First Time Engine Rebuild

[edd_barker]

Yes spins nicely.

The local mechanic has built and raced classic engines for many years, he was very happy that it was .003
If anyone would like a recommendation please let me know, he was recommended to me by Nicola our Minor Matters editor, in the Surrey/West London area.

I have ringed one piston, it has an extra ring below the gudgeon that wasn't on the originals. The oil control ring was a devil to get to sit properly, it does not seem like it will compress without overlapping, which is a no-no according to the instructions. I will have to see when i try to fit them. Does this look OK? Obviously making sure the gaps are all pointing different ways. The extra bottom ring has a funny square of what looks like piston ring stuck in there already, that will fill the ring gap when compressed.

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Not sure what the red dot is for? Its in the centre of two, off centre of one and not on the fourth.

I've made a few stupid mistakes, number one was this one:

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I took the main caps off to polish the crank further, and wasn't careful enough with them, so this little scratch happened. The engineers didn't have any in stock, but suggested just rubbing with soap and a cloth to remove the high spot and fit. Finding the rebuild more of a challenge than I expected, it's difficult when you have no one there who has done it before, and manuals/forums are not always as clear as you would like! I am pretty petrified of putting it all together and then catastrophe, with substantial £££ down the pan. Only time will tell...

Thanks for all your help,

Edd

[philthehill]

Edd
If you have any doubts about the oil control ring take the ring off the piston and insert it in the bore and see if the ring sits right without overlapping. If it does not overlap in the bore it will be ok.

The engine build will all come together and you will be as proud as punch when it fires up for the first time.
Have faith in your ability and workmanship.
We all make mistakes at some time or another.
Phil

[edd_barker]

Thanks Phil!

Better with fresh eyes today. Other 3 Pistons fitted no problem.

Annoyance now is two torque wrenches (both 30-40 years old) giving different readings, so trying to work out the true reading now with weights!

[bmcecosse]

Don't worry about the tiny scratch

[edd_barker]

Should the Pistons protrude from the block???

At TDC piston 4 is maybe 0.5mm proud of the block surface. Block was faced but not skimmed. I haven't fitted the others yet. I can measure the actual protrusion if this is key. EDIT it protrudes 0.008 (0.20mm). So it has the thickness of the head gasket compressed minus 0.008, will this suffice?

EDIT 2: just checked the box and they are 1098 Midget pistons 040. They have a much shallower dish than the ones that were in there before. Are they different? I don't want any higher compression than the head will already give me!! They are also 4 ring when the originals were 3....



Edd

[philthehill]

Edd
The piston should not protrude above the top of the block.
For a standard Minor block the top of the piston should be well below the top of the block.
Can you please post a photo of the piston at TDC for comment.
The measurements you are quoting are rather tight.
At worst you may have to have the tops of the pistons skimmed to bring the clearances back to normal.
Skimming the tops of the pistons is quite normal if the top of the block has been faced / skimmed. Only the minimum needs to be taken off the top of the piston.
It will not effect your compression ration very little.
The ring below the gudgen pin is quite normal and is there to control oil by scraping the oil off the bore before it gets to the oil control rings above the gudgen pin.
Phil

[edd_barker]

Photos as requested:

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[edd_barker]

Can only do one at a time on phone

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[philthehill]

Edd
I would have a word with your local mechanic before doing anything.
My gut feeling having seen the photos is that you will need to have the pistons tops skimmed so that they are at least level (if not slightly below) with the top of the block.
Do not forget the valve pockets are part of the combustion chamber and they will help lower the compression ratio.
Whoever did your valve pockets in the block did a nice job of it.
Phil

[edd_barker]

Ok, thanks Phil.

I have to go back to engineers AGAIN tomorrow to pick up a piston ring after I broke one, so I will assemble the 3 I have fitted, measure protrusion and then get them skimmed down.

What measurement should I aim for below the block deck for piston tops? Obviously piston can only take so much machining...they are just standard County ones.

I am amazed so much has been taken off the block face, the skim has not cleared up all the corrosion around some of the waterways.

Thanks,

Edd

Struggling to find measurements for Midget Pistons in case those happen to have a thicker deck? Does anyone know? When measured from inside gudgeon hole (not inside pin) they are virtually identical.

[edd_barker]

Going to have the Pistons machines down until .010 below deck height. Apparently there is plenty of spare metal in these Pistons to do that. If anyone knows the respective cc in the dish of a Morris minor 1098 piston and a MG Midget 1098 piston that would be very helpful info when working out compression ratios.

Will update!

