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 Posted: Mar 27, 2017 03:23PM
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US
We believe any bore past .060 makes the block too thin to reliably sleeve, certainly if trying to sleeve all bores. There's enough metal to sleeve one cylinder at .060... barely!  (by this I mean it's not advisable to sleeve adjacent bores)  We also don't think an open wall S block should go much above .060, but we do know that it is done!

If already at 73.5mm, the walls are just too close to bore over for a sleeve, which means if you just fit one, you'll have to remain at a MUCH smaller bore size going forward.  We occasionally find factory sleeved A+ blocks that are otherwise standard. If we bore to 72.5mm or bigger, the sleeves are gone!   We offset-bore anything above .060.  I specify moving numbers 2 and 3 outward .0075" to match the available good quality gaskets (TAM1521) which measures about .115 between 2 and 3.

Back to your pistons.... Is there a number on your "old" ones?  I've found that part number C-STR311 is actually the AE 20659-STD Piston used for the 1380 for decades. We show a set in stock.

 Posted: Mar 27, 2017 11:37AM
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The block has been offset bored.  I need to have the machinist mic the pistons in the bores now.

Dogscarf

 Posted: Mar 26, 2017 05:41PM
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AU
In Australia we have 1360 as well as 1380 oversize pistons available. If your 1360 has been offset bored and you want to use your 1380s all good.

 Posted: Mar 26, 2017 12:55PM
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Unless the block has been offset bored previously, a 74mm bore will make the wall too thin between #2 and #3 cylinders.

Ex factory, the bores were not evenly spaced, the 2-3 wall was always thinner than the others.

Kevin G

1360 power- Morris 1300 auto block, S crank & rods, Russell Engineering RE282 sprint cam, over 125HP at crank, 86.6HP at the wheels @7000+.

 Posted: Mar 23, 2017 03:29PM
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I will visit with my engine builder soon.  Now I am starting the think that it would be more cost effective to bore to 74mm and buy some 74mm pistons.  Then put the 73.5mm on Ebay or keep for the next engine build.  Sleaving and offset boring and fitting the 73.5mm pistons would be very expensive.

DOGSCARF

 Posted: Mar 22, 2017 03:05AM
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My suggestion would be sleeve the block back to std. and start over. I have a row of blocks run and done at 73.5 and 74. When it becomes reasonable ( value vs cost ) they will all be sleeved. I stick to .020 for a rebore and .040 if required. My first Cooper S block has been bored .020, .040 .060 and 73 mm. This covers it's life from 1971 to date. It will be sleeved to std. on it's next trip to the machine shop. As it's pulling my daily driver it may be a long time before that happens. 

As for pistons yes they are tapered and cam ground. They are not round and not straight sided. I'm working with a set of forged pistons now. I read what the box said, ask a couple of engine builders, the supplier and a former supplier. The on line info was not what I'd call a big help. My machinist and I talked it over and got out mikes out. We agreed on a size and were within .005 of what others came up with. There is something to be said about the way the engine will be used. CR, RPM, timing and fueling all play a part.  Steve (CTR)

 Posted: Mar 21, 2017 04:40PM
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The questions I would have is what piston did you start with? any brand/numbers stamped in the piston? Why not just replace the one piston with the same brand?  In the early days of this conversion to 1380 they talked of offset boring and I believe Triumph pistons but then they standardized on 73.5mm pistons with the next oversize at 74mm.  So what does the bore measure?

 Posted: Mar 21, 2017 01:47PM
 Edited:  Mar 21, 2017 01:56PM
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I always measure on the thrust side under the wrist pin, but as you state the bore size is dictated by the piston manufacturers stated required clearance for that particular piston. Pistons are not straight sided to allow for different expansion rates due to wall thickness and greater heat at the crown. Also the piston/bore clearance will be different for a Race motor as opposed to a road car which is where a good machinist/mechanic comes in.

I honestly can't remember of any case where the piston lower skirt measurement was used to determine bore clearance?

Edit............When I read the first post where it said the lower skirt I assumed perhaps wrongly that he was taking his reading at the lowest part of the skirt and not at the widest part of the piston as you say usually at the lower 3rd of the skirt below the pin.

Mini's are like buses they come along in a bunch

 Posted: Mar 21, 2017 09:34AM
 Edited:  Mar 21, 2017 09:45AM
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The skirt size is used as it is often the largest part of the piston.

Google-ing tells one the same thing.

Minimans, can you tell us how you determine bore size?

My understanding is piston-to-bore clearance is calculated at the tightest section, usually 90-degrees off from the piston pin near the bottom 1/3rd of the piston in the skirt. This is done to account for the expansion rate in the pistons when they are hot. if the skirt expands to a spec tighter than the bore has been honed to, the piston will seize or skuff excessively. Likewise, if the bore is too large, the piston will rock excessively, you'll get piston slap, and the rings will potentially not seal properly.

Different piston manufacturers specify different piston-to-bore clearance for many different reasons. Cast pistons for example will have a different piston-to-bore clearance specification than a forged piston. Different piston materials will have different expansion rates.

Forged race pistons are often sized looser in the bores because they expand more than cast street pistons. They often give noticeable piston slap until the engine is up to temp. I have no idea what powermax pistons call for in terms of piston-to-bore clearance.

For example: From Venolia //www.venolia.com/technical.html
PISTON CLEARANCE: With 2618 alloy, top of skirt .008 to .010. Bottom of skirt: .005 to .007". Air cooled engines usually require more clearance.

Some machine shops can knurl the piston skirts if they have collapsed to bring them back up to the proper spec and skirt to bore clearance. I've heard a tale from an old-timer that a certain company owner who had a fleet of trucks in town used to have brand new engines torn down to have their pistons knurled as he believed it would cause them to hold more oil than the factory micro-ridges would hold thereby adding longevity to his engines. YMMV

 Posted: Mar 21, 2017 09:17AM
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Why is he using the skirt size to determine bore size??  Google measuring pistons to educate yourself and him on correct techniques 

Mini's are like buses they come along in a bunch

 Posted: Mar 21, 2017 07:40AM
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Fellow Minoids,  I am in the middle of restoring my 1967 Aussie Cooper S.  I have a 1275 block, thicker flange that has been bored out to be a 1380 and has what I thought were 73.5mm pistons.  Well, one piston is broken so I am going to use set of new Powermax 73.5mm pistons.  My machinist measured and found that the old pistons are .010 larger at the bottom of the skirt than my new Powermax 73.5mm pistons.  He said then new pistons will not work as they are too small.  Now, .010 inches equals .254mm.  I am not aware of a piston that is 73.75mm (or .010 over a 73.5mm) piston.  I think that I want the machinist to measure the bore next.  This does not make sense to me unless the skirt on the pistons can vary by brand.  Too much money and labor to sleave all four bores and bore out which would come to $400.  Thoughts?

Dogscarf
1967 Morris Cooper S Aussie.
1968 Morris Cooper S.