Engine in bits for a rebore-anything else worth doing?

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ndevans
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Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Sun Feb 17, 2019 8:16 am

Morning.

As the engine is already in bits in readiness for a rebore (or will be once I've removed the last flywheel bolt-see "Flywheel bolts" post), is there anything else worth doing to it while it's apart?
I'm already thinking of fitting a rear crank seal kit-does anyone have any experience of these?
It's a 1275 out of a Marina that I put in 3 years ago.
Cheers N
cheers N
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oliver90owner
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Re: Engine in bits for a rebore-anything else worth doing?

Postby oliver90owner » Sun Feb 17, 2019 8:35 am

Depends. Got any precision measuring kit?

ndevans
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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Sun Feb 17, 2019 9:45 am

oliver90owner wrote:
Sun Feb 17, 2019 8:35 am
Depends. Got any precision measuring kit?
Only a couple of micrometers.
cheers N
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philthehill
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Re: Engine in bits for a rebore-anything else worth doing?

Postby philthehill » Sun Feb 17, 2019 9:50 am

From experience I would not fit a rear crankshaft seal kit.

The fly wheel has to spaced out from the rear of the crankshaft and the spacer metal is not always hard enough to cope. With the 1275cc Midget crankshaft the spacer lifts the flywheel away from the flywheel register so reducing the ability to centre the flywheel.
In my case the spacer as supplied was too soft and the flywheel came loose. Luckily I realised what had happened and stopped the engine immediately.

The spacer is required to allow the lip seal to rub on the flywheel mount flange and that flange rubbing surface has to be perfect so as not to tear the lip of the seal.

So long as the rear scroll and crankshaft are not worn and the breather system is effective there is no requirement for the crankshaft lip seal.

The 1275cc Maestro got it right in that the crankshaft has a longer precision ground flywheel mounting boss at its rear end so as to accommodate the lip seal without having to use a spacer.
https://www.ebay.co.uk/itm/1275-A-CRANK ... :rk:1:pf:0


ndevans
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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Sun Feb 17, 2019 10:22 am

philthehill wrote:
Sun Feb 17, 2019 9:50 am
From experience I would not fit a rear crankshaft seal kit.

The fly wheel has to spaced out from the rear of the crankshaft and the spacer metal is not always hard enough to cope. With the 1275cc Midget crankshaft the spacer lifts the flywheel away from the flywheel register so reducing the ability to centre the flywheel.
In my case the spacer as supplied was too soft and the flywheel came loose. Luckily I realised what had happened and stopped the engine immediately.

The spacer is required to allow the lip seal to rub on the flywheel mount flange and that flange rubbing surface has to be perfect so as not to tear the lip of the seal.

So long as the rear scroll and crankshaft are not worn and the breather system is effective there is no requirement for the crankshaft lip seal.

The 1275cc Maestro got it right in that the crankshaft has a longer precision ground flywheel mounting boss at its rear end so as to accommodate the lip seal without having to use a spacer.
https://www.ebay.co.uk/itm/1275-A-CRANK ... :rk:1:pf:0
Can this crankshaft be fitted to a Marina 1275?
cheers N
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philthehill
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Re: Engine in bits for a rebore-anything else worth doing?

Postby philthehill » Sun Feb 17, 2019 12:47 pm

I have to be honest and say I do not know.

The Maestro crankshaft is the same in all aspects except for the extended flywheel/seal boss.

The Maestro engine was (and somewhat still is) a good 1275cc conversion for the Minor but the conversion kits which were provided by Southam Mini Centre have dried up and no further kits are being considered.

The conversion utilised the Maesto seal housing and a modified flywheel.

The back to front SU car is correct for the Maestro.

See pictures below.
Maestro engine 1..JPG
Maestro engine 1..JPG (1.62 MiB) Viewed 1296 times
Maestro engine 4..jpg
Maestro engine 4..jpg (315.28 KiB) Viewed 1296 times
Attachments
Maestro engine 2..JPG
Maestro engine 2..JPG (1.65 MiB) Viewed 1296 times


ndevans
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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Sat Feb 23, 2019 9:04 pm

Ok, managed to shift the stubborn flywheel bolt eventually. Heat had no effect, half an hour of clouting with a lump hammer and cold chisel shifted it in the end!

The reason I am doing this is because the head gasket went between 2&3 cyls. On dismantling, I found a couple of small scores in no.3 bore, so decided to strip for a rebore.

So today, having got the flywheel, backplate and oil pump off, I got the pistons and con rods out, well, 2&3 so far. The top ring on both pistons was broken. No. 3 actually broken in two, and appears to have a piece missing, no. 2 has a small section broken off at the end. Photos below.
No3_IMG_20190223_205030.jpg
No3_IMG_20190223_205030.jpg (746.06 KiB) Viewed 1166 times
No2_3_IMG_20190223_205015.jpg
No2_3_IMG_20190223_205015.jpg (633.66 KiB) Viewed 1166 times
No2_3_IMG_20190223_205006.jpg
No2_3_IMG_20190223_205006.jpg (890.39 KiB) Viewed 1166 times
So I guess that explains the scoring in the cylinder wall, but why would the top ring break in both pistons? Bad batch?

