First Time Engine Rebuild

[bmcecosse]

Pretty sure the 1098 Midget engine DID use the standard pistons as on a Minor 1098. How they reconciled that with the larger chamber capacity 295 head is a bit of a mystery.... On the 998 Cooper they used 'raised D' pistons - but I'm not aware of any raised D pistons for a 1098.

[edd_barker]

The Pistons that came out of my engine had a dish with a diameter across almost the entire piston. From researching the forum, this dish is 7cc? The Pistons that I was supplied with 'Midget 1098' on the box has a much smaller dish, please see the images I posted earlier in this thread.

I think it is unlikely that the engine I rebuilt was a Low Comp engine, I will check the engine number though.

Does anyone have a photo of a standard Morris minor High comp 1098 piston?

Many thanks,

Edd

[panky]

Only the top of one.[frame][/frame]

[edd_barker]

Thanks for that!

Sadly hasn't cleared things up by much, so hard to see from a photo. This is what was in the engine before:

[frame][/frame]

As I said hard to tell! I really don't think it is a low comp engine as it had no rocker breather and a vented oil filler. I will check number tomorrow. When I did a very rough CC of dish with a tablespoon measure and a CD case it took more like 10ml. Again I will do it more precisely tomorrow.

BMC the midget piston has a tiny dish, 3cc from what I read.

Hopefully I will be able to clear it all up tomorrow.

Thanks,

Edd

PS a photo(s) of a low comp piston would be really helpful!!

[bmcecosse]

That would make sense then - shallow dish for Spridget pistons. There must have been some way to raise the CR against the large chamber 295 head.

[edd_barker]

Yes BMC I would concur, the problem arose as the County catalogue (who make all Pistons that you might buy from any of the usual Minor suppliers, unless specifically branded otherwise) suggests that the 1098 piston for the midget is also compatible with the minor, which is not the case. The Pistons I now have, which match the one I removed, are labelled as 8.0-1 compression, which is lower than it should be. Dish is 7cc as I measured. I think it is a catalogue error rather than anything else, but is worth bearing in mind for anyone ordering Pistons in the future.

When I first spoke to the engineers they said I could not just swap pistons, as they bore to the piston width, and so different ones might not be correct. What I didn't understand was that they do not bore individual bores to individual Pistons, as would make sense, but just to the size in general. With the cheaper county Pistons there is up to 0.00005 (half a thou?!) difference.
Their suggestion was to remove enough material from the piston top to generate the extra 4cc I need for my ratio, as the piston has to be skimmed anyway to sit .010 below the block. As you can imagine I didn't want to do this!

They then offered to order in a load of the County Minor Pistons and measure them all to find ones that matched the bore. Then, on measuring the 4 minor Pistons in the box, found they were all within tolerance anyway!

I have now dry built the bottom end and measured the protrusion from the block, (2 at 010, 1 at 011 and 1 at 015) so shall I have them all skimmed 025, or have the 015 one skimmed 025, then all the others to match that piston height from gudgeon to piston top, so all the Pistons are the same height from the gudgeon? Or skim each one to bring them all the same height below deck?

Either I have 4 Pistons all skimmed the same, or 4 with the same height, or 4 all at the same level!

Many thanks,

Edd

[philthehill]

BMC when boring a new cylinder block just bored it to the nominal 1098cc bore size - it was not always possible to get the bore size spot on and before the initial assembly was carried out the bore was measured and graded 1, 2, or 3. Pistons were also measured and graded again 1, 2, or 3.
Pistons were matched to bores i.e. grade 1 bore fitted with a grade 1 piston.
It is possible to find engines fitted with grade 1, 2 and 3 pistons.
Doing it the BMC way is alright so long as you have an unlimited supply of grade 1, 2 and 3 pistons.
Good engineering / engine reconditioning firms will require the new pistons to measure and will bore against the mean piston diameter. Extra good engineering / engineering reconditioning firms will bore to the individual piston but that is rare as the boring bar has to be re-set for every bore undertaken.
As regards the pistons you have - you need to look for and have constants.
Do the County pistons have a constant measurement from the top of the gudgen pin hole to the top of the piston?
I am afraid that you are stuck between a rock and a hard place if they do not.
I personally would skim the top of the highest protruding piston so that is 0.010" below the top of the block and then match the other pistons by the same distance from the top of the gudgen pin hole to the top of the piston.
What ever you do it is going to be a compromise between piston height and piston dish.
Whilst I doubt that your crankshaft been poorly re-ground (if it has indeed been re-ground) a poor re-grinding can put the crank throw out of sync and give a variation of piston height.
Phil

[edd_barker]

Hi Phil,

Thanks for that, I understand RE grading and boring. They have taken the mean piston diameter and bored to that, luckily the new Pistons fit within that mean measurement.

I have not had the crank ground, but it has been done at some point as is +020, and there were plenty of other bodges on the engine!

I am struggling to measure piston deck from gudgeon with my callipers, round hole to small flat surface means inconsistencies of the amount of difference I am looking for. I will ask the engineers to do as you've suggested, so in theory all the Pistons will be the same height, but there will be minute variations in the combustion chamber.

