Good day all,

I have one wrist-pin knocking in my B21F with 150 k. miles. I am certain of the diagnosis.

The engine, with a "K" cam, has 150 psi. compression in all cylinders.

I have a spare set of really excellent B21F used pistons/rings and rods, almost like new.

May I replace the faulty assembly with one of my better spare piston/rings/rod assembly?

I wonder mostly about the problems that might arise because of different wear patterns.

Thanks in advance to all...

If you have the engine that far apart to do one, you might as well do them all. I would give it a quick hone & a new set of rings & put all 4 pistons & rods in. How much wear do the 2nd hand pistons have? You can tell by measuring the top ring side gap on the top side with feelers gauges. .006" & bigger is Worn out.

Pierre you will be amazed at how forgiving engines can be. First, if the bore is nicely burnished with no vertical scratches, then don't bother honing it. If the "new" piston and rod assembly shows no scuffing on the piston skirt, and the rings stand out from the piston with good tension, and are clean, then install the piston as is. Make sure it is clean however. The ring gap placement is not as critical as one would think. The rings tend to walk around the piston in operation - this keeps the piston ring groove clean and distributes the wear evenly. Simply install the rings alternately in line with the wrist pin holes. Make sure they can move freely in the grooves. The rings will wear into the bore without any problem - if it burns oil at all it will do so only briefly. Use the rod bearings that came with the rod in the original locations top and bottom - do not swap used bearings between rods. That much is a no-no.
The bigger question is what is the original problem. If you do not have the piston out of the engine, then you will have to in order to determine the real cause of the noise. A small end bushing that causes noise will have noticeable play (once the oil has been washed out - use brakeclean or something like that). Hold the piston at a slight angle to the right when facing the front of the piston and rod, and rock the piston across the pin. Most small end bushings will pound out a few degrees from vertical. If the small end bushing has no or little play, then a rod bearing or collapsed skirt on the piston might be the problem. Piston slap can be seen as a carbon free zone above the top ring on the thrust surface of the piston (right side). Also the skirt may be scuffed. Don't put it together until you determine the cause of the noise.

Hi Rhys, I hope you are well...

I had the engine (250 k/kms) open just a few months ago and none of the bores had vertical scratches in them. The original factory crosshatch pattern was still clearly visible. The car has not been driven since.

The "new" used piston is...like new, with not a single scuff mark.

Brand new rod bearings have been installed. I definitely expect to find the damage on the small end bushing/piston.

Do you remember that I had my flywheel lightened according to your instructions?
It's now installed and I am looking forward to try it out, at last...

Once again, thanks for the detailed advice...

If you doing this on the cheap, I'd go ahead and swap the piston. I think you could also use the old rings too, but don't be surprised if some of the compression rings are cracked, in which case you'll need to use new rings. In either case, consult the manual for the correct location on the piston for the ring gaps. Usually they are staggered. If you use new rings, get the "moly" ones, not the chrome ones and you'll probably be able to break them in OK. If you're going to replace the piston from below with the engine in the car, and have to use new rings, you may not have enough room to use a scratch hone, in which case I'd be tempted to use some 320 grit wet/dry paper and kerosene to make a new pattern on the cylinder. A little rough and ready, and certainly not optimal, but will probably work. If you're going to pull the engine, use a scratch hone on the end of a drill. You may be able to borrow one from a machine shop. Good luck.

Jkordzi,

You and Don are teaching me a lot.

I am working on the engine in-car. You mention "replacing the piston from below... ". I guess you mean it can be done if the crankshaft is dropped. I just quickly checked on an opened spare engine that I have and cannot see how it can be done otherwise.

Or do you have a trick I should know about? It would be great to be able to change the piston without pulling the head or dropping the crankshaft.

I really have to go eat something before I can pursue this illuminating exchange, not to mention the job.

Hopefully, you and Don will watch this thread. I'll probably have some more questions for you both.

