Inline 250ci with a noise.

[Lugnutz65]

This is a 1971 250ci with a tapping noise.

VIDEO LINK

Dry cold compression test showed 125psi for cylinders 1-5.
I was happy until I tested #6. It showed 90 psi.

It runs good. Starts right up and doesn't smoke much.

Ideas? Suggestions?

[68gmsee]

Could be piston slap or piston rod bearings. Maybe lifters if it's higher pitch "clicking." With my bad hearing it's hard to tell from the video.

Either way, with the engine compression being off, it's probably time for an engine once over. Some people suggest using an additive in case the rings have seized but I've never had success in the past with that treatment.

[TJ's Chevy]

With the low compression on the last cylinder you have something wrong there for sure, you might dump a little oil in that cylinder and do the test again. If the compression goes up you have a problem with rings....if it doesn't you have a valve issue. The ticking sound sounds like loose rocker arms....exhaust leak....bad lifters all at once. haha In a video unfortunately it's hard to tell the difference. Course when my 305 v6 had a little piece of carbon fall into the cylinder you could hear the knock clearly in the video. Could pull a plug and peak down the hole with a flash light and see if you have any marks on the piston.

[chevy_mike]

So when I overheated my 250, this is what it ended up sounding like,

https://www.youtube.com/watch?v=OhEYYEN9SlY

When I tore it apart, the #6 cylinder piston had shrunk .012". I was getting piston slap. Compression was low on that cylinder too. #6 runs the hottest. Luckily I did not damage the bore and was able to use hone to to remove any glazing and replaced all the pistons and bearing. Ran great for another 3 years before I sold the car.

You're doesn't sound as bad as mine. Try taking the #6 plug wire off and see if the noise quiets down. If so, it's likely piston slap or a bad rod bearing.

[Lugnutz65]

Originally Posted by chevy_mike

So when I overheated my 250, this is what it ended up sounding like,

https://www.youtube.com/watch?v=OhEYYEN9SlY

When I tore it apart, the #6 cylinder piston had shrunk .012". I was getting piston slap. Compression was low on that cylinder too. #6 runs the hottest. Luckily I did not damage the bore and was able to use hone to to remove any glazing and replaced all the pistons and bearing. Ran great for another 3 years before I sold the car.

You're doesn't sound as bad as mine. Try taking the #6 plug wire off and see if the noise quiets down. If so, it's likely piston slap or a bad rod bearing.

Not being an "engine guy" I will need to do some reading about piston slap. Thanks. That's an easy test. I also plan to do a wet compression test today. I've run the engine enough now to work out any cobwebs from it sitting around and not having been run for several years.

[GRX]

Low psi in #6 is cause for concern. Sounds like valve train noise to me.

[Lugnutz65]

I'm on a short vacation, but I promise to follow up on this. Thanks for your thoughts.

[Lugnutz65]

Originally Posted by Lugnutz65

I'm on a short vacation, but I promise to follow up on this. Thanks for your thoughts.

As promised, here's a little follow up.

I've driven this truck now for maybe 75 miles with zero issues from the engine. I'm pretty sure I have a rear main seal leak, but other than that it starts, runs, idles just fine and temp stays 190's in this SC heat.

I repeated a cold/dry compression test yesterday on cylinder #6 = 90 psi.
I then added some oil down the cylinder for a cold/wet test and #6 = 90 psi. No change at all.

Next is a leak down test. I guess. I suspect I might have a broken valve spring on #6 since that's where I seem to hear the "noise" from.

Chevy Mike - I have not tried taking #6 plug wire off to see if the noise goes away.

Suggestions?

[68gmsee]

If there was no change at all in #6 it's possible you have a valve problem of some sort. A leak down test will help you determine that for sure.

Be sure and check to make sure the rocker wasn't tightened too much to keep the valve from closing. I've seen that before.

[Lugnutz65]

I did a cylinder #6 leak test today.
Throttle was 100% open and cylinder #6 at TDC.
Watch THIS VIDEO.

