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Sports piston rings orientation and order - dchw1975 - 06-02-2022
Hi,
I have 58mm pistons in my sports engine purchased some years ago and I am investigating my apparent low compression. Having taken two pistons out (to check they are all the same) the order the rings were installed was;
Top - Chrome plated ring with chamfered outer edge uppermost. (possibly has chamfer on bottom too)
Middle - Plain looking ring (which may or may not have an angled face)
Bottom - thick ring (it'll only fit there
)So my questions are;
1) Is this ordering correct, I would have thought the compression ring (plain looking one) would go on the top??
2) Since neither of the top two rings has any marks can I assume they don;t have a preferred orientation (i.e. top & bottom)?
3) I have 10 thou ring gaps when measured, is this a bit big?
4) knowing the ring gaps should be orientated 120degrees between each other is there a preferred orientation of the top and bottom gaps dependent on the thrust face of the cylinder etc?
Cheers
David
RE: Sports piston rings orientation and order - Bob Culver - 06-02-2022
Ring order seems correct. The third oil ring is usually slotted and drilled.
Opinions differ on gap arrangement and a look thru sundry "modern" workshop manuals reveals a myriad recommendations. The rings usually turn anyway. Has more influence with loose fitting pistons. Will little affect comp.
What causes you to consider the comp low? As long as at least one of top rings has a reasonable gap, and .o10 certainly is, comp is little affected, and running comp even less so.
With Sevens can try others on the handle to establish what is common, although many now have raised cr.
RE: Sports piston rings orientation and order - Stuart Giles - 06-02-2022
Ring gaps seem about right for a broken in motor; a piston manufacturer's recommendation I have gives a recommended ring gap of .003" per inch diameter of bore. How did you measure the low compression? did you use a compression tester or just 'feel' it on the starting handle?
Did you check the valve seats and tappet gaps before stripping the motor? Unlike pushrod motors, valve clearances tend to close up with use in sidevalve units leading to poor compression and burnt valve seats if left.
RE: Sports piston rings orientation and order - dchw1975 - 06-02-2022
I tested compression with a compression tester and all 4 cylinders came out at little more than 50psi, which seems v low for HC head etc. though I have yet to test the volume of the cylinder head. What should the combustion chamber volume be for an HC head?
Valve clearances are all good though as the head is off I will clean the block surface and valves and then regrind to ensure valve seats are good. The only thing there is that they are very very large inlet valves so the seats are quite thin...
This all started as its seemingly impossible to get it running so that the plugs are anything other than carbon black and I already have it well less than 2 turns of mixture on my HS2 carb. It goes well but also won't rev which is v irritating as you can imagine.
RE: Sports piston rings orientation and order - Graham Barker - 06-02-2022
At the risk of insulting your intelligence, I ask whether you tested the compression with the throttle wide open?
Regards
Graham.
RE: Sports piston rings orientation and order - dchw1975 - 06-02-2022
Should also say that I had oily bores and piston tops which made me think rings were upside down...
Yes, compression test done with throttle open, though it does have a myriad of adapters that need to be attached to get 14mm plugs so I am not 100% confident in it...
RE: Sports piston rings orientation and order - Bob Culver - 06-02-2022
Compression gauges came into fashion when crank handles disappeared. 50 psi is very low. Unless the rings are broken or gaps enormous should get a good "bounce" on the handle. The resistance depends much on the cr. As before can compare others at any meet.
If it has oversize inlets may have some modified cam. Has the cam been installed grossly out of step?
On the stock engine the valves "rock" about 7 crank degrees after tdc and most simple regrinds do not hugely alter that.
RE: Sports piston rings orientation and order - Colin Reed - 06-02-2022
HI DAVID,
Just to clarify you say the engine was purchased a couple of years ago so is this the first time engine has been started or has it been run in ,
this will make a difference to the type of answers you will receive
Cheers
COLIN
NZ
RE: Sports piston rings orientation and order - Ian Williams - 06-02-2022
David, I second Colin's question and add another, you say it was hard to start and that you have an HS2 fitted, what needle and spring are you using? I would suggest a 5 or 6 is a good starting point with blue spring, you will need to bed the engine in before reliably checking plug colour, new bores will let oil by confusing your readings. Certainly from your description that it will not rev I would suggest mixture and possibly timing are off.
RE: Sports piston rings orientation and order - dchw1975 - 06-02-2022
Hi,
The engine is very tuned with a newman race cam, largest possible inlet valves, reshaped followers, ported and gas flowed block and reprofiled head to allow the high lift cam without valves hitting the head. It has an HS2 carb with blue spring and #5 needle (I used a #6 last year until mid season but had huge issues with fouling plugs and misfiring though I did win both VSCC autosolos and the car will do > 65mph on the road). It is running an original Bosch 009 distributor (with the limit on centrifugal advance setup) and NGK plugs. Timing wise the its currently statically set at 7degrees before TDC but then adjusted slightly when running to find fastest idle and then retarded some.
The reason for my original question was that I put the engine together originally and following discussions about putting together another engine for my trials car (the piston rings for which have clearly marked top and the order in which they should be installed) I realised that I didn't realise that piston rings had a top/bottom so thought this could be the reason behind my low compression. Since I have also had issues with oil in the bores. As the flywheel has made a habit of coming loose and now running with too much run out then it appeared to be a good shout to take a look to confirm one way or the other at this point.
Since my A7 is the only car I have ever hand cranked then I don't really have anything to compare to when thinking about whether it has a good bounce. The only thing that I have thought about is that because I have an SU carb when I spin the engine over (all plugs removed) with the compression tester the dashpot is of course still very much down so should try with it removed.
Cheers
David