Thanks,

Edd

[philthehill]

Edd
Make sure that you mark the front of the piston (marker pen inside the piston) as that skim will take off the stamped FRONT.
10 thou will do nicely as you have the thickness of the head gasket as a buffer (not literally) to keep the piston head away from the cylinder head.
Phil

[bmcecosse]

These 'engineeers' seem to be ripping you right left and centre...... The pockets look enormous they should only be 40 thou deep at worst to protect the top ring (which is now going to be even nearer the surface after the block skimming...) . Is the gasket well clear of them? Any 1275 pistons I have seen are reversible - no 'front' marking - but I see these are so marked. Ideally each piston should be measured precisely to determine the protrusion - and machined appropriately to make them all the same depth (yes 10 thou will be fine) below the deck. Of course that will upset the balance weight if there any significance between them. The rings will need to come off to do that - in fact the pistons may have to come off the rods unless they have a way round that. First assembly is usually done with an 'easy fit' gudgeon pin to sort out all these difficulties before final assembly. These pistons (low dish soon to be near flat top) will give you high compressions - remains to be seen what that will be. What is the combustion chamber volume? And yes the pocket volume too (fill with plasticine then measure the displacement. Has the head too been skimmed ? Measure the thickness (you may have done this before - sorry can't remember) The +40 bore not helping here either. Not normal these days to go beyond 10/10.5 to 1 for road engines.

[edd_barker]

I will post pictures of head gasket clearance when I can get home on computer shortly this evening. There is clearance, although close in places it is the same as closeness to edge of the vote on the gap between 1&2 bore and 3&4 bore.

They are deep though, between .130" and .140" (3.3mm) I'm not sure why they would do this? Supposedly it was dry assembled, marked with engineers blue and then machined appropriately. Not sure what to do as they are unlikely to say sorry here's a new block on us. Wishing I'd never started!

I will work out CC of pockets now and post, along with gasket photos later this evening.

Thanks,

Edd

[edd_barker]

Figures as I have them:

Each pocket displaces 0.5ml water, so is 0.5cc.

Head thickness is 69.3mm (2.73 inches, so 20 thou skim).

Head face to exhaust valve distance is 0.265"

Head has been CC'd at 21.5cc

Block is overbored .040".

I will try and work out the compression ratio from this but if anyone is quicker please feel free. I can always give the midget Pistons back and ask for Morris minor ones.

Thanks,

Edd

[philthehill]

bmc
Why the reference to 1275cc pistons?
The block is a 1098cc one being fitted with plus 0.040" oversize pistons.
The pistons in the photo above are 1098cc with the gudgen pin held in place by circlips which can be clearly seen in the above photo.. Therefore the pistons can be easily removed for skimming.
If the same amount is taken off each piston head the piston balance will not be compromised.
The pistons must be fitted with FRONT facing front as the gudgen pin centre relative to the piston centre is slightly offset to compensate for thrust.
Whilst the pockets are deeper than you expected so long as the top ring is not exposed (and there is sign of that in the above photos) the rings/pistons will be ok. This is not an out and out racing engine just an improved 1098cc 'A' Series.
Phil

[edd_barker]

Phil for the sake of my fragile sanity I will assume you meant that there is NO sign of piston ring emerging from beyond the bore?!

I certainly could not see any, when it stops raining I'll measure distance from piston top to top ring and then deduct the pocket depth and protrusion, see how deep the top ring will be.

The pistons protrude slightly different amounts for each bore, so I was planning to have them all levelled to .010 below deck, however if this will affect balance I will just have them all reduced the amount required by the piston protruding the most, so the weight/mass is equal on all.

The bottom line is I will finish this engine! It may not be perfect but I will put it together and stick it in the car. If it gobbles up all its top rings in 500 miles then I will take some small pleasure in throwing it into the Thames.

Working out Comp Ratio now. Pictures of head gasket imminent...

Thanks all for your advice.

Edd

[edd_barker]

Head gasket sits like this. Engine on its side so tricky to hold it flat and photograph. I did fit all the studs to line it up

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And then the standard gap between bores for comparison:

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I'm struggling a bit with working out comp ratio as my yellow bible is back in the garage and I am at home, so if anyone feels like a quick mental work out that would be fantastic! I guess I don't really know what the CC of gap above piston will be as there currently is no gap, and then dish is going to be less. But then overall material will be removed.

Thanks again, much appreciated.

Edd

[edd_barker]

Info I have so far:

1098 overbored +040 - 1132cc / 4 = 283cc per bore.
Head CC - 21.5cc
1275 head gasket - 4cc
Pocket - 0.5cc
Volume above piston (once piston skimmed) - 1cc
Piston dish - I think 3cc as standard but obviously some will be lost on skimming. I can't find a source to confirm the 3cc.
Stroke 83.72mm

Can I work out CR from this? All the online calculators want bore diameter etc but I already know the volume. I may have to wait till I can read Vizard.

Thanks,

[philthehill]

Edd
To reiterate:-
There is no sight of the upper edge of the top piston ring being above the bottom of the pocket in your photos - which means that the top ring is below the bottom of the pocket. Your measurements will determine how far the top of that ring is below the bottom of the pocket
My recommendation is to reduce the height of the most protruding to 0.010" below the top of the block and then machine the other three pistons to the same height above the gudgen pin - that will ensure that the piston crowns are all at the same height. .
I doubt that the pistons were ever accurately balanced when produced.
Whilst we like to think that all pistons are the same dimensions they are not and that is why BMC graded pistons to bores - identified by a number 1, 2 or 3 in a Diamond on top of the piston.
Phil