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TR6 TR6 compression test

Terry_M

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Just completed a compression test, on a warm engine highest 105--lowest 105--Wondering is this acceptable I have low oil pressure at hot idle (2 lb) and thought refreshing the bottom would extend the life
opinions ,please
Terry
 

TR3driver

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Those readings are a little too good; makes me wonder if the gauge is broken. Easy test, squirt a small amount of oil (like one pump of the can) into a cylinder and check again. I've never had an engine that didn't show at least a 1 psi variation between cylinders, even right after a rebuild. With the oil, the reading should come up at least a little. If it comes up a lot, then the rings are bad.

You didn't give any history; but if the low oil pressure is due to worn bearings in the lower end, then new bearings would certainly pull it up. OTOH if you have a broken oil pump or a big leak somewhere (like maybe something caught in the pressure relief valve, blocking it partially open), then new bearings won't help much.

How do you even read 2 psi on a TR6 oil gauge? Have you checked the gauge for accuracy?
 
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Terry_M

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My bad--the reading for the compression test were from 90-105 (90,100,100.105.105.105 )--No major oil leaks ,new pressure relieve valve The car has 81000 miles on the odometer and i have not driven for three summers . When starting cold at idle it sits at 50 as the motor runs to temp the oil gauge slides to slightly over zero (why i said 2 LB) revving and it comes up to 40-50 and the idles down to zero plus 1-2
I can't say the oil gauge is accurate ,i do believe it indicates a problem The question is what is a acceptable number for a compression test--- I will try the comp test with oil again this week and get back to you
Terry
 

poolboy

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Does the engine smoke when you accelerate /
How many miles can you go before you find you are a quart low ?
 
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Terry_M

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hello
as i said the car has been resting for three summers, can't remember how far until a oil top up ,i get white smoke from the tail pipe on revving I rebuilt the carbs and the final adjustments are proving to be challenging ( i am now far too rich) The oil is 20-50 fresh change
terry
 

poolboy

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It's possible that the piston rings are just stuck around the pistons, having sat so long without movement.
 
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Terry_M

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I thought the car sitting would affect the comp test ,but it shouldn't effect the oil pressure should it ?- To put the car on the road is insurance and licence tags then find out i am running the risk of damaging the motor
 
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Terry_M

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I did the comp test to determine if it was cost effective to freshen the bottom of the engine,which i believe i can complete myself--Low compression in my mind is motor rebuild,which is beyond me and require a professional
Terry
 

Zip960

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I bought a 1971 rust bucket parts car for the A overdrive and did a compression test on the "warm" engine before removal and the numbers were: 140, 135, 132, 130, 130, 140. It was a CF engine.

All are within 10%. So the engine is GOOD! Almost all maintenance books I have read tell you not to concentrate on the numbers, to just ensure they are all within 10%. That said, I can't understand how you're numbers can be that low. If it was me, I would borrow a gauge and do the test again.

HOWEVER, And after reading the rest of your comments....white smoke, almost non existent oil pressure, 20-50 oil...sounds like a "tired" engine :( Can't rebuild, find a used one on CL and make sure you see it run before you buy!

CHEERS
 
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Terry_M

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hello Zip
i do appreciate the reply with some help other than ,whatever I was looking for other sites for help-- I thought the motor was tired ,was prepared to rebuild freshen the bottom ,the compression was a worry Purchased a new gauge and will try again As i said i rebuilt the carbs and still trying to get the mixture down to acceptable levels .August i plan on reinsuring and driving for a month or so and try the compression again
regards
terry
 

TR3driver

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My bad--the reading for the compression test were from 90-105 (90,100,100.105.105.105 )
Ok, that's still reasonably good. The lowest is only about 10% below the average. No doubt you're losing some power, wouldn't want to go racing with that motor, but it should run fine for the street. I've certainly driven much worse.

When starting cold at idle it sits at 50 as the motor runs to temp the oil gauge slides to slightly over zero (why i said 2 LB) revving and it comes up to 40-50 and the idles down to zero plus 1-2
I can't say the oil gauge is accurate ,i do believe it indicates a problem
Well, the old rule of thumb for minimum oil pressure is 10 psi per 1000 rpm. Since you're looking at a 40 year old version of a gauge that wasn't all that accurate new, it could easily be the gauge off by that much. But I'll agree, it could probably use some new bearings, and check the oil pump while you're in there.
The question is what is a acceptable number for a compression test
The absolute numbers are not really important, since they are affected by all sorts of things including compression ratio, temperature, even humidity and altitude. What you want to look at is the variation between the numbers. +/- 10 percent is considered about the limit for a "healthy" engine; but they will usually still run well enough at +/- 20 percent (although idle may be a little rough). When I got my previous TR3A, one cylinder was down around 85 while the other 3 were around 130! I drove it that way for about 6 months, until I had time & money enough to stuff a set of rings in it. (Then another 2 years before I put in the set of liners & pistons that it really needed.)

If the low compression is from leaking valves, then continuing to drive it may do some damage to the leaking valve seat, possibly pushing it from repairable to needing replacement. But even replacing a valve seat is fairly cheap compared to the overall cost of an overhaul. And if it's just rings that are leaking, they aren't likely to do any more damage than you already have.

That said, changing the rings and lapping the valves isn't that hard for a "shade tree" mechanic, nor does it require a lot of special tools. (It does require some, just not a whole lot.) And it can be done with the engine in the car (so you don't need a hoist, place to store it, etc.) If you just want to squeeze a few years use out before doing a proper overhaul, you might consider rings along with bearings. In my case, one of the rings was actually broken in the cylinder with low compression, so new rings helped a lot even though the cylinder wall was in such bad shape. The rings had apparently rusted to the cylinder during storage (before I got it), leaving a row of rust pits around the cylinder. The bore was visibly oversize through the section with the pits, plus some of the pits were still there. I put probably 30,000 miles on it that way (drove it to work almost every day and all over the place on weekends).

PS, never did get around to doing a proper overhaul on that engine. The OD had quit and I bought another TR3 just to get the OD out of it. Its engine ran better, so I swapped the engine along with the OD. Never heard back from the guy who bought the TR3 with my old 3A engine in it, but he seemed anxious to buy even after I explained what had happened, so I guess he was happy.
 
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