara coming to the rescue with a pack of Lock-In-Serts coming next day via post. A plug if I may. Apart from having a very useful website, Mick at Cross Tools also helped me as if I was only customer in the world, going through my problem and the potential fix and options in great detail. Turns out there are "thin" and "thick" thread inserts and Helicoils are usually "thin" which means they can pull out or fail rather easily in Alloy. They're preferred by mechanics because they're fast and easily available, but that doesn't mean they're the right thing for every job. With the amount of metal I had to play with it was recommended I go big. Really big.

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These Lock-In-Serts need a frighteningly large 27/64" drill bit (that's slightly under 11mm) and have a chunky 1/2" - 13 thread. In other words a really big surface area compared to a "thin" Helicoil to spread the load. They come pre-applied with locktite and are designed to be removable and replaceable in the event that the M8 thread wears or fails. All I know is that drilling the hole to fit was slightly nerve racking.

A useful tip from the Nephew was to stuff every nook and cranny in the head with newspaper so as to avoid any swarf (that's the metal bits from drilling and tapping) falling into places where it shouldn't. An addition of my own was to liberally apply axle grease to stick the paper into place and 'catch' any swarf life flypaper. While it was a little messy, it worked!

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Another useful tip. Use brand new or sharpened drill bits. Go slow, really slow. The new hole needs to be straight!

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Machinery House helped out with a new tap set and the drills I needed. Each tap is different and is designed to be used in succession to give the desired result.

A nerve racking 10 minutes later, and the new thread is in.

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Putting it all back together from here was pretty much the reverse of pulling it all apart. If you can follow a workshop manual you're sweet. The 300tdi is now silent by comparison and running very smoothly and very sweetly. Mechanical noise is pretty much gone apart from a little diesel knock and injector noise at idle. Bliss!

Total cost of the Lock-In-Serts, new taps and drills was just over $150. It took about 2 hours going really slowly from start to finish.

My take home message (if I'm permitted one) is that given the amount of metal available, the larger Lock-In_Serts might be preferable to Helicoils as the larger surface area means they're less likely to fail.

Good job, $150 is cheaper than a new head which would have been the only other option.