457 replies to this topic

[JonnyAlpha]

Pistons on the way 

[JonnyAlpha]

Ok so the pistons have arrived and in the end I opted for the P21253's over the Omega's (AC Dodd is going to be screaming at me now).

 

Based on using either an MG Metro Cam, Kent266 or EVO001 I have been advised that with today's fuels I should aiming for a CR of 9.75:1 although I have also been advised that if using RON98 and a Distributor tuned for optimum advance, a CR of 10:1 would be fine.

 

Anyway the way I figured it, I have decided to get the head modified as there is little point in doing all of this without allowing the engine to breath properly. In doing the head I am hoping to achieve the correct CR?

 

However before I take the next step which is to get the Crank Polished / Ground and Rebore done, I need be clear in my head regarding the Compression Ratio. And when I make the final decision I need to give the machinist the clearance figures (which I have asked Mini Spares but not received a reply)

 

Several people have given me figures and I have found several calculators on the web which are giving me different results.

 

Using the formula provided in this thread by Cooperman, I have deduced the following figures for my build however there are some figure which are not confirmed, these are in red and it is those that I need help with.

 

Based on an early MG Metro (1989 Mk2) 1275 +20 Thou Rebore (1293) (2.801 Inches / 71.145mm) and standard 1275 A+ Crank Stroke of 81.3mm

 

A - Piston to Top Deck Height (0.001 Inches / 0.0254mm) =  1.01cc

B - Piston Dish (P21253) = 8cc

C - Cylinder Head Chamber (To be confirmed) = 21.4cc

D - BK450 Head Gasket Compressed Volume = 3.8cc

E - P21253 Ring Landing = 0.75cc 

 

So can anyone confirm the figures above (especially those in red).

 

If the above figures are correct I have calculated the following:

 

Swept Volume (SV) = 323.25cc / 4 = 80.8125cc (Swept Volume of one cylinder) (Not sure why this calculation is here as it is not used in the formula anywhere?)

 

A+B+C+D+E = Unswept Volume (UV) 35.16

 

CR = (SV+UV) / UV 

 

CR = (323.25cc + 35.16cc) / 35.16cc

 

CR = 10.19:1

 

So based on a target CR of 9.75:1 the above (as I have already been told) yields a CR that is slightly too high!

 

  

[Cooperman]

That 10.2:1 will be fine. The old A-Series does love high compression and the only issue is modern petrol. If you always fill with 97 or 98 RON petrol and the dizzy is correctly curved and set up you will have no problems. If necessary you can add some octane-booster to each tankful of petrol, but that is usually only necessary with C.R.'s of around 11:1 plus. The key to running higher compressions is fuel quality and accuracy of distributor curve to match the fuel and octane. Just make sure the engine is set-up correctly and that there is no 'pinking' at any point when accelerating with full throttle.

[JonnyAlpha]

That 10.2:1 will be fine. The old A-Series does love high compression and the only issue is modern petrol. If you always fill with 97 or 98 RON petrol and the dizzy is correctly curved and set up you will have no problems. If necessary you can add some octane-booster to each tankful of petrol, but that is usually only necessary with C.R.'s of around 11:1 plus. The key to running higher compressions is fuel quality and accuracy of distributor curve to match the fuel and octane. Just make sure the engine is set-up correctly and that there is no 'pinking' at any point when accelerating with full throttle.

 

So as long as everything is as above I should be OK, incidentally is the ring landing on the P21253's as I mentioned above?

[Cooperman]

I normally work on ring land of 0.7 cc, so you are OK with what you have.

Edited by Cooperman, 22 June 2019 - 09:17 PM.

[Steve220]

https://jscalc.io/ca...RNEglVkKLQir11T

 

Compression ratio calculator if its of any help.

[JonnyAlpha]

https://jscalc.io/ca...RNEglVkKLQir11T

 

Compression ratio calculator if its of any help.

 

Yeh I have used that one but the 1275 gasket on it is 3.4cc not 3.8cc and to add a ring landing you need to add it to something else

[ACDodd]

21253 pistons can easily be machined to get the CR where it needs to be.

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Edited by ACDodd, 23 June 2019 - 05:32 PM.

[ACDodd]

10.2:1 is not 9.75:1.

Ac

[JonnyAlpha]

Took the Pistons to the machine shop for a quote on machining, still nothing back yet. Looks like locally we are a bit stuck here in the SW :-(

 

Anyway, in anticipation of getting the rebore done I need some help with the Piston Clearances.

 

The pistons are the P21253 +20 from Mini Spares, I have had a look around and found this posts by Alex-95 , it seems the Nural recommended clearance of 0.03mm (1.2thou?) is considered to tight and that 0.04mm(1.5 thou) is better. 

 

I just need confirmation please. 

 

This will be a fast ish road engine, details on the proposed build below:

The Engine is from a Mk2 Metro and from the cylinder head. it has been identified as a possible early MG Metro.

CAM: As it stands this Engine will be using either a standard MG Metro Cam, a Kent MD266, Evolution001 Cam or BP255 Cam (All similar profiles?).

CR: CR will be as near to 9.75:1 as possible. 

Cyl Head: The cylinder head will be lightly modified with 3 step valves, smoothed valve recesses and possibly Valve Guide bosses cut back.

