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1275 Metro Rebore


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#1 JonnyAlpha

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Posted 05 March 2019 - 08:21 PM

This is one of many threads associated to a Metro 1275 Engine that I have salvaged from a car that has been sitting around outside a barn for about 15 years. I am planning to refurbish the engine and have been doing lots of research and have obtained several quotes for machine work.

 

I managed to speak to another local machine shop yesterday, this guy has been around for ages and has done plenty of A series Engines. He was recommended to me by one of my other Mini acquaintances.

 

Following lots of peoples advice on here I am going for a rebore to 1293, which is stated as being +20. 

Am I correct in my thinking that +20 means +20 thousandth of an Inch, written as 0.020 inches? And that these would be the correct pistons?

 

To help what is the bore diameter of:

1275

1293

1310 

 

On speaking to my machinist I said I am looking for a +20 Thou rebore to 1293 to which he replied that +20 was not 1293?? 1293 being more like 100 Thou?? Not sure if I asked or phrased the question incorrectly?

 

As I will be doing all of the build work myself I have confirmed the following plan with the machine shop:

After stripping everything apart from the Crank.

 

1. Rebore to 1293 - this will be done with pistons / rods removed but the crank still in place to avoid having to strip and refit (which will be done after the dry build).

2. Dry fit new pistons and measure piston to deck height.

3. Strip Crankshaft.

4. Hone bores and skim block face to deck the pistons.

5. Build short engine.

6. Refurb head (to include trimming Inlet Valve Guide Bosses and Valve Guide Bases).

7. Calculate chamber capacity to calculate CR.

8. Skim head as required to gain correct CR.

 

One of the bores has suffered some rusting as a result of coolant being left sitting in the bore, I cleaned it up with some W&D but I can still feel a roughish area (The pistons are still in and the crank rotates smoothly and any lubricant is forced out on the upstroke).

20 thousandth of an inch is 0.5mm and my concern is that a rebore to 1293 may not be sufficient (can't quite picture exactly what 0.5mm would look like on the bores) and so I guess I shouldn't order the pistons until after the rebore in case we need to take it out to 1310?

 

Thanks for taking the time and bearing with my dumb questions :-)


Edited by JonnyAlpha, 05 March 2019 - 09:08 PM.


#2 JonnyAlpha

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Posted 05 March 2019 - 08:27 PM

You need to get your new pistons & the block needs to be rebored to give the correct size/clearance for the pistons used.

 

 

Standard bore on all 1275 engines is 2.780" (70.6 mm).
So a 1293 is +0.020" which is, of course, 2.800"

 

Just found these replies to a 9 year old thread, which answers one of my questions ref bore size and also reminds me that I need to get the pistons before the final hone is done as we need to also make sure that we get the correct clearance - does this vary for different piston types and again varies based on finished engine build / use?

 

What would the clearance be for a Fast(ish) Road engine. This will have minor head work as noted above Stage One Kit with HIF 44 and an Evo001 Cam (to ensure the CAM is newer than the existing MG Metro CAM that's one 50K.



#3 KTS

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Posted 05 March 2019 - 08:35 PM

don't even think about reboring with the crank in place - take it out 

 

once you've removed it you'll be able to examine the journals to identify what remedial work may be required, or whether it's scrap

 

you'll probably be wanting to replace & ream the cam bearings while you're at it



#4 Steve220

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Posted 05 March 2019 - 08:40 PM

As above. With the amount of swarf produced, the block ideally needs to be stripped then cleaned out after. It also means you can have the crank bearings checked for wear.

Edited by Steve220, 05 March 2019 - 08:41 PM.


#5 absx2

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Posted 05 March 2019 - 08:52 PM

To even think about leaving the crank in place sends alarm bells ringing.

Engine building is a clinical procedure requiring super accuracy and meticulous cleanliness.

I politely suggest you entrust the work to a trusted engine specialist as it will save a lot of heartache and give you piece of mind as well as a guarantee.        



#6 JonnyAlpha

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Posted 05 March 2019 - 09:06 PM

don't even think about reboring with the crank in place - take it out 

 

once you've removed it you'll be able to examine the journals to identify what remedial work may be required, or whether it's scrap

 

you'll probably be wanting to replace & ream the cam bearings while you're at it

 

 

As above. With the amount of swarf produced, the block ideally needs to be stripped then cleaned out after. It also means you can have the crank bearings checked for wear.

 

 

To even think about leaving the crank in place sends alarm bells ringing.

Engine building is a clinical procedure requiring super accuracy and meticulous cleanliness.

I politely suggest you entrust the work to a trusted engine specialist as it will save a lot of heartache and give you piece of mind as well as a guarantee.        

 

Sorry it's late and I am tired, i'll re word my post above, this is an experienced machinist, after the initial rebore we will put the conrods back on with the new pistons. Measure the deck height. 

Then remove the pistons, rods and crank before skimming the block and honing the bores.

The crank will be ground (if required) and polished (and balanced when I can find someone that is not going to charge the £160 that I have been quoted by two local shops).

 

The short engine will then be rebuilt with new main, centre and thrust bearings etc.