Thanks,

Edd

[philthehill]

Edd
One way to measure the distance from the top of the gudgen pin hole to the top of the piston is to place the piston upside down on a sheet of glass - slide the gudgen pin sideways so that a portion of the gudgen pin is exposed and with a spring calliper measure the distance between the underside of the gudgen pin and sheet of glass. Measure the distance between the outer faces of the spring calliper with your calliper.
Do all four pistons/gudgen pins and see what the differences are if any.
Of course better done with a surface plate and a lever dial gauge of the type in the link.
http://www.ebay.co.uk/itm/Dial-test-ind ... SwstxVFkie

Phil

[bmcecosse]

The difference in protrusion may not be in the pistons -indeed - I find that very unlikely. These will be churned out on a CNC machine to exactly the same dimensions. More likely is variation in crank stroke and/or con-rod length. As I explained before - ideally you need to do a mix and match with conrods and pistons (can't do much about the crank stroke now) to get the most even protrusion - and then machine them to suit - which will give you different dishes...... The other possibility is that the block has been skimmed at a slight angle..... Interesting to see if the protrusion is the same on both sides of each piston! Would be so much easier to just have flat top pistons.

[philthehill]

Edd
I have come across this small bore block with pockets.
http://www.ebay.co.uk/itm/Mini-998cc-En ... Sw-4BXa7Eh
The pockets as regards depth do not look any different to the ones you have in your block.
You can see where the top ring rubbing surface stops and there is still quite a bit of cylinder wall between the top ring and the bottom of the pocket.
Phil

[edd_barker]

Phil,

998 has a shorter throw yes? So less piston travel? I actually think those pockets look a fair bit deeper, hard to say from a picture. Interesting comparison! Only time will tell I suppose. i asked the engineers why they were so deep they said they had never had a problem and do it all the time.

The proof will be in the running!

Although my measuring with a steel rule and feeler gauge was not ideal, I did feel like one side of the piston protruded more. The engineers said that those Pistons vary quite a lot, although probably not as much as I have got. If engineers can measure accurately then at least I will have 4 even pistons.

Thanks all,

Edd

[bmcecosse]

Pockets are made to suit the camshaft/rocker lift expected - for a 1098 cam, 40 thou (+ whatever has been skimmed from the head) is all that is necessary. Indeed - with nicely 'recessed' valves no pockets at all may be fine. I had huge pockets in my 998 - but I was running a 649 cam and the S head had been skimmed - but it was a Comp engine - and I could see the top ring... I believe all the pistons will be EXACTLY the same size in bore and compression height when they leave the CNC machine. Try swapping the 'most protruding' piston in place of the 'least protruding' piston and see how that affects the readings.

[edd_barker]

Hi all,

Engineers recommended to have all the Pistons skimmed to .010 below deck height. They suggested that the differences this would create in piston compression height (measurement from gudgeon to top) would not matter, as the Pistons are so light the weight/mass difference would be negligible, but then at least all the combustion chambers would be the same. Although having said that, they would all have a slightly different dish!! They said the difference could easily come from con rod, small end big end etc.

Should be done by Monday so we'll soon find out!!

Cheers,

Edd

[amgrave]

They have agreed with what BMC said then and he suggested trying different combinations of conrod/pistons to get the best match between them which is similar to what the factory did originally.

[bmcecosse]

My view -these 'engineers' have led you down the garden path - and are now taking the easy way out. I hate to think what you are paying them to mess about like this. Was there not some talk of bore size variation too ???

[edd_barker]

Hi BMC,

You'll have to elaborate a bit, do you mean bore diameter? This was discussed earlier in the thread regarding boring to the mean piston size. Or do you mean it could affect why the Pistons protruded differing amounts? Should I not have had them all machined to 010 below deck height as I can probably call them tomorrow and change that?

I do not think they have set out to deceive me, but I have learnt with engineers that there are two options. Either you say "hello, please recondition my engine", give them the whole thing and they do the job as they see fit, and you pay accordingly. Or, you ask for specific things, reboring, grinding etc and you tell them exactly what you what and how much by. I have been learning as I have gone along, and therefore have not been specific, and so they have done what they think, some of which has turned out to not be ideal, but that is the beauty of hindsight! As Phil advised me, as soon as you deviate from standard things get complicated!

I will be looking on the bright side. Another classic A series given another lease of life. If it blows up then at least I will have a good story to tell.

Having said that, all advice to prevent it blowing up much appreciated!

Thanks all,

Edd

[bmcecosse]

I can't be bothered re-reading all the thread- but i thought there was a variation in the bore sizes mentioned at one point. This has nothing to do with piston protrusion - but everything to do with how well the engine will run in the long term. In 'competition' use the bores would be finally honed to an exact size - often with quite big clearances - but they will ALL be the exact SAME. Anyway - if this is just a mild road engine, I guess as you finally assemble you will carefully check that the rotating assembly spins over easily by hand as you fix each conrod/piston in place. Use lashings of 20W50 as you do so - and don't forget to fill the (new?) oil pump with oil... I'm sure it will all work out fine.

[edd_barker]

Ok I misunderstood your meaning when mentioning bore variation there.

No there is no bore variation, block bored to mean piston size (0005" variation in piston diameter). Phil explains this approx 10 messages up.

Yes new oil pump already filled with a mix of correct oil and engine build cream.

I will of course check for binding. I am not expecting it to turn over easily by hand with all 4 Pistons in though? It spins with two fingers just the crank in there, but when I have had head off other engines it turns on flywheel but not on crank pulley?

[bmcecosse]

It should turn easily by hand - but no spinning. With all this work - I hope you are finishing it off with a nice crankshaft damper on the nose.