Standard Rockers.

Bolt Ons: Stage One Kit with HIF44, Re-Curved Metro Electronic Ignition.

 

Many thanks 

[gazza82]

One man's Stage One is another's Stage 2 or 1 1/2! Do you mean the manifolds, needles, air filters, etc. All can have an impact on what you want to achieve: too large an exhaust and power will suffer in just the same way a small diameter will.

 

I don't know all the answers yet .. or I'd be a rich man .. and I can assure everyone I'm not .. but I am learning! (And also that Person A's ideas might not be to Person B's liking .. you know, you can't please all the people all of the time. etc).

 

I'm building a 1330 with Minispares Mega pistons (C-AJJ3378-60), a 12G940 12G1805 Cooper Mk III head and MD266 cam. I'm still to work out the CR I want/need (9.75:1-10:1 seems favourite), a 1.75" exhaust system, HIF44 carb, ... and rockers? I've got to choose from a set of 1.5 rollers, A+ sintered, Forged Coopers (I picked up for silly money), and the standard pressed rockers .. all new bearings/studs/bolts/nuts/screws/etc ... it was a pretty bare block and crank/caps but in 100% condition.

[JonnyAlpha]

One man's Stage One is another's Stage 2 or 1 1/2! Do you mean the manifolds, needles, air filters, etc. All can have an impact on what you want to achieve: too large an exhaust and power will suffer in just the same way a small diameter will.

 

I don't know all the answers yet .. or I'd be a rich man .. and I can assure everyone I'm not .. but I am learning! (And also that Person A's ideas might not be to Person B's liking .. you know, you can't please all the people all of the time. etc).

 

I'm building a 1330 with Minispares Mega pistons (C-AJJ3378-60), a 12G940 12G1805 Cooper Mk III head and MD266 cam. I'm still to work out the CR I want/need (9.75:1-10:1 seems favourite), a 1.75" exhaust system, HIF44 carb, ... and rockers? I've got to choose from a set of 1.5 rollers, A+ sintered, Forged Coopers (I picked up for silly money), and the standard pressed rockers .. all new bearings/studs/bolts/nuts/screws/etc ... it was a pretty bare block and crank/caps but in 100% condition.

 

Sorry late reply.

I will be fitting a Mini Spares Stage One kit with a needle to suit my Engine. The head is probably going to get done by the Car Kitchen as follows:

 

Stage 2 basically you get the port bowls/throats ported around the guides and valve areas (the guide boss is partially removed)

It will come with the chambers modified and smoothed, 3 angle valve seats, unleaded inserts, all surfaces faced off, new mg metro spec Valves, double valve springs, caps, collets, viton stem seals and then finally painted in your choice of colour

 

All for around £400 (which I don't have yet ). 

[JonnyAlpha]

Well after hearing nothing from the machine shop I popped in to see what the delay was. They said that it would be too difficult to machine the pistons, as reducing the bowl by 2cc was uber difficult!! I told the fella that I had given clear instructions when I dropped them off that all they needed to do was to measure in 5mm on the crown above the Gudgeon Pins and machine down to the lowest point of the bowl. They responded saying that I would be better off getting someone who has done it before to do it. At this point I lost my faith with them (although I have no doubt that they are a reputable company), needless to say I would not be getting them to do my work.

I then asked if they had checked the crank - they responded "oh yeh its fine" - is it, I thought, had they even looked at it?

 

So after mulling over all of my options, two more local machine shops or sending it away, I bit the bullet and on Wednesday I dropped the block. crank and pistons off at another "Old School" machine shop. 

He measured the crank and found that the main bearings were ok but the journals were just outside of minimum wear. So advised that the journals needed 0.0010" removing and for another £20 advised to get the Mains done as well. That way everything would be sweet. There was some staining and what looked like microscopic scoring on the timing end main so this makes sense too me.

I also asked the machinist to check the Con Rods, but forgot to ask him to remove the old pistons and press on the new ones :-( maybe he'll do it anyway

 

So now I need to order some +10 Mains and Big End Bearings, these are the standard ones not ACL Race Bearings as I have no intention of racing the crap out of this Mini.

 

Once the rebore is done, we will make up the crank, rods and pistons, do a dry build and measure the Piston Deck height to see how much to remove from the face of the block.

 

Questions:

 

1. Do I re-use the thrust washers, depending on End Float? 

2. Should I get the Oil Galleries threaded? Avon Bar Racing do a set of Gallery plugs for £6.25? 

[Retroman]

Providing the crank thrust washers are used in the same side as previously, are in good condition they can be reused [ assuming the float is 0.002" to 0.005"]

 

You don't really need the galleries threaded, just use the standard brass jobbies

[JonnyAlpha]

Providing the crank thrust washers are used in the same side as previously, are in good condition they can be reused [ assuming the float is 0.002" to 0.005"]

You don't really need the galleries threaded, just use the standard brass jobbies

Just dug our my notes during the strip down and the End Float was 6 thou so I’ll need to get new Thrust Washers then anyway
Ref the gallery plugs, a bit like the Core Plugs, I find it odd that the holes are just blocked with friction fit caps, holding back 60 PSI (I think) of oil and whatever of water?

Edited by JonnyAlpha, 04 August 2019 - 09:46 PM.