Jump to content


Photo
- - - - -

Skim A Block


  • Please log in to reply
11 replies to this topic

#1 whistler

whistler

    Camshaft & Stage Two Head

  • TMF+ Member
  • PipPipPipPipPipPipPipPip
  • 1,699 posts
  • Location: Cardiff

Posted 18 August 2017 - 02:39 PM

How much can be safely taken off a Metro A+ 1275 block? Mine has some corrosion on the deck due to inclement weather outside in the yard?
I'll probably fit low compression pistons (21251) to compensate for too high a compression ratio.

#2 ACDodd

ACDodd

    Up Into Fourth

  • Mini Docs
  • PipPipPipPipPipPipPipPipPip
  • 2,646 posts

Posted 18 August 2017 - 03:16 PM

They can have lots skimmed off, your only issue is achieving the right compression afterward. The 21251 is a weak piston and good for standard use only. The question we need to know is what spec is the engine you intend to build?

Ac

#3 whistler

whistler

    Camshaft & Stage Two Head

  • TMF+ Member
  • PipPipPipPipPipPipPipPip
  • 1,699 posts
  • Location: Cardiff

Posted 18 August 2017 - 04:25 PM

Thanks AC. It will be a mild road car with probably just an LCB and an alloy inlet manifold. It already has a 35.6mm head and because of corrosion it might have to go to +60. I don't want to go above about 10:1 CR so I'll have to do some calcs when I know how much the block is being skimmed before it finds solid metal.

#4 ACDodd

ACDodd

    Up Into Fourth

  • Mini Docs
  • PipPipPipPipPipPipPipPipPip
  • 2,646 posts

Posted 18 August 2017 - 06:29 PM

You will likely need to use a piston that can stand a little removing from the crown while maintaining a reasonable dish volume. You might also have to consider opening the chambers out in the head to maintain a decent sane compression ratio.

Ac

#5 whistler

whistler

    Camshaft & Stage Two Head

  • TMF+ Member
  • PipPipPipPipPipPipPipPip
  • 1,699 posts
  • Location: Cardiff

Posted 18 August 2017 - 06:42 PM

You will likely need to use a piston that can stand a little removing from the crown while maintaining a reasonable dish volume. You might also have to consider opening the chambers out in the head to maintain a decent sane compression ratio.

Ac

Think I may have to look at different pistons then. Ones with a bit of meat on the top but still low CR. I think the chambers are as factory so a little grinding would help with the CR as well. Any suggestions of piston? My other 1293 uses 21253 but think the CR would be too high in this case.

#6 Northernpower

Northernpower

    Mr. 7-Port

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 2,409 posts
  • Name: Graham
  • Location: North Yorkshire (God's County)
  • Local Club: Its out there somewhere

Posted 18 August 2017 - 06:53 PM

 

You will likely need to use a piston that can stand a little removing from the crown while maintaining a reasonable dish volume. You might also have to consider opening the chambers out in the head to maintain a decent sane compression ratio.

Ac

Think I may have to look at different pistons then. Ones with a bit of meat on the top but still low CR. I think the chambers are as factory so a little grinding would help with the CR as well. Any suggestions of piston? My other 1293 uses 21253 but think the CR would be too high in this case.

 

Machine the 21253 pistons to give the correct compression ratio along with a little work on the head, or, look at the C-STN100.



#7 Spider

Spider

    Moved Into The Garage

  • Admin
  • PipPipPipPipPipPipPipPipPipPipPip
  • 13,843 posts
  • Location: NSW
  • Local Club: South Australian Moke Club

Posted 18 August 2017 - 08:57 PM

I've use countless sets of these;-

 

http://minisport.com...et-hypatec-1330

 

while are a "9.7:1" CR Piston, they can have loads machined out of the crown quite safely, I'll post up a pic later.

 

They also use more common imperial rings, so you have a much wider range of choice of brands and types of rings available. The AE Pistons I have to say, I went off years ago as the rings are odd-ball metric types, which only AE have and the design / manufacture of their rings isn't remotely what it was. I also found their pistons (last time I had some) varied a lot in manufacture.



#8 Cooperman

Cooperman

    Uncle Cooperman, Voted Mr TMF 2011

  • TMF+ Member
  • PipPipPipPipPipPipPipPipPipPipPip
  • 23,014 posts
  • Location: Cambs.
  • Local Club: MCR, HAMOC, Chelmsford M.C.

Posted 18 August 2017 - 09:22 PM

You may well be OK with the famous 21253. It is unlikely the block will need more than about 0.030" skimmed from it. It may be that the pistons would have been sitting around 10 to 15 thou down the bores, so you would only need to take 15 thou to 20 thou from the piston top, which is entirely OK.

The standard head would still need skimming to get a reasonable CR of, say, 10:1, especially after a bit of chamber improvement, so you are very unlikely to have any problem.

 

It is unlikely you have any engineering problem here. Even if the piston had a lot machined from it and the chambers were already small due to earlier skimming, it is easy to increase the capacity of the chambers by 2 or 3 thou each. 



#9 whistler

whistler

    Camshaft & Stage Two Head

  • TMF+ Member
  • PipPipPipPipPipPipPipPip
  • 1,699 posts
  • Location: Cardiff

Posted 19 August 2017 - 11:23 AM

Thanks Peter, Moke and AC et all for your clear advice on what is possible. Much appreciated.

#10 ACDodd

ACDodd

    Up Into Fourth

  • Mini Docs
  • PipPipPipPipPipPipPipPipPip
  • 2,646 posts

Posted 19 August 2017 - 11:41 AM

If you are near Salisbury, I can takes look, and skim it for you if needed.

Ac

#11 whistler

whistler

    Camshaft & Stage Two Head

  • TMF+ Member
  • PipPipPipPipPipPipPipPip
  • 1,699 posts
  • Location: Cardiff

Posted 19 August 2017 - 11:58 AM

If you are near Salisbury, I can takes look, and skim it for you if needed.

Ac

Thanks, I'm in Cardiff and currently go use Dale at Pogsons but if the weather's nice, you never know.

Edited by whistler, 19 August 2017 - 11:58 AM.


#12 ACDodd

ACDodd

    Up Into Fourth

  • Mini Docs
  • PipPipPipPipPipPipPipPipPip
  • 2,646 posts

Posted 19 August 2017 - 01:27 PM

Evenings and weekends are best for me. Can also do the piston dish and crown machining, and boring.

Ac

Edited by ACDodd, 19 August 2017 - 01:29 PM.





0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users