A series vs. A Plus

[wantafaster1]

Can any experienced engine builders or engineers tell me if the blocks are different heights? I've seen that the pistons have different deck heights but why is that?

Also can any crank be used in any block, am I right in thinking the thrusts aren't the same?

[Spider]

Are we talking 998 or 1275?

[wantafaster1]

I'm sorry, 1275. I have been told that A plus are .030" taller. Have to decide which motor to strip to go for machining next week.
Got nothing here capable of measuring it, The standard style pistons have different deck heights, but why is this? Need answers folks, Blyton looming fast lol

[251 ENG]

All 1275 blocks are the same heights .

The A+ type slipper pistons have different compression heights to give different compression ratio,s.

The 21253 MG metro piston is the largest , they will quite often stick out of the top of the block if it has been faced .

Most of the other A+ pistons are only suitable for std low compression road engines so you are stuck with using 21253,s if you don,t want to go to omega,s .

[Spider]

As 251 ENG has aluded to, there are a few heights due to Compression Ratios offered and the deck to Crank Centre line heights are (theoretically) the same, however, read on.

With the A Series, there was only 3 factory CRs, 8.0, 8.8 and 9.75:1, this was done only by the CC of the dish in the pistons, being 15.8, 11.1 and 6.6 CC respectively. As far as I am aware, only the 8.8's and 9.75's were fitted to Minis. The Pin to Crown Height of all the pistons were the same.

When the A+ came along, they did a few more CR varitaions, however to improve combustion (and possibly emmisions) the Pin to Crown Heights varied, in doing so also varied the Squish of the Combustion Camber. The CRs that I know of in the A+ were 8.8, 9.4 & 10.5 (and quite likely 9.75):1.

Crank Centre Line to Deck heights are - theoretically - the same. I say Theoretically, because the factory machining on the A+ (the early ones in particular) were really, REALLY bad to put it midly, I have seen variations in excess of 0.030" that you've mentions, but that the good bit, none have been anywhere near parallel to the Crank Centre line, many I've had here have been as much as 0.022" off (and the bores leaning over to a similar angle), as time went by they did get better. Many of the faces that are supposed to be flat (for gasket sealing for example) have been anything but.

1275 A Series, NON Cooper S Cranks and 1275 A+ Cranks again - theortically - interchange - however - be aware that the running clearances are different. This always catches out a few bone-headed (so called) engine builder in these parts, and then they blame the plain style bearings of the A+ for being a crap design Thrusts are the same, unless you want to fit a Cooper S Crank.

[wantafaster1]

It's a brand new forged crank with small journal big ends and a set of Graham Russell pistons. Confused a bit because the paperwork from Keith Calver says they are A+ compression height, but I thought Australia never had A+ engines.
Do I then need a different set of main bearing shells to use it in the newer block?

Thanks again

[Spider]

Was there any information supplied with the pistons? (and I think I already know what the answer is).

We sure did get A+ Engines here, fitted in production.

Who manufactured the crank?

[wantafaster1]

Yes there is information supplied regards pistons to bore clearance and also regarding trial build if used in old style blocks, I'd be doing that anyway.
Crank came from Minispares.

As an aside, do you heat the small ends before fitting the pin?

[ianh1968]

Do I then need a different set of main bearing shells to use it in the newer block?

The A+ type mains have a grooved shell in the block and a plain one in the cap.
The locating tag is in the centre on the plain shells.

If you are using A+ shells in a pre-A+ block it's just a case of filing a second
slot in the correct position. It would also be possible put pre-A+ shells in an
A+ block, but I cannot think why anyone would want to do this.

I do use a bit of heat when pulling gudgeon-pins - a paint stripper gun...

For my last "dry build" I used some old cam followers as dummy gudgeon-pins.
The outer diameter needed a very light go on the bench grinder to make them
a non-interference fit in the rods. Because they are much shorter than the
standard length pins, this meant less time was spent getting the diameter right.

Ian

[ianh1968]

With the A Series, there was only 3 factory CRs, 8.0, 8.8 and 9.75:1<SNIP>

Spider, please clarify that this was in relation to "large-bore" engines.

There were some other 8.x values but these were small-bore.

Ian

[wantafaster1]

Want to go pre A+ if I can, it's more period looking in a 1970 car, since I already got mains for it and it means my existing distributor can be used again. I know its feasible just buying some more parts but the money would be better spent on a steel flywheel and such. Have also been told that the blocks are the same height!
Minefield, only decided to build up a new engine after getting ripped off with other stuff

[Spider]

I Used to heat the little ends, but I've never liked doing them that way and in fact the factory don't recommend it. I press them in but also carefully observe the pressure needed to do so, I forget what the conversion is off hand, but anything less than 1100 on my press, I'll reject them as a press fit, when you heat them, you really have no way of checking how much interference fit you have.

With the A Series, there was only 3 factory CRs, 8.0, 8.8 and 9.75:1<SNIP>

Spider, please clarify that this was in relation to "large-bore" engines.

There were some other 8.x values but these were small-bore.

Ian

Yes, there was 8.0:1, 8.8:1 and 9.75:1 in the Big Bore Pre-A+ Engines from the Factory.

It seems that the only ones to have the 8.0:1 were an In-line Engine, I think fitted to the Sprite and possibly the Marina.