#7 wile e coyote

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Posted 05 March 2019 - 09:07 PM

If you're concerned 20 thou wont be enough  to get a decent bore then get it to your machinist and let them make the call

 

There no specific "clearance" for a fast road engine - if you're questioning piston to bore - different pistons (by manufacturer or sometime in the same batch!) may be a few thou out of spec (pistons normally supplied with a data sheet anyway) so never have the block bored with a view to sourcing pistons later,,, supply block and pistons to the machine shop and let them sort the spec snug fit (best to use your machine shop to push the pistons onto the rods anyway)......

 

As the engine in bits anyway the crank has to come out - at the very minimum - if you're very lucky,to renew the big end & main bearing shells (I've never heard of a rebore being undertaken with it in - machinist wouldn't be able to get the block flat) as well as being a v bad idea as already outlined!!! - most likely the journals will need a grind down that'll necessitate  different size bearings to compensate in any event



#8 Cooperman

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Posted 05 March 2019 - 11:09 PM

If you measure how far down the bores the existing pistons are sitting, you can then measure the old piston from gudgeon pin to crown and compare that dimension with the new dimension on the 21253 pistons. From this you can calculate how much to machine from the block deck to sit the new pistons at between 0.003" and 0.007" down from the deck at TDC. The tolerance is sufficient to allow for any very small mis-measuring and is OK at between those figures.

 

An over-bore to +0.020" normally removes any scoring, bore wear or damage, but if you leave the fitting of the pistons until after re-boring, should any scoring not be removed it is not too late to exchange the +0.020" pistons for larger ones.

 

The trial build is vital to establish the correct measurements, establish the cam timing, set the crank thrust float and check the deck height (already calculated).

 

Personally I would have the crank re-ground to the next size.



#9 whistler

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Posted 06 March 2019 - 10:17 AM

Block needs to be totally stripped because most machine shops will acid dip the block prior to machining. The 21253-20 pistons are fine. They're what are in my 1293. I bought mine from bullmotifminispares but any of the major Mini parts suppliers would supply them.

Make sure the machine shop keep the engine number plate for you.



#10 Moke Spider

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Posted 06 March 2019 - 06:21 PM

It is possible and routine done on big engines to Bore and Hone a Block with the Crank in place, but for any Automotive Engine, there's no reason to do it.



#11 JonnyAlpha

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Posted 06 March 2019 - 08:03 PM

OK;

 

So assuming I do strip down the block completely, before having the initial rebore done, when doing the dry build do I do it before the Crank has been ground / polished and use the old bearings or do I dod it after the Crank has been ground and polished and assemble the dry build with the new bearings (Centre / Mains / Big Ends?

Do I install the Thrust washers.



#12 Cooperman

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Posted 06 March 2019 - 09:12 PM

The trial build should be done after the crank has been ground and with the new bearings. That way you can, if you wish, use 'Plastigauge' to check the bearing clearances, you can check the crank end float, check the ring gaps (when trial building using the new pistons with the rings removed, fit the cam and check/set the timing, etc.

 

Then after measuring and doing the calculations, the block can be returned to the machine shop for final deck skimming, the combustion camber sizes can be calculated for the desired comp ratio and the amount to be skimmed from the head measured. The head can then also go back to the machine shop for final skimming.

 

Once you have done all this properly, you will realise why a well-built engine is so expensive to have built by a specialist. You will also wonder how some so-called 'engine-builders' do it so cheaply until you realise the real answer!



#13 JonnyAlpha

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Posted 10 March 2019 - 02:40 PM

So to continue with this thread I finally received my DTI gauge through the post, missed the postman so had to go to the depot grrrr. I picked it up yesterday and did the measuring this morning.

 

Bore Sizes

Using a set of vernier digital callipers I can see that the bores are standard at 70.06mm.

 

Piston to Deck Height

Using the DTI gauge I found TDC and then moved the DTO gauge to the block face and set the DTI gauge to zero, I then carefully twisted the DTI gauge so that it sat on top of the piston crown and found that on each piston it was around 15 thou. Does that sound about right for a standard engine?

 

dvdxHg7.jpg

 

 

Jtk8lSK.jpg

 

End Float

Current end float of the existing crank measures 6 thou when I insert pressure using a screwdriver on the centre main and 7 thou at one of the outer main bearings

 

v5t6Ewt.jpg

 

oWElWaF.jpg

 

Block Height 

Measuring the depth of the block using a ruler it is about 26cm from top to bottom. 

 

I am going to start a separate Engine Building Project thread rather than add it to my Mini Project Who and i'll link in all of the threads where I have asked and received info on doing the engine refurb / upgrade.


Edited by JonnyAlpha, 10 March 2019 - 02:44 PM.


#14 ACDodd

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Posted 10 March 2019 - 03:52 PM

Is there a stamped letter or letters at the rear of the block between cylinders 2&3?

Ac

#15 JonnyAlpha

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Posted 10 March 2019 - 05:28 PM

Is there a stamped letter or letters at the rear of the block between cylinders 2&3?

Ac

 

Nothing that I can see:

 

MOZJjkK.jpg

 

m42Betq.jpg

 

4Uq7jSb.jpg






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