Low compression #1 cylinder, piston or ring or block problem

[Trelisong]

Well about 10,000 miles ago I had my engine rebuilt, I had a machinist do the work and also assemble the small block, I took the block home and assembled everything from there. Ross Racing Pistons 20 over were used in the rebuild.

A week or two ago I did a compression check because my car had been running horrible for a while and I found the #1 cylinder had low compression, 30 psi to be exact.

I suspected valves or headgasket so I had the head checked out at my local machine shop they refaced and lapped in the exhaust valves to be safe but he said he didnt see anything wrong, I was hoping it was either the head or gasket.

I reinstalled. I still only have 30 PSI on #1 cylinder.

Tomorrow I will be pulling the head back off and dropping the oil pan and pulling the piston to see what went wrong.

I have never installed a piston myself. I have pulled them once before though.

What do you think my chances are that its just the rings.... The only other two things it could be are a damage piston or a damaged cylinder bore.....

I am really praying that all I need are rings. Is it advisable that I replace the bearings while I am there, I would think mine would be fine since I only have 10,000 on them, but this is the part I have never done myself before.

If I do end up needing a new piston, can they be purchased individually someplace... possibly if i gave a call to extreme psi, or some other vendor? what do you guys think?

 

[centralboost90]

do you know what type of rings were put in it. chances are its just the rings, but t-gauges and micrometers will tell the truth on the cylinder.
the bearings should be fine. i would advise measuring oil clearences just incase. and i also advise you replace all of the rings, like every cylinder. what head fasteners are you using, ARP? or reusing stock bolts?

 

[Trelisong]

Yes it was the rings that come with the pistons when you order the pistons from slowboyracing.

Slowboy Racing :: Engine :: DSM Engine :: Pistons :: Ross Racing Pistons :: Ross Racing Pistons 6 Bolt 4G63 8.5:1

I purchased the rings back in October 2005, and have only put 10,000-11,000 on the engine since then.

The engine has ARP Mains and Head bolts.

 

[centralboost90]

hmmm. maybe its just a defective set of rings. a hone wouldnt hurt either, a ball hone would help with cross hatching with helps ring seal.

 

[Trelisong]

Is this what you mean by T gauge? cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=350114534720

and is this what you mean by micrometer? cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=220301747271

 

[Trelisong]

Ive been doing some reading and I am worried that I might have a damaged ring land which would mean I would need a new piston, with a compression of only 30 PSI, is it possible to be only the rings, or is it more likely that I also have a damaged ring land.

Either way I will know tomorrow when I pull the head and then the number one piston.

 

[centralboost90]

yes those are t-gauges and a micrometer, you can do alot of engine blue printing with those, cyliner taper and out of round, and pretty much every oil clearence in the entire engine. if it was a ring land then i would expect that is was detenating. how much boost are you running, any tune? it is possible to be just the rings. but we will see. let me know when you find out.

 

[BoredDSM]

I honestly do not like the t-gauges. They aren't as accurate as a bore gauge and they tend to not touch the ends of the cylinder walls completely. But its better than nothing.

 

[tumtum411]

Get a leakdown test

 

[centralboost90]

I honestly do not like the t-gauges. They aren't as accurate as a bore gauge and they tend to not touch the ends of the cylinder walls completely. But its better than nothing.

this is true, but t-gauges are cheaper. and they are also better then plasti-gauge.

 

[centralboost90]

Get a leakdown test

he has already determined that it must be blowby the rings, wheather its the cylinder or rings a leak down test wont tell us much. that is if the person he said checked the head out knew what he was doing and did the right tests. which it sounds like he did.

 

[Trelisong]

I just called and talked with the Machinist I took the head to, I told him that I added oil to the cylinder and it did not go up any, to him this is very odd.

He is simply puzzled because by adding oil the rings should seal up and he just got done doing the head so everything should be working.

He recommended I pull the lifters from the number 2 and swap them with the lifters on the number 1.

He said if my compression goes low in #2 and goes back up in #1 that it must be my lifters.

He said that if the lifters over pump that they could hold the valve open when they shouldn't.

I am going to get a leakdown tester from harbour freight tools if this does not end up being the case, just to be sure.

 

[Trelisong]

UPDATE: I forgot to bleed the lifters.

I took them back out and bled them, compression is the same with #1 lifters swapped with #2. only 30 PSI on #1 and #2 still at 180

 

[Trelisong]

I will be running to Harbor Freight Tools tomorrow to get a leak down tester.

I will let you guys know tomorrow what I find out.

 

[toyotaturbo]

Having the same problem. Our race head only has 30 psi in #1 and good compression for the rest. After we had the head milled, we got it back from the machine shop, strapped it on and the car ran terrible. The head "looks" fine, but something's wrong with #1 cylinder that has to be figured out by the machine shop.

The head was warped and we had some milling done to even things out.

 

[Trelisong]

So your problem was a warped head. I just ran to harbor freight since they were still open.

I have the leak-down tester now. I am going to go do a leak down test and see what I can find out.

 

[Trelisong]

just got done leak down testing it.

The machinist did not trim the valve height correctly or something.

one of the two exhaust valves were leaking really bad on #1, so I pulled the rocker for that valve and sure enough the leak practically disappeared.

Everyone agree? valve stem height must be off?

I am going to call the machinist in the morning and after I talk to him I will pull the head and take it in.

I almost want to take it someplace else but he has done great work for me in the past.

 

[centralboost90]

that would make sense. its allways the things you never expect go wrong. let us know what the machinest says

 

[jlr2196]

Why would it be fine for 10,000 miles and then all of a sudden the valve heights be off?

 

[Trelisong]

Actually the car has always ran a bit rough, and I already pulled the head once and he relapped in those valves that were giving me the problem, I have a feeling it was always a little to tall on the valve stem height and then when he relapped in the valve it made it even more apparent by opening the valve that much farther

 

[jlr2196]

Gotcha ya, hopefully he does a little more research and measuring of the installed valve heights this time around. Perhaps he ground the seats a little deep the first time the head was done and didn't tip (grind) the ends of the valves accordingly. Good luck

 

[Trelisong]

just following up on this, the problem ended up being that the valves on the #1 piston were barely!! bent like almost not noticeable, and the first time that he relapped in the valves he must have missed it. I ended up replacing just those two valves with new ones, and problem was solved.

 

[b00stAdict]

just following up on this, the problem ended up being that the valves on the #1 piston were barely!! bent like almost not noticeable, and the first time that he relapped in the valves he must have missed it. I ended up replacing just those two valves with new ones, and problem was solved.

glad to hear everything turned out good. for future reference before you pull the head off id do a wet compression test and see if you get a higher number and if you do then its your rings.