11 replies to this topic

[blakie]

Im after a few thoughts on these pistons if anyone has any? Is anyone running them/ ever ran them?

Ive done a search around to try to find some more info on them but I didnt have much luck.

Cheers

blakie

[Cooperman]

The 21251 is a Hepolite piston for the 1275 engine for lower compression engines. it differs from the higher-compression 21253 by having a lower gudgeon pin height which gives a greater distance between piston crown and block deck and this gives the lower compression ratio.
It's not a bad piston, although not as strong as the 21253 due to it having larger oil drain slots, as opposed to oil drain holes in the 21253. I have an innocent which has the 21251 as the previous owner had the engine re-built and the 'cheapskate' builder fitted low comp pistons. To bring the CR up again I skimmed the block and now have about 10.2:1. I wouldn't want to go any higher with 21251 pistons, but it now runs very well.
There is nothing basically wrong with the 21251 so long as you don't want to run CR's over about 10.4:1 or high sustained revs over around 6000 rpm. I wouldn't like to run them in 'detonation', with too lean a mixture or with the ignition very retarded either.
The number after the piston ID of 21251 is the oversize. i.e. 020 indicates that it is for an engine bored to +0.020" which would give 1293 cc. Mine are 21253-060 which is a +0.060" bore giving 1330 cc.
I hope this helps.

[blakie]

Yeah I understand the +40 etc. Was just after view on quality etc. How would these run without a skimed block?

[Cooperman]

Yeah I understand the +40 etc. Was just after view on quality etc. How would these run without a skimed block?

With a low compression, but otherwise fine. I worked out that the CR on my Innocenti was about 8.4:1 as re-built, so i skimmed 0.065" off the block to remove the excessive combustion volume bringing it up to 10.":1.
You can work yours out. The piston dish on the 21251 is 8.4 cc and allow 1 cc for every 0.010" the piston crown sits down from block deck level at TDC. Ring land volume is 0.6 cc and gasket volume is 4 cc. You will have to measure the chamber volume of your own head, but it is likely to be about 21 cc. You don't really want the CR to be below 9:1 for decent running efficiency, but you can skim the head a bit to help which is cheaper than skimming the block. Mine was completely stripped, so block skimming was an easy option.

[blakie]

WOW cooperman, think I will ave to read that a few times before it makes sence. Well the pistons are in the block and I currently do not have a head. Thinking of going stage 3. So I will have to get the head skimmed to get the best ratio I can.

[Cooperman]

Yes, you will need to measure and calculate the CR and measure the head chambers to see how much needs to come off to get it right. You really can't just guess. However, if you get the calculations it should be possible to arrange for the head supplier to supply one with the correct cubic capacity. If you care to measure EXACTLY how far down the bores the piston crowns sit at TDC I'll be happy to do the calculations for you and let you know what head chamber volume you will need for, say, 9.75:1 comp ratio. To measure this, bring a piston to TDC, put a straight edge across the bore and measure from the top of the straight edge to the piston crown, then deduct the depth of he straight edge from that figure. Use a digital vernier caliper to do this. Expect a figure in the region of 0.055" to 0.070". My Inno was 0.065" as a guide. That must be accurate to within about +/- 0.003", and with this done I can calculate the required chamber volume.

Peter

[blakie]

Your a gentleman!!!!!

Ill get the figures and PM them to you this week, then I can get onto the head supplier to get it ordered to a desired spec.

blakie

[Cooperman]

With those pistons I would go for a nicely gas-flowed head with 35.6 mm inlet valves and 30.5 mm exhausts. The expression 'Stage xxx' means nothing in cylinder head flowing terms. You won't be using high revs with those pistons anyway, so those valve sizes wiill be fine. You'll need a decent inlet and exhaust system to obtain better power too and an LCB with 1.75" bore would be best, perhaps with an RC40 twin-box system. As for induction, an HIF44 carb on a decent alloy manifold would be great. If you want a really good head at sensible money, properly done with combustion chambers to the size you specify, try Rob Walker Engineering near Witney in Oxfordshire (I think he has a web site). He does a great job and his work really is top quality. None of this 'Stage thingy' nonsense either with him.

[blakie]

I live in Carterton near Witney, may well give him a buzz.

As for the spec you listed I have most of what you listed along with a piper 285 cam.

Chris

Edited by blakie, 27 October 2010 - 06:28 PM.

[Cooperman]

I've not really ever worked with Piper cams, but I believe they are on a par with the Kent Megadyne range, which is excellent.
The rest sounds good and you will have a good engine. Just don't rev it too much with those pistons. If you really want to rev it and make the most of the rest of the engine mods, consider using 21253 pistons. I've used them in a 1330 up to regular 7000 rpm in competition. You should get a new set for around £145 plus fitting onto the rods at c.£5 per rod.
Rob Walker is your man in Oxon. Much better than the well-known sales outlets for cylinder heads who charge a fortune for their 'Stage this 'n' that' heads. You could ask Rob to fit unleaded inlet valve seats as well with 35.6 mm inlet valves.

[ministar]

Cooperman seriously knows his stuff and is a great asset to the mini forum.

to him!

[Cooperman]

Cooperman seriously knows his stuff and is a great asset to the mini forum.

to him!

And I sincerely thank you for that compliment.
I've loved Minis since the day they were launched back when I was an impoverished very young engineering apprentice in the aircraft industry but couldn't afford one. Rallied in my first one as a navigator in 1961, bought my own first one in 1963, bought my first new car, a Cooper 998 in 1964 and got my first Cooper 'S', a Mk.1 1275, in 1969. Still own 2 Coopers, a 1964 rally car and a 1973 Innocenti 1300 Export. My wife says it's an obsession; well, maybe, but I just call it enthusiasm. I try to help all our local Mini owners and I build a few engines for a bit of retirement income or for friends. My 15-year old Grandson is currently restoring a 1997 MPI as his first road car as well.
We are all Mini-mad here in Cambs.