Fitting New Pistons and Rings

Post by **AJT** » Sun Sep 07, 2008 1:53 pm

Hi All,

I'm thinking of fitting new pistons and rings to my engine: it's not been re-bored. I seem to recall that there is a risk that the new top ring may be damaged by hitting the small ridge left on the bore by the last ring - does anyone have any experience of this being an issue? or are the top rings 'stepped' to avoid this happening?

Whilst the engine uses very little oil, the compression isn't great and so I'm thinking of doing this whilst the head is off.

Thanks
Alan

Post by **Alec** » Sun Sep 07, 2008 3:19 pm

Hello Alan,

stepped top rings are (or used to be?) available but I'm not sure if new pistons are available with them.
However, I would see what condition the engine is before committing to by them. Also your poor compression may just be valves and you would not really benefit by replacing the rings.

Alec

Post by **bmcecosse** » Sun Sep 07, 2008 3:41 pm

If you decide you really DO need new rings - then you MUST remove the ridges at the top. Had a mate do this recently with his MGB - he left the ridges and the result 1000 miles later is many broken rings and damaged pistons. In any case - you must inspect the pistons - and if the ring grooves have 'belled out' or are showing any other signs of damage - then you need new pistons. With either rings or pistons (and rings obviously) you should hone out the bores (after removing the top ridges by hand) - this can be done with an electric drill and a hone but can also be done with a sheet of emery and a slim hand! Just rub up and down the bores in a corkscrew motion, clockwise and anti-clock - the aim being to 'bust' the glaze on the bores and leave a light cross hatch pattern on the bores.

Post by **8009STEVE** » Sun Sep 07, 2008 5:30 pm

stepped top rings are (or used to be?)

They were called CORD rings.
Not sure if they are still available.

Post by **bmcecosse** » Sun Sep 07, 2008 6:27 pm

I think CORD were more famous for their multi-ring oil control set-up - which they supplied to revive oil burning engines - and yes, they included a 'stepped' top ring to avoid the wear ridge.

Post by **kennatt** » Mon Sep 08, 2008 8:29 am

if you are going to the cost of new pistons and rings I would have the block rebored The engine needs to be stripped down to do the pistons so why not rebore then you know that all will be well .for the additional cost of a rebore you have peace of mind.Different if you were just doing rings into the old pistons but you say you are fitting new pistons and rings. Your choice but I know which route I would take.But before that I would have a compression test done that will show where the problem lies rings or valves. If you have a compression tester test each pot dry then put some oil in and retest if the compression goes up dramatically then its rings that are worn if no great improvement then its burnt valves. Do this first before stripping out the pistons.Could save you a lot of work and money. Good luck with it

Post by **AJT** » Mon Sep 08, 2008 9:18 am

Hi All,

Thanks for your very useful replies. I think the job is worth doing since the engine runs at 60psi when hot and is otherwise in good nick so I'd prefer not to take it out of the car for a full re-build - I've only done 5000 over the last 5 years as it's 'sunny day' convert. so a 'moderate' re-build will keep it going for some years (the head is off for a de-coke as tends to run-on). Having completed a compression test incl oil in the bores it confirms that the rings are not what they used to be. It could be that I can re-use the exiting pistons - to be confirmed.

In terms of removing the top ridge, (bmcecosse), is there a best approach? emery paper?

Thanks
Alan

Post by **PSL184** » Mon Sep 08, 2008 9:27 am

I wish I had 60psi when cold let alone hot!!! Are you sure that you aren't just trying to find work for yourself.... Sounds like bottom end is OK to me. I'd just lap in the valves, replace head gasket with copper one and stick it all back together.....

Post by **Kevin** » Mon Sep 08, 2008 1:13 pm

In terms of removing the top ridge, (bmcecosse), is there a best approach? emery paper?

Hi Alan I don't wish to sound off putting but if you dont have an engineering background this not an ideal job to do for the first time.

Post by **AJT** » Mon Sep 08, 2008 1:23 pm

Don't worry Kevin - I've owned old cars for 20+ years incl a Spitfire which I changed the pistons in - it's just a long time since and at the time I could get 'stepped' rings which solved the issue.

