TR2/3/3A 60 TR3A Compression test

[Tinkerman]

Just finished a compression test on my rebuilt engine, and the numbers are:
#1 210 PSI #3 200
#2 170 PSI #4 195

The factory workshop manual listed 120 PSI. I do have the larger pistons and a shaved (mild)Head. I am going to reset the valve lash because I think I did it wrong last time. And recheck. In a rebuilt engine I would expect the numbers, no matter what they are, to be fairly close, right?

Dick

 

[sd80mac7204]

Would I be correct in guessing that the engine has not actually run yet?

 

[Tinkerman]

The latest compression test results, after resetting the valve lash per a corrected Haynes manual are:

# 1 190 psi # 2 170 psi
# 3 170 psi # 4 160 psi

Yep the engine has actually run. I had it up to 2000 rpm but the vibration was so bad that I shut it down ASAP. We still have some carb issues but I expect that to go away after the incoming carb parts are installed.
I would really like some help interpreting the compression numbers.

Thanks, Dick

 

[TR3driver]

IMO, those numbers aren't telling you anything useful. The spread is a little high for a freshly rebuilt engine, but certainly none of them are bad enough to keep it from running smoothly. And it is at least reasonable to hope that they will improve as the rings bed in.

These engines aren't the smoothest in the world, but if the vibration was that bad then you've got a more serious problem than compression off by a bit. My guess is that you have one or more cylinders not firing at the right time (or perhaps at all); so I would proceed to check the firing order one more time, and then start pulling plug wires to find which cylinder(s) aren't running.

Of course if your carb issues are really serious, they might be the explanation.

 

[TRTEL]

Dick, What type of gauge are you employing / threaded with an O-ring seal/ all plugs out at same time / throttle propped wide open/ was engine hotter on the first go around / kept cranking on starter till psi # stabilized? And lastly (I hesitate to ask)could this be a new more agressive (overlapped) cam?
Tom Lains

 

[Tinkerman]

Tom, all good questions. The gauage is threaded with an O-ring seal. Three plugs in, engine not running, cranked until psi stabilized. Way I was taught when I worked in a garage. Back then gas stations actually fixed engines and other automotive stuff, heh! It is a re-ground more agressive cam. I also have delrine spacers between the rocker arms. Forgot about those. Got the cam from a chap in CA that used to race TR's and builds racing engines(TR). He called it a mild street cam. Company is called British Frame and Engines (NFI). Came highly recomended.
I am concerned by the 190 on #1 and the 160 on #4. Realize that it may be operator error. Been there before and have the T-shirt
Going to check those two cylinders again, but am really waiting for the carb parts I have coming in. Both of my carbs are high mileage and both failed the "piston drop test". Once that is fixed I will go back after the compression tests.
Thanks for everyones input!
Dick

 

[poolboy]

I wouldn't be too concerned until the rings had a chance to seat in the cylinders and that takes running the engine a few hundred miles..at least.

 

[TRTEL]

Interesting. Do you have the numbers the came with cam (open, close, before, after,lift, blah....blah)? Even with no break in time, that much spread is a bit abbie normal. I would suggest pulling the other plugs, it's easier on the starter and lets it crank faster. I've measured enough cams to find curious things on regrinds. And having seen a few shops, think it's as often the machinery (read old like me) as the operator. I'll keep an eye on the thread.

 

[CJD]

Need to seat the rings, like everyone said above. At this point you do have compression, so you are golden.

The quickest way to seat rings is to lug the motor in high gear from about 40mph up to about 60mph. Then let the car coast back down to 40. Repeat a dozen or so times and they should be seated. The lugging loads the rings heavily and the coasting sucks oil up to cool them. This method is straight out of the 1960's auto shop manuals.

John

 

[Deleted member 8987]

No.....you can crank and crank until you get numbers you like.
First, ALL plugs out. Free wheeling, so the engine speed on the cylinder being tested is not being affected by the compression in another.
Tie the throttle open, all plugs out, count the "hits". I use 5. Every one the same. DO NOT oil the cylinders yet.

 

[Mickey Richaud]

No.....you can crank and crank until you get numbers you like.
First, ALL plugs out. Free wheeling, so the engine speed on the cylinder being tested is not being affected by the compression in another.
Tie the throttle open, all plugs out, count the "hits". I use 5. Every one the same. DO NOT oil the cylinders yet.

Yup.

 

[Member 10617]

"ALL plugs out..." That's a new one on me. Never heard that before. So, for a better reading on compression, ALL the plugs should be out as you check each cylinder by cranking until psi stabilizes (engine not running)?

Am I reading this right?

And why do you tie the throttle open?

 

[Mickey Richaud]

Short answer is that you don't want anything contributing to or taking away from simple compression. The only compression you want to "read" is from the opening and closing of the valves.

Someone a bit more erudite at this hour will surely give a more thorough comment shortly...

 

[Brinkerhoff]

Everyone is making great points about checking the compression. His compression readings as is indicate the engine should start and run higher than 2000 rpm without vibrating. His cam timing is off, or his firing order or he has something grossly wrong with a carb. like they are not flowing fuel to the engine. It is extremely easy to mistime a camshaft when installing it , especially one that has been reground ,reprofiled etc. , without using a degree wheel and determining accurately where TDC is. A stock cam is not a big deal to install just by aligning the marks on the gears with a straightedge through the axis' of the crank and cam. Even then you can be off a tooth if not careful. Randall brought up a good point about pulling plug wires off while the engine is running. There are special pliers for this but will tell immediately if the firing order is off. When I was 16 I rebuilt the engine in my Bugeyed Sprite and logically installed the plug wires in order ( 1,2,3,4) . The engine vibrated and wouldn't rev but I could still drive the car around the block only running on one cylinder ! Kevin

 

[Tinkerman]

Mornin All. Kevin when the engine was vibrating so badly, the valve lash was way out of whack. I have since reset the lash and am waiting for the carb parts to arrive. Once they are installed I will have another go at it.
Thanks for your input, Dick

 

[Tinkerman]

I'm happy to report that after tinkering with the lash setting on cylinders 1 & 4 that my compression is now:


#1 170psi #2 170psi

#3 170psi #4 165psi

Happy with those numbers will get on with the process of starting as soon as the carb parts are in.
By the way the man that did all of the machining told me that with that compression I will have to us Hi grade gas rather than regular.

Cheeers, Tinkerman