Rod knock Qs

[Dan_Gyoba]

I'm pretty sure that my 7M-GTE has rod knock. Kinda bummed about this, but it is what it is, and I've got decisions to make.

I haven't touched bearings in a very long time, and never on a 7M. Not looking forward to doing this, either, but if it's what I've got to do, then it's what I've got to do.

Option 1. Repair/rebuild current engine. Oil pressure has never been high on this engine in the time I've owned it, so a new oil pump is definitely in the works if I rebuild. Currently the oil pressure is on the low side, but is still (barely) within TSRM spec. From this I'm going to make an optimistic assumption that machine work on the crank will be minimal, and I should be able to get a set of off-the-shelf replacement bearings and install, with appropriate machine work done. Obviously rod bearings need to be done, and will probably do main bearings as well if I'm in there. Upside: I've replaced all of the seals and gaskets in this engine in the past couple of years. I'd have to re-do the head gasket, which means work on the head and block, but the valves/seats and stem seals are all good.

Option 2. Find another 7M-GTE without rod knock. I'm pretty sure that this would be cheaper. The engine that I have is an '87, and I'm entertaining the thought of finding a newer block, maybe with a 7M crank instead of the 6M crank. Of course the engine management will remain the same for now, and I'll have some sensors to swap over for the wiring harness. Upside: newer block with "improvements". Possibly get oversized valves this time. Downside: get all of those seals done again.

Option 3. Start over with another car. My car is no spring chicken, with over 500,000 km on the clock, the chassis is showing its age, and the car is pretty rusty. I'd rather not, since the car has a lot of sentimental value, but the reality of the situation is that another car will be far easier to make into the car that I want to have. There are certainly a lot of parts on this one that I'd want to keep and transfer over, but they're just parts.

I guess what I'm looking for is advise for option 1. Have people had good success rates with a rod knock repair without launching into other problems afterwards? I realise that this can turn into a major high dollar project pretty quickly with a complete rebuild as a possibility, too. I'm not sure that I want to spend that kind of budget, given that the body of the car will need a hefty budget for repair/restoration too. If that's what I'm in for, and option 2 would be cheaper/better, then that's the avenue I'll pursue. I'm also keeping an eye out for a car for option 3, though really, that's not really my preferred route.

 

[Skeezix]

With option two how do you really know how long those bearings will last? I went the long way on mine. I got rod knock on my 87 turbo and it had some bad rust issues. Driving around town found a mint 86.5 bought it, rebuild my motor and swapped everything over. $3500 later I'm in a 97% rust free car and boosting hard. Just make sure you toss out your old oil cooler, or if you feel confident in washing it out do so but it cost me an extra $400 and another crank. I was gonna replace my oil pump too but its only ten years old and it was pretty much dead on tight with the TSRM specs

 

[Dan_Gyoba]

Yeah, that's kind of my fear with another engine.

Regardless, I don't plan on keeping my oil cooler, I plan to go with an aftermarket cooler coupled with an oil filter relocation kit.

 

[radiod]

I wouldn't go for option 2, too many unknowns in a used engine. If you go with option 3, I'd still look towards finding a shell as cheap as possible (but still what you're looking for) and still rebuild. With the age of these cars, there's just too many potentially expensive issues to be confident in someone else's unknowns.

 

[Dan_Gyoba]

Okay, so I'll take option 2 off the table for now. At least until I see what the bearings in this engine look like.

There's a really nice condition NA for sale around here, and I'll keep it in consideration just because it looks so nice. It's an '87, but since I have all of the '89 stuff, I can swap over mirrors, nose, tail lights. Probably even interior parts, since I like the '89+ interior pieces better, too. Boatload of work though. At least the '87 engine/ECU that I have would go in with no issues.

so I guess I need to see the crank before I'll know if it's re-useable, or should I be looking for a good one first, and if so, should I be looking for another 6M crank, or should I look for a 7M (counterbalanced) crank?

The 7M cranked engines came with a lower redline, 6250 instead of 6500, but were rated for (Very slightly) more power. I was under the impression that the lower redline is because of the crank, and the higher power was a change in the turbo. Since I'm using an aftermarket turbo, that's not going to affect power delivery, but with the 7M crank, would that lower redline? (Of course, the rev limiter in the ECU will stay at 7k as it is now, so does this decrease the safety margin that the rev limiter imposes?) Is the pulley assembly different with the 7M crank?

Balancing the rotating assembly also seems like it would be something to do while I've got things apart like this, but I do need to keep costs down somewhat.

Of course MHG and ARP studs will be installed, as well as new timing belt, just because.

What else should I be looking at?

 

[mk3-4-me]

Well I would NEVER use bearings from one engine and put them in another even if they "look" good. Just get new bearings or use the N/A block if you don't want to do the work.

Check to make sure the crank journal is fine with a mic or take it in to get it checked.

Make sure you mark which piston/rod went into which cylinder. If you pull those out, I would get new piston rings to.

If your plannin on fixing the problem, I would rebuild the whole engine which will get a little expensive. Otherwise if you just replace the bearings, it may just mask the problem that caused it. The metal flakes could get into passages and cause another RK.

Im on my 4th build all from RK! Lol it's a pain. Do it right the first time and you will thankyourself later.

Goodluck!

 

[NashMan]

on side note if you have rod knock (pending one how bad and for how long) your rods are most likly out of round and may fail again in shorter period of time.