What next? Any other tests I should do before I remove the rocker cover?

[68gmsee]

Yo, Lugnutz, good video. I would definitely remove the valve cover. Last time I did a leak down test I had the valve cover off. Primarily because I first checked to make sure the rockers were not adjusted too tight or had other visible problems.

You're on the right track. Worst case, you'll need to do a complete head rework. Last time I had one done some years ago it was close to $200. If they have to do extensive work it may double that.

['68OrangeSunshine]

I had a broken valve spring on my '68 L6 292. I noticed immediate power loss, as well as bad noises. The first time, we thought it was a fluke, since the engine had only been rebuilt recently, and replaced the broken exhaust spring in No 4 cylinder. When it happened again a year and a half later, we replaced all the valve springs. Now it runs great.
Mileage was under 40,000. If I ran in to that problem on an old engine, I'd rebuild it.
Here's a link to that thread:
http://www.67-72chevytrucks.com/vboa...d.php?t=727863

[Lugnutz65]

My son and daughter-in-law came over for dinner tonight. He just bought a bore scope and wanted to try it out. Didn't have to twist my arm. Nope!

We looked down #6 plug hole. I'm sure we'll make better videos with a little more experience, but THIS VIDEO is the best we could do for now.

We noticed a small very symmetric looking chip at the edge of the #6 piston. You can see it at the 30 second mark on the video.

Doesn't look serious. I doubt it's the cause of my leak since the dry and wet compression tests were exactly the same = 90psi.

**Edit** The "chip" I referred to above is actually a factory casting mark to indicate the front of the piston. A friend explained that to me. I then went out to my garage and found an old piston from an L6. Sure enough, it has the same "chip", so that's NOT the problem.

[TJ's Chevy]

Both valves not sealing....also the cylinder that runs the hottest....could be torched or recessed valves..could also just be tight valves...hopefully just tight. I'd loosen them both and adjust to 1/2 a turn pre-load.

[Lugnutz65]

Someone suggested that my problem with low compression in cylinder number six, and the leak in both the intake and exhaust valves is because I have a crack in the head between the intake and exhaust ports.
What do you think?

['68OrangeSunshine]

How many miles are on this build? Or is it the original 1971 engine and head?

[GRX]

Removing the valve cover would have been my first move when hearing that noise. Do that and pay attention to spring & retainer height on #6. Good chance you have sunken exh. valve seat, and all the valve train wear to go with it. Especially if this is original to 1971.

[Lugnutz65]

Originally Posted by '68OrangeSunshine

How many miles are on this build? Or is it the original 1971 engine and head?

Bought this 63 and found out the engine had been swapped. Engine stamp indicates it's a 1971 250ci. Mileage unknown. I'm encouraged that cylinders 1-5 all show exactly same compression at 125psi. Cylinder #6 looked ok with the bore scope. Keeping my fingers crossed.

['68OrangeSunshine]

Originally Posted by Lugnutz65

Bought this 63 and found out the engine had been swapped. Engine stamp indicates it's a 1971 250ci. Mileage unknown. I'm encouraged that cylinders 1-5 all show exactly same compression at 125psi. Cylinder #6 looked ok with the bore scope. Keeping my fingers crossed.

If I'm not mistaken, Chevy Trucks didn't have the 250 until MY '66, so your original engine was probably a 230. '71 is an excellent L6 vintage.
If it was my truck, after a leakdown test, and a check of lifter/rocker operation with the valve cover off -- if it didn't come around -- I'd pull the head and get a valve job from a machine shop. The machinist can magnaflux the head and tell if it's cracked or rebuildable.
If it's your daily driver, and you're broke, well I understand. BTDT. But sooner or later, [to quote Gary Cooper] ''Sometimes a man's gotta do what a man's gotta do.''

