Piston problem

JohnG
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Joined: Thu Jul 02, 2020 5:44 pm

Piston problem

Post by JohnG » Tue Sep 29, 2020 8:54 pm

I recently removed the cylinder head from my 1946 Rover 12 to carry out a de-coke. I have discovered that the number 1 piston appears to be loose. There is slight circular movement (probably less than 1mm) about the axis of the piston. There is also vertical movement (maybe a couple of mm) if I turn the crank and then push down lightly.

The other pistons appear to be stable and everything appears to be in good shape otherwise. Pistons 1, 2 and 3 are of the same type, stamped with the number 7768RS, while 4 is of a different type - it looks older and has a much narrower clearance to the cylinder wall (see attached images).

I am assuming the movement in the first piston is bad news and was wondering whether anyone has any ideas about what the problem is and how tackle it. I understand that in theory it’s possible to extract pistons without removing the engine but is it feasible in practice with these cars? I’m keen to do as much work as possible myself, but having only carried out simple bolt-off, bolt-on jobs on cars in the past, I’m wary of getting out of my depth.

Any advice much appreciated.

Regards,

John
Attachments
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IMG_2731.JPG (59.8 KiB) Viewed 1254 times
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4.JPG (54.43 KiB) Viewed 1254 times
1.JPG
1.JPG (51.05 KiB) Viewed 1254 times

GOY189
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Re: Piston problem

Post by GOY189 » Fri Oct 02, 2020 9:31 am

Hello John, what prompted the decoke? Was the engine noisy (rattling or knocking?). If there is vertical movement, the worst case scenario is that the big end bearing has failed, given the radial movement, the small end bearing or the gudgeon pin may also be worn. It is possible to remove pistons/ rods with the engine in the car.

The con rod will not pass through the cylinder, so once the bearing cap is removed (2 castellated nuts secured by split pins) you will get a view of the state of that big end bearing and the crankshaft journal.

The piston with rod attached should be pushed upwards until the shoulders of the con rod are in the cut outs at the bottom of the cylinder when the gudgeon pin can be removed and the piston lifted out and the con rod removed downwards.

The 10 and 12 hp cars big end bearings are white metalled in the conrod and bearing cap, not bearing shells. so if the big end bearing needs replacement,it is a skilled job.

If there is an engine reconditioning business nearby, it would probably be a good idea to consult them.

Hope this helps

Mike

JohnG
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Re: Piston problem

Post by JohnG » Fri Oct 02, 2020 7:22 pm

Hello Mike,

Thank you your very much for your guidance about piston removal. it's a big help towards understanding what to do.

I decided to decoke because compression was poor and based on what I know of the car's history I suspected it probably hadn't been done for some time, which looked to be the case once I'd removed the head. I also thought it would be a good chance to check whether there were any signs of trouble. When the engine was running previously there was a kind of metallic clanking noise at low revs but I couldn't figure out exactly where it was coming from.

I will have a go at taking the piston out as soon as I can to see what is amiss.

Thanks again for your advice.

Regards,

John

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luli
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Re: Piston problem

Post by luli » Sat Oct 03, 2020 4:54 pm

From what I see one piston is .01 oversize while the other is 0.04. Looks like a major mistake in engine assembly. Such a mistake justify taking the engine out, taking it apart and measure and examine everything.
If you like to see a series of posts (30+) describing restoration of such engine see here: https://wp.me/pXLKy-21p
Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD
http://lulisml.wordpress.com/

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luli
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Re: Piston problem

Post by luli » Sat Oct 03, 2020 5:33 pm

7768RS is a Hepolite piston.
Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD
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Rubythursday
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Joined: Thu Oct 09, 2008 6:30 pm

Re: Piston problem

Post by Rubythursday » Sat Oct 03, 2020 9:46 pm

Both look +040 to me.
Tony
1948 Rover 75, 1987 Range Rover Auto, 1987 Volvo 240 Auto 1999 Rover 75 Connoisseur Auto
http://www.tonysimons.me.uk

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luli
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Re: Piston problem

Post by luli » Sun Oct 04, 2020 7:41 am

Sorry. My mistake.
Rover 10 1946 RHD
Rover 10 1947 LHD
Rover 12 1947 tourer LHD
http://lulisml.wordpress.com/

RobHomewood
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Re: Piston problem

Post by RobHomewood » Sun Oct 04, 2020 11:50 am

But one cylinder looks very much tighter in the bore than the other ie the bore must be different? Had one been sleeved or bored out and not the other?

TonyG
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Re: Piston problem

Post by TonyG » Sun Oct 04, 2020 2:04 pm

John,

There is no denying that you have uncovered a serious issue. They key thing is that you have found it before anything too disastrous has occurred. Luli is right that you will most likely have to remove the engine for a strip down, inspection and remedial work. However, before doing that, I’d suggest dropping the sump and taking a close look at the condition of the bottom end. As Mike alluded to, it’s quite straight forward and all big nuts and bolts to play with. First thing is to get the car up off the ground a bit so you have easier access. Invest in several cheap lead lights so you can see what you are doing and take pictures as you go along, which will help during reassembly.
It seems that No1 is a problem so have a good look at that from the top and bottom before removing the big end cap to see just where the movement is occurring. You may be lucky and find that the problem is the little end bearing and some broken rings. However, you will want to take off the end cap and examine for wear and, at this point, you may wish to remove a main bearing to help decide if the crank needs grinding and the bearings re- metalling. If they do then it’s definitely an engine removal job and you may as well wait until it’s out before looking at the pistons or passing onto a specialist for a rebuild. Probably best that it’s all in one piece if you are going down that route, but it won’t be cheap.
Should the bearings appear ok and if the oil pressure had been fine when it was running the bottom end might be ok and some wear is to be expected. If it doesn’t look too bad (you can always compare No1 end bearing with another that has no movement. Don’t muddle any bits up though.) proceed to remove No 1 piston to see what is wrong with that one and if a repair in-situ is feasible.
It’s easy to dive in and end up with a heap of bits so I always feel it’s wiser to go forward in small steps.
Taking the engine out is best achieved in unit with the gearbox, using a hydraulic lift which allows you to wheel it away. I bought a new one from eBay for just over £100. Happy to lend it to you if you are near me. You will need to remove the bonnet, rad and shell to gain access and getting it all back and aligned is a challenge, as I’ve recently discovered! However, it’s a great opportunity to refurbish the engine bay and paint the engine before putting it back. Wise to fit a new clutch and release bearing too.
This must all sound quite onerous to you but you appear to have made a good job of cleaning up the pistons etc and removing the head so no reason why you can’t take the engine out and, once done, you will certainly know your car.

Good luck and fingers crossed it’s just No1 piston and that it can be resolved more easily.

Tony.
Tony Gilbert

P1 12 Tourer
P2 12 6 Light Saloon
Discovery 3
Discovery Sport

JohnG
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Joined: Thu Jul 02, 2020 5:44 pm

Re: Piston problem

Post by JohnG » Mon Oct 05, 2020 8:37 pm

Hello Tony,

Thank you very much for taking the trouble to post such a thorough explanation and advice about what to do next. It certainly makes me more confident about tackling the job and will definitely bear all the points you made in mind as I proceed. Whereabouts are you, just in case I needed to take up your very generous offer of borrowing your hydraulic lift?

And thanks also to everyone else who responded. On the question of cylinder bore raised by RobHomewood, the cylinders all appear to have a diameter of two and three quarter inches, while the piston crowns on 1-3 are slightly narrower than on 4 but the rings protrude more (if that makes sense).

Regards,

John

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