Low compression in #1 (1 Viewer)

[40cruiserdad]

Contunuing work on my sons Fj40 to discover the cause of low compression (60 psi) in the #1 cylinder has led to removing the head and oil pan. I pushed the piston out the top to discover both compression rings are shattered to pieces! Any ideas what would cause this? The cylinder walls are smooth as silk with no visible signs scratching so thats good.

 

[Pitter Patter]

Overheating is the most likely cause. It’s tough to know what happened to the engine if this 40 is new to you. It’s entirely possible that someone set the timing wrong and ran the truck at 80mph with rust for coolant. Lots of prior owners treat these vehicles like they’re indestructible.

Seems like you may have dodged a bullet by digging in when you discovered an issue, new rings aren’t expensive if your tolerances are still good and you aren’t attempting a full rebuilt to get it back in good running condition.

 

[DougAustinTx]

Contunuing work on my sons Fj40 to discover the cause of low compression (60 psi) in the #1 cylinder has led to removing the head and oil pan. I pushed the piston out the top to discover both compression rings are shattered to pieces! Any ideas what would cause this? The cylinder walls are smooth as silk with no visible signs scratching so thats good.

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[DougAustinTx]

I have never seen a 2F piston that took FOUR (4) rings like in your photo. Am I missing something? Is this from some other engine?

 

[middlecalf]

Yeah is that another oil ring with the zig zag?

 

[Steamer]

I had the same thing on several pistons on my F.5 when I got it 21 years ago. There was no scoring on the cylinders, but they had a slight ridge on the thrust side. Some of the ring pieces fell out and some were locked in with carbon. Bores were slightly oval, but I did not have the means, time, or money for a rebuild.

I borrowed a ridge reamer but never using one before, It scared me when I set it up and thought I would do more damage than good. So, I didn’t use it. I just took a 3 piece stone hone to the cylinders till the ridge came out and left a nice cross hatch. The bores were just over spec measuring on the thrust side. I cleaned up the pistons and put in a set of standard rings. The rod bearings looked good, but I replaced them anyway since it was apart. It’s not a daily driver, it’s a weekend trail only rig but I’ve still got decent compression in the 145-152 range.

I know the PO, and the people who wheeled with him told me he overheated often and was known to deal with boil overs with swamp water. That led to the horrible condition of my cooling jacket when I got it and probably caused the rings to break up like they did.

 

[Steamer]

I have never seen a 2F piston that took FOUR (4) rings like in your photo. Am I missing something? Is this from some other engine?

It's an F. I have four rings on my F.5 also.

 

[Steamer]

Yeah is that another oil ring with the zig zag?

Yup! The zig zag oil ring is in 3 pieces.

 

[40cruiserdad]

I have never seen a 2F piston that took FOUR (4) rings like in your photo. Am I missing something? Is this from some other engine?

Yes, the engine in this rig is a 1973.

 

[40cruiserdad]

So I pulled all the pistons and there were 2 others with broken rings as well. Looks like i had a couple pistons overheat. One shows signs of rubbing on the cylinder wall as well as a stiff wrist pin joint. The other has no signs of rubbing but has a slight stiffness when rotating on the wrist pin. I assume these two pistons need to be replaced. Cylinder wall shows some scratching that can be felt with with my fingernail. Probably needs to be rebored huh . Ran a hone down the cylinder and the scratches are still there and I can feel them but my nail does not catch on them.

 

[Pitter Patter]

Serious bummer and I agree you probably won’t get the compression you’d like in that condition. Hopefully you can find a decently priced machine shop. Seems like everything is getting expensive. Fortunately oversized pistons and rings are still available through Toyota. —edit— just realized this is a 73, not certain those O/S pistons are available but there are aftermarket options. I just placed an order for a 2F and the only things that I wanted but couldn’t get are main bearings and a new cam, but there’s an aftermarket for those as well.

 

[Steamer]

Ran a hone down the cylinder and the scratches are still there and I can feel them but my nail does not catch on them.

Just curious, what style hone did you use?

 

[40cruiserdad]

Just curious, what style hone did you use?

A 3 stone style rented from O'Reilly's.

 

[40cruiserdad]

Serious bummer and I agree you probably won’t get the compression you’d like in that condition. Hopefully you can find a decently priced machine shop. Seems like everything is getting expensive. Fortunately oversized pistons and rings are still available through Toyota. —edit— just realized this is a 73, not certain those O/S pistons are available but there are aftermarket options. I just placed an order for a 2F and the only things that I wanted but couldn’t get are main bearings and a new cam, but there’s an aftermarket for those as well.

The only pace I've found .5mm ovesized pistons so far are at SOR for $99-$135. I'm not sure what the diference is between the two? The ones they offer for the 1973 and newer are not flat top?

 

[mrboatman]

Give Clegg a try. They might have what you need.

 

[40cruiserdad]

Give Clegg a try. They might have what you need.

Thanks! I'll check with them tomorrow.

 

[40cruiserdad]

Sooo...I've had a revelation of sorts that makes me question everything I thought I knew about my cruiser for the last 30 years! After contacting a few places looking for some pistons and i could not find a flat top piston for a 1973. I asked SOR about this and they said if its a 73 it should have domed top pistons. This led me back to my engine block and the production stamp the number F 334___. From my fellow mudders I learned SOR has a table that can tell me the year my engine was made. While I could not find a F 33..... number series on the table it seems to fall either late 1970 or Jan 1971....wait...what! This whole time I thought I had a 73 since it came from a 1973 cruiser from Bothel Wa. I was young (18) and my Dad made the purchase and had the engine rebuilt, so I just ran with it, untill now!

 

[40cruiserdad]

Anyone know if the piston rings I bought for the 73 will fit the 70/71? I'm guessing not. Maybe SOR will let me return and exchange for the 70/71 rings.

 

[middlecalf]

Engine build date is on driver’s side of block toward the front.

 

[40cruiserdad]

Engine build date is on driver’s side of block toward the front.

Thanks, the numbers are 01202 with 71 below it. Must be 71 then right?