Compression and ring gap tolerances

fletch

fletch

New member
Location
Australia
Excuse my ignorance! My old 3C (No 1399) and I are on a learning journey.
A compression test revealed results of 75, 60, and 45 psi. What should I expect? (Standard pistons)
Obviously something wrong so heads and barrels off to check piston tolerances. Ring gaps in the bore were around 0.65mm. Book says max should be 0.4mm. Oil ring was about 1.5mm. Max should be 0.5mm.
But then I read in the text of "the new green book" that rings in good condition will operate well with gaps of up to 0.9mm.
Do I need to go oversize?

My appreciation in advance for expected deluge of wisdom!

Fletch
 
You should be expecting at least 100-120psi, healthy engines muster 150 and more, your engine seems well and truely knackered worn out.  Ring gaps way are out of spec, both barrels and rings will be well past their best.  Don't be fooled by piston condition, they always look as if they could be re-used, even when bore measures up to be (just) within tolerances.  The top ring groove is usually very well worn and the piston alloy has collapsed, ripe for the scrap bin.

piet
 
A few questions
Did you check valve clearances before you stripped it down ?
Was this cranking pressure or running pressure ? ( might be a silly question)
I would expect better than 110PSI cranking but in any case within 10% of each other max.
Was there much blow by on the breather ?
At 45psi i'm surprised there were any rings in there to gap ? I don't know much about Lav engines but i did pull a Duc engine down the other day and found the rings were all in upside down. That didn't help  :D but more to the point the valves were not shutting/sealing properly either.
 
One blindingly obvious question - did you have the throttle wide open when you did your compression test?
And another - did you try putting a little oil down the plug holes to see if that made a difference?  I'm slightly in awe of you stripping the top half without an apparent second thought - or question here first!
 
Rob said:
One blindingly obvious question - did you have the throttle wide open when you did your compression test?
And another - did you try putting a little oil down the plug holes to see if that made a difference?  I'm slightly in awe of you stripping the top half without an apparent second thought - or question here first!
Click to expand...

Doesn't make much difference Rob, ring gaps point to a well-worn engine.  :D Not going to be much fun in that condition...

piet
 
You're right of course Piet - I guess I would have attempted a bit more diagnosis to start with! Superfluous as things have turned out here
 
Check your valve sealings.
I agree with what Rob says, and would certainly have done at least a wet test, if not got hold of a leakdown tester. Moot point now but.
 
Thanks for the prompt replies. Some great advice.
The head is good (just reconed by Redax) but bottom end is an unknown. Valve clearances were a little tight at about 0.2mm ex and 0.15 inlet. Not sure if that is enough to cause the problem.
However I will now treat the bike to some new pistons.
Maybe I need to post some questions BEFORE I perform drastic mechanical surgery.
(The collective knowledge of this site is quite remarkable)
Cheers, Fletch
 
Rob said:
One blindingly obvious question - did you have the throttle wide open when you did your compression test?
Click to expand...

A valid point and one I only learned myself a few months ago... :-[ :-[ :-X
 
AndyW said:
A valid point and one I only learned myself a few months ago... :-[ :-[ :-X
Click to expand...

There's knowing and then there's remembering and unfortunately as I get older there seem to be more things that I know I should do then forget them.
 
Are you saying you put a good head on the engine without lifting the barrels while you were in there to check the pistons and bores? just wondering....
 
Hey Fletch you are the first person I have heard of with a 1300 numbered triple well done, where are you in the world.
With compression that low "if your test was accurate" then it must have been difficult to start what were the symptoms?
 
With compression that low "if your test was accurate" then it must have been difficult to start what were the symptoms?

 
Click to expand...

  When the soft valve seat problem began (1978) we could often tell when the valves were being kept off their seats  and reducing compressionas the starter motor would whizz motor over like the plugs were out - engines had a much "softer" sound too.
          With the pistons I had made - I always specified what rings I wanted to use and source the rings myself. I used ring sets that were 0.10mm  (80.4mm piston with 80.5mm ring set for example) larger diameter than piston diameter. This made the top two rings usually within spec with a nice smaller gap - the oil ring would need filing/grinding down to suit individual motors - this did lead to me making a ruck of extra money as the amount of rings that were broken by owners was substantial. ::) ::) - Regards - Phil
 
One thing I'm going to say is that Ring Gap is more critical for Performance, Endurance and Long Jeopardy  then Piston Diameter to Cylinder Bore Clearance - Specifically in an Air Cooled Engine.

This is something I hole hardly attend too are my ring gaps when assembling an Engine.

For this reason I always buy a set of rings .010" even to .030"  over size and fit Rings for each Cylinder.

Never buy a Set of Rings from a Manufacture that claims to fit at specific bore - Buy Over Size and File to Gap the individual bore.

No Bore is Perfect or Identical to the Next.

There's a method and takes hands on teaching from an experienced to learn how to Ring Gap.

Bore Consistency Vs Ring Gap for that specific cylinder is critical if you want to Win Races.

HINT; A Bore Caliper may help but all you really need to know is how to use a Feeler Gauge to determine an average where the Ring Gap for the Compression Ring - Secondary Ring and Oil Rings fitted on that Piston do NOT FLUTTER against the Bore with the tightest possible seal without binding or breaking a ring - Rings fit the Bore not necessarily the Piston ;o)
 
Rather Simple: Even from your most trusted Machine Shop, using a Sunnen Hone Bore Machine, you will find discrepancies from one cylinder to the next just by using an old set of rings and noting the Ring Gap along the Bore of each Cylinder with Feeler Gauges, hence you will gap your rings for that Specific Cylinder. That's how accurate Ring Gaping can be. No 2 Cylinders are the same even though how you attempt them to be so. It's the Ring Gap of each Cylinder that Ultimately Defines a great percentage of your Engine's Performance and life expectancy.

Ever hear of a Company that advertized Perfect Sealed Rings? Unique Design but I would sooner do it my way and I believe Ring Seal plays an important ideo in regards as to why many of our 3 CLY Members have problems in setting up their Carbs and Timing.
 
I've had good service from "Total Seal" piston rings with other engines. They will make a custom set for you. I had a set of rings made up for my 3c back in 2008 and then decided to fit 77mm Ross H/C pistons. The Ross pistons will be my winter project this year. Don't need the Total Seal ring set.

Back in 1977 I had 1st over 75.4mm H/C factory pistons fitted. Those pistons are still in the engine.

Specs on the Total seal rings. 2.970 bore. 1.5mm top rings, 2.0mm two piece middle rings and 4.0mm three piece oil control rings.

Total Seal has some specs for "file to fit" based on bore size, ring location and application (street use, nitrous, blown gas,etc)

If they have any value to anyone out there..let me know.
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