Thank you very very much, for now...

Head removal is mandatory, the piston comes out from the top. No reason to remove the crankshaft, just the pan and the rod cap. If there is much of a ridge, you might break a ring (or ring land on the piston) in the removal process. Solution is to remove the ridge with a ridge reamer. I have had decent luck breaking the cylinder glaze with emery paper as suggested earlier. New rings are not that expensive, especially if you can get them for one cylinder, and will probably mean the difference between success and failure of the project.

Good luck! And make sure ALL the emery dust gets cleaned out of there...

Chris

Chris,

thanks for the tips. I'll see if I can get rings for the one piston only.

It's unlikely that the used rings will seat properly in the (probably glazed) bore.

You'd probably be better advised to use new rings on the piston and run a glaze breaker through it first. You might need a ridge reamer, too, but Volvo don't usually have much of a ridge.
--
Don Foster (near Cape Cod, MA)

Thanks Don...

Being too broke to afford new rings, do you think I could transfer the rings from the old piston to the "new" one,then?

Also, might there be problems in the fit between "new" used piston and bore? The piston is like new.

Thanks again...

Not a good idea. Once you remove a piston from the bore, it's impossible to reposition the rings in precisely the same orientation. So even if you remove a piston and intend to reinstall it, you should replace the rings and run a glaze breaker.

"...might there be problems in the fit between 'new' used piston and bore?"

Good point. You must be sure that the replacement piston is NOT an oversize piston. Likewise, you must confirm that the original bore is not an oversize bore -- unless it happens that the bore and replacement piston are the same amount oversized. However, it's likely that everything's the original stock size.

Keep in mind that if the donor motor is a turbo, the pistons are different from a non turbo motor.

How did you confirm that you have a worn wrist pin? That's an unusual problem for a B21.
--
Don Foster (near Cape Cod, MA)

Don,

Bad wrist-pin confirmation ?

I just put in new rod bearings and "almost new" (!) main bearings. That quietened down the engine very much.

But I still had a "tock-tocktock" somewhere, a knock pattern that I read was indicative of wrist-pin problems.

I listened to the engine with a stethoscope and cylinder no. 2 sounded to be the culprit. When I pulled the spark plug wire to it, all knock disappeared. I repeated the procedure a few times. The stethoscope also told me clearly that the bearings were quiet. The knock definitely came from the upper portion of the block.

Thanks for your support!

- When I pulled the spark plug wire to it, all knock disappeared.


Are you sure you are not having pre-ignition in that cylinder?

Sometime a dirty combution chamber (say a thick deposit) will raise compression pressure to the point of pre-ignition. All is needed then is a small carbon glow and you have this.

Years ago when I was using leaded gas something similar happened to me on my 144 1972. The very steady knock made me conclude I had to open the head and block to find the loose part. An old experienced mechanic looked at it and he poured slowly half a quart of engine oil keeping engine running. That was at night in the old days. Today one could be arrested for polluting the environment. That is why I suggest you do not do this. There is better stuff on the market today.

I would use a combustion chamber cleaner in a spray can bottle and rev it up a little so it does not die when using it. I would then replace the spark plug with a new one and see if there is difference.

I agree with Don, wrist-pin faillure is not frequent on these engine.

Thanks for the "heads up", Roland...

I think we can pretty much rule out carbon deposits this time.

The head has 0 km. on it since I rebuilt it. It has mirror-polished combustion
chambers, valves and piston tops. Deposits-wise, it should be immune for a while...

Still, before opening the engine, I'll make sure, one last time, that the knock doesn't stem from something else than the wrist-pin/big-end bushing/piston.

My head has been milled .070", for approx. a 10.75:1 CR, making the engine an "interference" type with the high-lift "K" cam. The bad wrist-pin might be explained by a collision, months ago, between the valves and the pistons. My camshaft had seized and the timing belt was destroyed at crank-up...

Thanks for your kind help.