12 replies to this topic

[Chippy1950]

Good morning, I am new to the forum and have some questions regarding the machining process on the rebuild of a 1275 A+. The engine will be used for road use although I’m thinking of upgrading to the MD266 cam.
The cylinder head is currently split form the block and before I go any further I plan to measure the thrust play in the crank and also the piston crown to deck height.
Moving on from that I will then need to decide if a hone or rebore is required. To confirm this is I will measure the bore diameters or Is it common practice just to go to +0.020” 1st oversize instead of honing?
If I rebore I will use the mini spares P21253-20 pistons which they have suggested require a piston to bore clearance of 0.003”
Am I correct in planning the build as follows
1. Carry out honing or rebore to 1st oversize
2. Carry out regrind of crank if it’s outside of the limits and install new bearing shells
3. Dry build using new pistons and measure crown to block deck height.
4. Skim deck of block (what is the minimum depth this can be?)
5. Carry out the CR calculation to work out head chamber cc’s required and what requires skimming from the head (I’m basing this on a 9.5:1 ratio, is that correct for road use?)
If anyone is able to offer any advice on whether my plan is correct or any other route to take I’d be great full. Thank you

[nicklouse]

First things first.

 

pick your machine shop.

 

get them to measure things. They have the correct tools. They will then advise if a hone is enough. Or if it needs a reborn and to how much it is needed due to possible wear.

 

same with the crank. Similar with the piston to deck height. It depends. Once that has all been done you can than workout what CCs you need in the head to give the CR you need for the cam.

[KTS]

i would add checking conrod big ends (and resizing if required) to the list of things to be done before the dry build

[Chippy1950]

Thank you both, I have been in touch with a couple of machine shops locally to me but just to ask the costs for doing the work listed along with a chemical clean.
I have access to Dti’s, bore gauges, micrometers and verniers so I can get an idea of the measurements I currently have, but I will take your advice and speak with the machine shop about the measuring.
Would I be correct in thinking that if a hone is possible it is still advisable to replace the pistons for new and not just replace the rings (if they are in fact still available)
@KTS With regards to the big end bores. Do you mean to measure them for ovality with the bearing shells removed but the Con rod assembled and torqued?
The current pistons are 20142 G2
Thanks again

[Lplus]

Good morning, I am new to the forum and have some questions regarding the machining process on the rebuild of a 1275 A+. The engine will be used for road use although I’m thinking of upgrading to the MD266 cam.
The cylinder head is currently split form the block and before I go any further I plan to measure the thrust play in the crank and also the piston crown to deck height.
Moving on from that I will then need to decide if a hone or rebore is required. To confirm this is I will measure the bore diameters or Is it common practice just to go to +0.020” 1st oversize instead of honing?
If I rebore I will use the mini spares P21253-20 pistons which they have suggested require a piston to bore clearance of 0.003”
Am I correct in planning the build as follows
1. Carry out honing or rebore to 1st oversize
2. Carry out regrind of crank if it’s outside of the limits and install new bearing shells
3. Dry build using new pistons and measure crown to block deck height.
4. Skim deck of block (what is the minimum depth this can be?)
5. Carry out the CR calculation to work out head chamber cc’s required and what requires skimming from the head (I’m basing this on a 9.5:1 ratio, is that correct for road use?)
If anyone is able to offer any advice on whether my plan is correct or any other route to take I’d be great full. Thank you

It might be worth asking minispares what CR those pistons give with the standard 1275 A+ head.  It might not be too far from what you need and will save a lot of messing about.

[KTS]

Would I be correct in thinking that if a hone is possible it is still advisable to replace the pistons for new and not just replace the rings (if they are in fact still available)
@KTS With regards to the big end bores. Do you mean to measure them for ovality with the bearing shells removed but the Con rod assembled and torqued?
The current pistons are 20142 G2
Thanks again

 

yes; exactly that

 

with regard to honing;  if the pistons are still in good shape, and the piston-to-bore clearance within tolerance after honing i don't see any reason why they couldn't be re-used with a new set of rings.

[Chippy1950]

Thank you for your advice guys👍🏻

[gazza82]

Thank you for your advice guys

Not sure where you are in Buckinghamshire but I can recommend CKM on the Cressex Industrial Estate in High Wycombe. They checked out my 1275 engine and crank and gave it a good clean, and fitted the new pistons to the rods for me ..

http://www.ckmotorengineers.co.uk

Edited by gazza82, 09 September 2022 - 09:00 AM.

[Chippy1950]

Hi Gazza, I'm actually in Prestwood so only 15mins from Cressex. I’ll give them a call.
Cheers

[gazza82]

Their website has been under construction for a couple of years .. hopefully concentrating n the real task ... engines!

[Chippy1950]

The block is now at the machinists and it has been determined that a hone is all that is required in the notes. The original pistons installed are the Hepolite 20142’s. Can piston rings still be purchased for these as they are in good condition and will reuse them if I can.
I’m having trouble finding any info on ring availability?
Thanks

[whistler]

Try Minispares or Somerfords. If all else fails go to https://www.fwthornton.co.uk/

[Chippy1950]

Thanks Whistler👍🏻