Discussion
Well after a thorough going over, what was sold to me as a good engine looks like it needs a re bore. The crank etc is good but I don't wish to put it all back together with new shells pump cam etc and find I have to rip it out again in 12 months. So here we go again.
So, recommendations for re boring a 1275 A+ motor. I read about 73.5 pistons and 1380, is that expensive compared to +10, +20 etc. Recomendations please and does anyone have a set of oversize pistons in good nick, that they might want to sell, to keep this extra cost down a bit?
Cheers
Tony H
So, recommendations for re boring a 1275 A+ motor. I read about 73.5 pistons and 1380, is that expensive compared to +10, +20 etc. Recomendations please and does anyone have a set of oversize pistons in good nick, that they might want to sell, to keep this extra cost down a bit?
Cheers
Tony H
A set of pistons will be more-or-less the same price for any oversize. The MiniSpares MegaPiston range is pretty good for the money.
The additional cost when "going large" comes from having to put the boring tool in multiple times. Now I realise some machine shops may not do it this way, but when Mick did my replacement block he put 4 cuts in each bore, first to 20thou over, then 40thou, then 60thou, and finally out to 73.5.
Other than that costs should be about the same, I mean all the consumables cost the same unless you are "tricking" out with ARP and the like.
The additional cost when "going large" comes from having to put the boring tool in multiple times. Now I realise some machine shops may not do it this way, but when Mick did my replacement block he put 4 cuts in each bore, first to 20thou over, then 40thou, then 60thou, and finally out to 73.5.
Other than that costs should be about the same, I mean all the consumables cost the same unless you are "tricking" out with ARP and the like.
With the bigger bores, i.e. for 73.5 mm pistons, it's common to 'offset bore' the block which, of course, is more expensive. As Phil rightly says, the boring tool has to make more passes.
The size I usually go for is +0.060" giving 1330 cc. I do like the Hepolite 21253 low-drag slipper pistons. They cost about £140 a set, depending on what sort of discount srtucture you get. It's a good piston with an 8.1 cc dish. There are holes rather than slots inside the bottom ring - always the sign of a tough piston.
In fact I'm about to fit a set to my 1991 Enduro rally car at +.060".
It seems that the most important thing with fitting pistons is to make sure that they come right to the top of the block when the engine is assembled. It's common for them to sit up to about 0.030" down the bore at TDC, so i have a set of old gudgeon pins which are linished to be a light push fit into a standard piston and I do a dummy build using these after boring, then return the block for final deck skimming after measuring the distance of the piston crowns down the bores. Remember to measure all 4, as with tolerances you can get a few thou difference.
When you do this you not only get a beter C.R., you get better 'squish effect' in the combustion chambers.
Most modern pistons are really very good.
The size I usually go for is +0.060" giving 1330 cc. I do like the Hepolite 21253 low-drag slipper pistons. They cost about £140 a set, depending on what sort of discount srtucture you get. It's a good piston with an 8.1 cc dish. There are holes rather than slots inside the bottom ring - always the sign of a tough piston.
In fact I'm about to fit a set to my 1991 Enduro rally car at +.060".
It seems that the most important thing with fitting pistons is to make sure that they come right to the top of the block when the engine is assembled. It's common for them to sit up to about 0.030" down the bore at TDC, so i have a set of old gudgeon pins which are linished to be a light push fit into a standard piston and I do a dummy build using these after boring, then return the block for final deck skimming after measuring the distance of the piston crowns down the bores. Remember to measure all 4, as with tolerances you can get a few thou difference.
When you do this you not only get a beter C.R., you get better 'squish effect' in the combustion chambers.
Most modern pistons are really very good.
Gassing Station | Classic Minis | Top of Page | What's New | My Stuff