I guess that anyone changing the pistons / rings will have this issue and I'm just keen to ensure that I remove the ridge most effectively.

Having been through several pretty terrible 'MM specialists'. I don't trust them anymore. The last one left my lovely new chassis legs very heavily dented from careless jack usage and I had to return it twice to get the orginal job, (type 9 gearbox installation), completed properly. I'd prefer to trust my own work in future even if it does take me more time, particularly having spent five years improving the car.

Post by **bmcecosse** » Tue Sep 09, 2008 10:41 am

Yes - emery paper and elbow grease will take the ridge away - or at least taper it enough to do no harm. Just make sure you wrap the big ends up to keep grit away - and stuff oily rags down the bores. Then carefully clean up afterwards. I agree with you - for low miles use car new rings will be fine. Rebore means an awful lot more work and should be avoided unless absolutely necessary.

Post by **newagetraveller** » Wed Sep 10, 2008 1:39 pm

If the engine is not burning oil but the compression is poor I would just regrind the valve seats, put the head back on and see what the compression is like then. It is unlikely that pistons and rings are so bad that they are affecting your compression but that the engine is not burning oil at the same time.

Post by **bmcecosse** » Wed Sep 10, 2008 1:41 pm

Well - it can happen and new rings and a glaze bust can keep it better for a good few thousand miles.

Post by **XDB** » Wed Sep 10, 2008 9:12 pm

A proper cylinder hone isn't a lot of money and does a good job, I have bought piston rings and complete pistons and rings recently neither had stepped rings. If I had a choice of doingthe job having taken the engine out or lying on my back doing it in situ, I would take the engine out.

Post by **youngun** » Wed Sep 10, 2008 9:52 pm

Blimey CORDS! Ive heard all sorts about those things. Apparently it was a right little weeze to rejuvinate a knackered engine, for a time.

The ring set i got had a choice of rings in it. I had a choice of either fitting the conventional single piece oil rings or fitting rings made up of an expander, a concertina shaped centre piece and then two rings one to go on top and one to go on the bottom. On the recommendation of this site i fitted the latter. I have yet to find out if they are any good though because i aint run the engine yet!

Post by **bmcecosse** » Wed Sep 10, 2008 10:19 pm

Yes - the multi piece oil rings will be fine, provided you fitted the thin rings the right way up!
Don't understand 'on your back' doing the honing - it would surely be done from above ? Honing is one thing - to glaze bust the hard skin on the bores and to put a cross-hatch pattern there to hold a little oil, but any ridge at the top of the bores must be removed or at least tapered away on the underside or the top rings will break!

Post by **youngun** » Thu Sep 11, 2008 9:21 am

Yes the thin rings were fitted the correct way up. They were marked for orientation which was handy as i couldnt tell which face had the grooved edge!

Post by **AJT** » Thu Sep 11, 2008 2:26 pm

Thanks all for your advice. Well last evening all became clear...resulting in a trip home attached to an AA van!.

As mentioned before, I was planning to take the head off anycase this weekend because of occassional running on and the car seeming to idle slightly unevenly. The compression test was simply to test if the rings were worn - and adding oil did raise the numbers a bit, though none hopeless.....but last night the head gasket blew...total loss of compression on two cylinders so I guess it has been partially blowing for a while and finally packed up as I tried to climb a hill (power has been a bit down for a few weeks).

In all probability I doubt the rings will need replacing but we shall see! Also explains why oil consumption is fine.

Alan

Post by **PSL184** » Thu Sep 11, 2008 2:38 pm

See I told you to fit a copper head gasket

Post by **AJT** » Mon Sep 22, 2008 11:35 am

Well new gasket fitted and did the timing chain / gears whilst at it (the starter motor / wrench trick worked a treat).

The engine is now incredibly quiet, pulls well: I think the gasket had been partially blown for some time.

My long running issues with 'pinking' have also been resolved allowing me to advance the timing very considerably so I wonder if in fact the problems when hot were always gasket leakage related.