[Lugnutz65]

Originally Posted by '68OrangeSunshine

If I'm not mistaken, Chevy Trucks didn't have the 250 until MY '66, so your original engine was probably a 230. '71 is an excellent L6 vintage.

Yes, I also believe that 1966 was the first year for the 250ci.

Your plan is a good one, but I have more info that might change your recommendations. I own 2 additional 1971 250ci engines. They are both complete.

Here's the back story, if you like stories.

My #1 engine was painted up nicely with Flame Blue engine paint and it ran for a while in my 1965. It was a free engine I got when I parted out a truck and it "seemed to run good". Then it started having low oil pressure issues. The dash warning light would come on at the traffic light and the engine burned a lot of oil. However, it drove around just fine and had plenty of power. I was even getting 18MPG on the highway with my T5 tranny. So I made a plan to replace it. Live and learn as they say.

My #2 engine was purchased as a replacement. Seller claimed it was a "good engine", but we all know that it ain't always so. My plan was if #2 passed all the tests, I'd re-seal and paint it. Then do the swap into the 1965 truck.
If it DIDN'T pass, then I was going to have it rebuilt for extra power. I would send it to the machine shop, then rebuild with 307 pistons (flat top), bigger valves, and a little bit of decking to increase compression. I've never taken time to test this #2 engine (see next paragraph).

My #3 engine is a 230ci. Within 2 weeks of buying engine #2, I'm contacted by a forum member about a 230ci he has for sale. He had a 1964 230ci rebuilt by a machine shop and ran it for less than 4000 miles and then did a SBC swap because he wanted more power to pull a trailer. I could have the 230ci for $400. No brainer. VIDEO of the 230ci on my run stand. So I swapped that 230ci into my 1965 where it lives and breathes today.

My #4 engine is the one in my 1963 truck and has the low compression in #6.

So, I have the luxury of multiple options. If the valve springs look OK in engine #4 and I can't find anything obviously wrong with the valve train or head, then I could do any of the following.

1. Check engine #2 for compression and a leak test. If it passes, then I'd run it on the run stand and check oil pressure etc. If it proves to be a better engine than #4, I'd re-seal it, paint it and do a swap.

2. I could use the head off of #1 if it passes a leak test. It already has new valve seals. However, I fear it burned oil because of worn valve guides.

3.Pull the #4 head and have the machine work done as you suggested.

['68OrangeSunshine]

Originally Posted by Lugnutz65

Yes, I also believe that 1966 was the first year for the 250ci.

Your plan is a good one, but I have more info that might change your recommendations. I own 2 additional 1971 250ci engines. They are both complete.

Here's the back story, if you like stories.

My #1 engine was painted up nicely with Flame Blue engine paint and it ran for a while in my 1965. It was a free engine I got when I parted out a truck and it "seemed to run good". Then it started having low oil pressure issues. The dash warning light would come on at the traffic light and the engine burned a lot of oil. However, it drove around just fine and had plenty of power. I was even getting 18MPG on the highway with my T5 tranny. So I made a plan to replace it. Live and learn as they say.

My #2 engine was purchased as a replacement. Seller claimed it was a "good engine", but we all know that it ain't always so. My plan was if #2 passed all the tests, I'd re-seal and paint it. Then do the swap into the 1965 truck.
If it DIDN'T pass, then I was going to have it rebuilt for extra power. I would send it to the machine shop, then rebuild with 307 pistons (flat top), bigger valves, and a little bit of decking to increase compression. I've never taken time to test this #2 engine (see next paragraph).

My #3 engine is a 230ci. Within 2 weeks of buying engine #2, I'm contacted by a forum member about a 230ci he has for sale. He had a 1964 230ci rebuilt by a machine shop and ran it for less than 4000 miles and then did a SBC swap because he wanted more power to pull a trailer. I could have the 230ci for $400. No brainer. VIDEO of the 230ci on my run stand. So I swapped that 230ci into my 1965 where it lives and breathes today.

My #4 engine is the one in my 1963 truck and has the low compression in #6.

