33 replies to this topic

[1977 Loud_Mini]

Hi everyone, i recently installed a rebuilt 1380cc engine in my mini, after having numerous problems which most are now fixed i have another obstacle in my way. The engine was burning oil really badly so much so that it used the oil from the max mark on the dipstick to the minimum in around 15-20 miles. In the end the engine wouldn't stop smoking really thick blue smoke so i took the engine back out yesterday. Upon inspection the pistons rattle backwards and fowards (front in this case being the oil filter side etc) in the bore a heck of a lot. They are the 20659 Powermax 73.5mm pistons with 9cc dish from minispares or similar and they just seem way too small. I had a quick measure with a tape measure of the bore and it seems to be 74mm, i know this isn't the most accurate of ways to measure the diameter of the bore but it's all i've got.
I've seen some mini pistons 74mm on ebay by a place called Jonspeed. I'm not sure if these are what i need? Also i asume the block has been bored out beyond the 1380cc which some blocks will not allow as it goes through the cylinder wall so it may now be 1400cc?

Any help would be of great assistance.
Thanks, Evan.

[xBR4DLEY]

Weird , because the block cant be bored out to more than 73.5mm.
Blue smoke could be anything , but in your case most probably piston rings as they wont be sealing properly if the bores are bigger as you suggest.
Could it be your conrods that are causing the slapping or something to do with the crank? Not too sure on that bit mate.

[bmcecosse]

I would be asking the person who 'rebuilt' the engine what is going on! If the bores are 74mm - you obviously need 74mm pistons. The actual capacity is not important - but the pistons must fit the bores!! You may want to consider sleeving the block back to a more standard size.............

[mini13]

you need to get a proper measurment of the bore, with an internal micrometer or bore gauge and external micrometer.

last set of 73.5 powermax's I measured came out to 73.12mm (at the "A" measurement just below the pin), and were very loose in the 73.5mm bore they were put in,

Trouble is some people assume that the bore needs to be bored to 73.5 and have the block bored to 73.5, you need to measure ethe pistons and have the block bored to fit the pistons.

I have just had a block bored for 73.5mm accralites, the actually measured 73.42mm so the block needed boring to 73.475mm

Also its worth mentioning that pistons are tapered, barrel shaped, and eliptical all at the same time, There can be 25mm difference between the top and bottom of the piston, essentially they are shaped so the fit the bore when they are at running temperature,

[1977 Loud_Mini]

I'll take the block down to my local engine centre on saturday to get it measured. The piston movement compared to my 998 engine and standard 1275 engine is huge. The engine didn't smoke when first run, i'm asuming this is because the piston rings were just pushing on the bore enough and as i've driven it the rings have worn slightly and therefore letting a lot of oil past them. There was a pool of oil in each piston and the valves etc are fine as i was running the cylinder head on my 1275 a couple of days before the 1380.
I always thought you could get the block bored to 74mm as pistons are available, it depends if the block has been offset bored correctly to allow the next oversize without going into water/oil ways.
I didn't get the engine built. I bought it with pistons in as they had new rings which i checked and there was minimal movement within the bores but the rings must have worn as the bores are too big.

Edited by 1977 Loud_Mini, 11 January 2011 - 11:50 PM.

[danrock101]

I know it seems like the long way around but the cheapest and quickest option is get a new block bored out to the correct size of the pistons, ideally use a block that actually needs a rebore.
This is providing the bore is actually too big for the pistons.

[1977 Loud_Mini]

Yeah i was thinking of doing that, i have a spare 1275 but the only problem is the flywheel won't come off. I'll take the pistons out of the 1380 and then take the rings off, place the rings into the bore and measure the gap to see if they are definately too small.

[danrock101]

You'll need a flywheel puller and a big breaker bar. If the bore is too big, it will cost about £80-90 for a re-bore, then it's up to you if you want new core plugs, threaded oil plugs, new cam bearings. Personally I'd get it checked out at a engineering shop so they can check it quick then if the bore is too it doesn't leave you with much of a choice,
Or maybe you can get some second hand 74mm pistons cheap and sell your 73.5mm pistons on.

[danrock101]

IIRC the fiesta pistons are 73.7mm

[1977 Loud_Mini]

Yeah i've got a flywheel puller and used it for the 1275 and left it very tight for about a week, have heated it but only with a blow lamp and it wouldn't come off, oh and also hit it through the starter hole with a big lump hammer. I'll definately get the bores measured so i can see whats what. I didn't know about the fiesta pistons being used but i think some people use the triumph ones which may be 74. something mm?

[pogie]

Weird , because the block cant be bored out to more than 73.5mm.

That's incorrect. 74mm used to be quite a common overbore as it gave 1399cc on a standard stroke crank and was useful for the up to 1400cc class in motorsports. You can go even further, I once bought a ex-rallycross engine which had 74.7mm Triumph Pi pistons on a stroked crank that gave 1426cc.

[danrock101]

I'm just thinking though, what's strange is how it was fine before, surely if the bores are too big it would be smoking not long after you first started it.

[Sir Cheat]

Have a look on here at all the sizes you can have...'Topic Link'

[Sir Cheat]

I'm just thinking though, what's strange is how it was fine before, surely if the bores are too big it would be smoking not long after you first started it.

Its very strange...

[Cooperman]

I had almost exactly this a couple of years ago when a machining company had bored to 1380 but had gone about 0.005" too big on each bore. The options were to either try to bore to the next size up or to get another block and bore it correctly. As the block was at 1380 there was a risk that a bigger bore could go through the bore walls which would still scrap the block, but would still be charged at standard machining costs for re-boring and still require another block plus the boring costs for that as well. We used another block.
One way to check is to take a piston off a piston and slip it into the block. Then measure the ring gap at about 1" down from the block top. The gap should be between 0.010" and 0.015". It could be slightly more, but on the engine I had with this problem it was between 0.025" and 0.028" which indicated an oversize of bore of around 0.015"/3.142 = approx 0.005".
Of course, the best way is with a bore micrometer which will give the actual size.
This is the problem with going to 1380 as it may not be possible to go any bigger.
As stated above, it may be possible to sleeve back to 1275, but the outside diameters of the sleeves will be getting awfully close between 2 & 3 if you try to sleeve it and I've never seen a 1380 block successfully sleeved, although I'm not saying it's impossible. I think it would be impossible if the block had not been offset bored.
If we keep on scrapping 1275 blocks in this way we'll not have any left in future years!