The bores were honed 2 years ago, and I re-used the pistons (standard size, it's a 1275 from a Marina), with new rings.

Cheers N
Last edited by ndevans on Sat Feb 23, 2019 9:33 pm, edited 1 time in total.
cheers N
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oliver90owner
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Re: Engine in bits for a rebore-anything else worth doing?

Postby oliver90owner » Sat Feb 23, 2019 9:32 pm

Ring grooves worn, wear ring left in bores, pinking or detonating running, cracked on installation, rings fitted wrongly, use of ether starting aid, overheating - to provide a few possibilities. Likely some more.

Have you checked the head and deck with a straight edge?

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Re: Engine in bits for a rebore-anything else worth doing?

Postby philthehill » Sat Feb 23, 2019 10:02 pm

When fitting new rings to old pistons the piston ring groove to piston ring clearance must be checked to ensure that the ring does not go up and down in the groove - usually indicated by the rings pumping oil and heavy engine breathing.

The piston ring gap must also be checked to ensure that the ring has enough room to expand in length.

The piston rings out of the bores look as if they have a piece broken off. Compare the rings against each other to confirm either way.


ndevans
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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Sat Feb 23, 2019 10:05 pm

OK ta. Will the same apply to new pistons? Or will they come with matched rings?

The head and block were both squared off when I rebuilt it 2 years ago, so should be square.

cheers N
cheers N
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philthehill
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Re: Engine in bits for a rebore-anything else worth doing?

Postby philthehill » Sat Feb 23, 2019 10:39 pm

If you are purchasing the pistons/rings separately to the re-bore you do need to take the pistons/rings to the machine shop so they can re-bore to suit the pistons.

When BMC were building engines they graded the pistons to the bore now you have to grade the bore to the piston and they should be marked according to the bore they are to be used in.

Even though the rings will be matched to the pistons - the rings should ideally be individually checked for ring gap before fitting.

If the ring gaps are not correct you then have the problem of adjusting the ring gap. You can only make the gap bigger you cannot make it smaller and you do need a special tool to adjust the piston ring gaps.

You will need the machine shop to fit the pistons to the con rod as the gudgen pin is an interference fit in the con rod small end and unless you have the special equipment the home mechanic will be unable to fit the 1275cc pistons to the 1275cc conrods.


ndevans
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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Sat Feb 23, 2019 11:06 pm

Ok ta, useful info there.

Do I need to renew the con rods as well, or can I re-use them?

Cheers N
cheers N
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philthehill
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Re: Engine in bits for a rebore-anything else worth doing?

Postby philthehill » Sat Feb 23, 2019 11:10 pm

You can re-use the conrods.

To remove and refit the gudgen pin/pistons and do the job properly you would need a tool like that shown below (which is similar to the BMC special tool) to remove and fit the con rods.
gudgen pin remover.JPG
gudgen pin remover.JPG (1.46 MiB) Viewed 1121 times
Many just heat up the small end and slip the conrod through but you can get caught out in that if the con rod cools and you have not been quick enough the gudgen pin can be stuck half in and half out.

Heating the small end (and it has to be hot) can damage the conrod so I would never recommend using heat.


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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Tue Feb 26, 2019 10:59 pm

Ok thanks, noted. I'll get the machine shop to remove and refit the new pistons.

A couple more questions!

What are the rebore sizes for a Marina 1275? I'm pretty sure the pistons it has in are standard pistons, so what's the next sizes up?

If I go for a new camshaft, a 266 as suggested above, is it worth renewing the camshaft bearings as well? Is this a specialist job, or is it a DIY job?

Lastly, the main and big end bearings look good, and were new in 2016. The engine has done a bit less than 10000 miles since. Is it ok to re-use the bearing shells?

Thanks,
Neil
cheers N
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oliver90owner
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Re: Engine in bits for a rebore-anything else worth doing?

Postby oliver90owner » Wed Feb 27, 2019 8:39 am

You are asking questions from the wrong end.

Rebore might depend on how much metal needs to be removed to clean up the scored bores. New pistons might, these days, be in imperial or metric sizes ((larger than nominal) but the cylinders should be matched at the boring stage. Tolerances have generally been reduced since the first MInors were made (think here why Japanese engines tended to last better than similar products from the UK motor manufacturers of the time).

Camshaft bearings, as per the rest of the engine, may or may not require replacement. Measuring is the usual means of checking if they are not obviously scored or othnerwise clearly worn out. Replacement is a engineering process, but whether it is a DIY job may depend on the DIYer’s ability or experience.

Same with crankshaft bearings. Have you heard of plastiguage? Damaged crank surfaces are rather more expensive to correct than replacing the shell bearings. Checking for ovality, along with general wear, is important before considering replacing shells - no point in putting new bearings into an engine with oval crank journals.