So, I have the luxury of multiple options. If the valve springs look OK in engine #4 and I can't find anything obviously wrong with the valve train or head, then I could do any of the following.

1. Check engine #2 for compression and a leak test. If it passes, then I'd run it on the run stand and check oil pressure etc. If it proves to be a better engine than #4, I'd re-seal it, paint it and do a swap.

2. I could use the head off of #1 if it passes a leak test. It already has new valve seals. However, I fear it burned oil because of worn valve guides.

3.Pull the #4 head and have the machine work done as you suggested.

Interesting dilemma. No clear choice.
I personally wouldn't want a 230, but I'm a 292 guy, so maybe that makes me a displacement snob. The 292 is the ''Big Block'' of the Chevy InLine world. I even think 250s are small.
The heads are all the same for the Gen III GM L6s - 230, 250, 292. Running the numbers, I found out the head I'm currently running on my 292 was once on a 230. It's now got 1.84'' [V8 307] exhaust valves, and new Sealed Power valve springs, with a Crane 260H cam, .030 bore, etc. in the '68 block. [Original to the truck, but rebuilt once.]
You could pull off the head from any of your engines not currently mounted in trucks and try a swap. You have an advantage with more doghouse tunnel room and a lower engine deck height, than a '67-'72 with a 292. [We had to modify the spring compressing lever with a big pear-shaped hole to get the valve spring retaining clips on Cylinder #6 -- no headroom.]

What I would do is take any of those heads to the machine shop for a rebuild, and bolt it on. If the engine still has issues, rebuild the lower block too. Then you have a fresh start. And you will know who built it, with what parts and when it was done. Junkyard acquisitions of machinery over 50 years old can't be too reliable, even if it was built to be bombproof then.

[Lugnutz65]

Originally Posted by '68OrangeSunshine

I personally wouldn't want a 230, but I'm a 292 guy, so maybe that makes me a displacement snob. The 292 is the ''Big Block'' of the Chevy InLine world. I even think 250s are small.

I take no offense. If indeed you are a displacement snob, you need those of us who like the smaller L6 engines. You actually NEED us. Otherwise you would have no one to feel superior to. LOL!

I'll take the rocker cover off this weekend and see what's what. I really need to get my 1965 back on the road. That means I will not be swapping heads on the engine in the 1963 until my 1965 stepper is road ready, and that's several months away.

Stay tuned. I appreciate all the advice as I try to diagnose the low compression issue.

['68OrangeSunshine]

230s and 250s use the same block. Is is a difference in rod length or bore diameter, or both, that yeilds the extra 20 cubic inches?
I've heard of racers making a ''Cheater 292'' out of a 250 block. I think they used a 292 crank, and maybe rods, clearanced everywhere, and very fat pistons. Getting around racing regulations would be the only reason for so much machine shop work.
A 230 would be ''period correct'' for the truck. What are they like for torque and horsepower?
Yes. I fully admit to 292 snobbery, since God favored Me, by putting a 292 into the '68 truck I was about to buy in 1973. [JK]

[TJ's Chevy]

Originally Posted by '68OrangeSunshine

230s and 250s use the same block. Is is a difference in rod length or bore diameter, or both, that yeilds the extra 20 cubic inches?
I've heard of racers making a ''Cheater 292'' out of a 250 block. I think they used a 292 crank, and maybe rods, clearanced everywhere, and very fat pistons. Getting around racing regulations would be the only reason for so much machine shop work.
A 230 would be ''period correct'' for the truck. What are they like for torque and horsepower?
Yes. I fully admit to 292 snobbery, since God favored Me, by putting a 292 into the '68 truck I was about to buy in 1973. [JK]

230-250-292 all have the same bore, the strokes were different.

['68OrangeSunshine]

Originally Posted by TJ's Chevy

230-250-292 all have the same bore, the strokes were different.

OK. I still don't get the concept of the Cheater 292 in a short deck block.