Remember that a new top end overhaul may well stress a poor bottom end beyond its limits. I think you really need to entrust all the work to a reliable reconditioning workshop. That way, you may get a warranted repair.

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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Wed Feb 27, 2019 10:05 pm

Thanks once again for the advice, much appreciated!

I'm pretty confident in the main and big end bearings-the crankshaft was reground and new big end and main bearing shells were put in 3 years ago, and having got it apart now, all looks good.
The camshaft bearings look good too, no excessive scoring, just the usual wear patterns. What I don't know is how worn they are, or what the allowable wear is on a 1275.

Kent Cams have advised that an MD246 would be a suitable camshaft for a 1275 with large valve head and HIF44- does this sound reasonable? Has anyone any experience of this? Is it a worthwhile improvement on a standard cam?

Cheers N
cheers N
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oliver90owner
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Re: Engine in bits for a rebore-anything else worth doing?

Postby oliver90owner » Thu Feb 28, 2019 8:11 am

the crankshaft was reground and new big end and main bearing shells were put in 3 years ago,

It certainly helps if you mention all the facts.:)

Check them with plastigauge, is my advice, before reusing them (as long as they have been retained in exactly their original positions). Bearing shells are cheaper than crank repairs and new shells!

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Re: Engine in bits for a rebore-anything else worth doing?

Postby BobHood » Sun Mar 03, 2019 7:40 pm

ndevans,

The reason your top rings have broken probably has something to do with putting new rings in old bores. I've attached a picture to this post that shows you how the wear ridge in the bore is slightly rounded at the top. Meanwhile the top of the top ring is square. The square top edge is slightly higher than the top of the rounded wear ridge, as that was slowly scraped away as the top of the ring itself slowly rounded.

This mismatch in position where the two meet puts strain on the new ring every time the piston reaches TDC. It will eventually cause the ring to fail before it has a chance to wear to a rounded shape to fit the existing wear ridge in the barrel. That's why you should always rebore and hone a cylinder before fitting new pistons and rings if there is an existing wear ridge, i.e. one that you can feel with your fingertips.

This picture shows in exaggerated form what the new ring looks like at TDC when put against a worn bore.
ring and bore wear.jpg
ring and bore wear.jpg (13.21 KiB) Viewed 864 times
If there is no wear ridge then you may be able to get away with fitting new rings in worn bores, but years ago when I was in the business full time, we used to be able to supply stepped rings especially for fitting in worn bores for some engines. I don't know if anyone supplies them now, but I do remember we used to use Hepolite oversize pistons, and if we couldn't get a match from them we'd go to Ferrari's Piston Service in Cricklewood, London. FPS are still around, so you could do worse than asking them if they can still get hold of stepped rings. I doubt if they can somehow, as the evolution of modern oils has more or less done away with the need for them.
Bob's Boring! (or so I've been told) :D but I do honing as well!
See here: https://www.carandclassic.co.uk/car/C1053818

ndevans
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Re: Engine in bits for a rebore-anything else worth doing?

Postby ndevans » Thu Mar 07, 2019 11:45 pm

BobHood wrote:
Sun Mar 03, 2019 7:40 pm
ndevans,

The reason your top rings have broken probably has something to do with putting new rings in old bores. I've attached a picture to this post that shows you how the wear ridge in the bore is slightly rounded at the top. Meanwhile the top of the top ring is square. The square top edge is slightly higher than the top of the rounded wear ridge, as that was slowly scraped away as the top of the ring itself slowly rounded.

This mismatch in position where the two meet puts strain on the new ring every time the piston reaches TDC. It will eventually cause the ring to fail before it has a chance to wear to a rounded shape to fit the existing wear ridge in the barrel. That's why you should always rebore and hone a cylinder before fitting new pistons and rings if there is an existing wear ridge, i.e. one that you can feel with your fingertips.

This picture shows in exaggerated form what the new ring looks like at TDC when put against a worn bore.

ring and bore wear.jpg

If there is no wear ridge then you may be able to get away with fitting new rings in worn bores, but years ago when I was in the business full time, we used to be able to supply stepped rings especially for fitting in worn bores for some engines. I don't know if anyone supplies them now, but I do remember we used to use Hepolite oversize pistons, and if we couldn't get a match from them we'd go to Ferrari's Piston Service in Cricklewood, London. FPS are still around, so you could do worse than asking them if they can still get hold of stepped rings. I doubt if they can somehow, as the evolution of modern oils has more or less done away with the need for them.
I will get round to replying, I promise! Up to my neck in umpty-nine top priorities at the moment!
cheers N
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King Kenny
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Re: Engine in bits for a rebore-anything else worth doing?

Postby King Kenny » Fri Mar 08, 2019 4:02 pm

The last time I did an engine rebuild for my Traveller I fitted CORD piston rings rather than have a rebore. That was over twenty years ago and she still runs well.


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