47 replies to this topic

[silky]

OK - so I have spoken to Bill. And yes, you were all right! I'm looking at double my (rather hopeful) estimate. >

Thanks goodness for the bank who accepted me for a new credit card!

He decided to re-bore to 1330, with the expensive pistons, duplex timing(?), and loads of other stuff I did not understand. Oh, and most importantly, he's painted it!

He is putting in a Piper 286 camshaft (apparently I have 1.3 rockers). Not sure what this does for performance - can anyone explain?

He also mentioned that he had 11 studded (I think) something - can anyone explain this too?!

[dklawson]

Others can tell you about the cam, I don't know what the proper application is for a 286.

The "11 stud thing" is likely a reference to the cylinder head. The typical cylinder head is held in place with 9 studs. The Cooper-S sneaks in two more, one at each end of the head. Most blocks and heads can have these additional fasteners/holes added. It's just a little extra insurance for the head gasket and it's done/added to most big-bore engines as part of a performance upgrade. It shouldn't add too much to your overall cost.

[silky]

Thanks for that - sounds more relevant now. He did mention that it was to do with the cylinder head. Glad he knows what he's doing!

[Jammy]

Yea, he certainly does know what he's doing, but the cost of it doesn't surprise me, nor does the fact that he's basically built you a fast road engine without you asking him to!

[silky]

Yep, he seems to get a bit carried away! He's a true enthusiast, which I suppose means he'll do a decent job.

Dr Jammy, as the source of all knowledge, can you explain to me about the rockers and cam? What difference in performance is there between a 1:3 - 1:7 rocker (or do they serve a different purpose)?

[Jammy]

The lift the valves more. Basically you can make the valves open further, the valves open for a long time (duration), or both. You can set both with the cam, but the rocker assembly only affects the lift, meaning for the duration that the valve is open, it can allow more air/fuel mixture into the cylinder, resulting in a bigger bang and more torque/power.

Now, its not always a case of bigger is better. Standard rockers are about 1.25:1, then you can get 1.3s, 1.5s and 1.7s. 1.3s are quite a popular upgrade as they give a bit more power with little downside, 1.7s are race only really as I think they lose you torque lower down the rev range. Its quite a complicated relationship because of the siamese ports of the 5 port mini cylinder head.

I won't go into the relationship between the rocker ratio and the cam you chose, mainly because I don't know enough about it, but also because Vizard dedicated about 3-4 chapters of his popular book to the subject, so whatever I could write hear wouldn't really do it justice! 'How to Modify the A-series' by David Vizard is a very good read!

[silky]

You really are the source of all knowledge - hence the "Dr." title, I suppose.

I'm sure whatever I've got is fine. Bill would have changed it otherwise!

Can't wait to get it back now, I have next week off work and will be painting her (done all the necessary body work). Going to do it the old fashioned way with rollers and Rustoleum (my Dad used to be a coach painter - handy).

Hopefully she'll be a nippy little bee-atch. Although I have read on the forum that the 286 cam doesn't like idle much - is the true or is it down to how the carb, etc is set?

[Jammy]

The 286 is a bit off a funny one! An immensely popular cam over the years, in some engines it idles, in some it doesn't. I've heard people say its down to timing, down to the weight of the flywheel, the size of the engine, the cam. Truth is its probably all and none of these! lol

Nah, the 'Dr' part is due to a discussion with the gf over what constitutes a Marmalade, hence the title under my avatar on the left!

You should get yourself on the Xmas Kent run - (Linky)

EDIT: In depth article by Keith Calver on rockers - Linky

[Guess-Works.com]

If you're not worried about performance, but more interested in reliability, just get yourself a low mileage 1275 and drop it in.... let someone else have the hassle of sorting out the old engine if they want to...

If you do want to rebuild the existing engine, then possibly better looking at an engine kit and exchanging yours in...

[Sprocket]

286 will be fine on the 1380. Give a nice torque band

[silky]

Sounds good - I've always wanted a good torque band. I take it that's the amount of 'pull' I'll get?

Dr Jammy - thanks for linkies. Will refer to my diary (the missus) and see if I can make the Kent run.

[Flat6]

Silky, I went to the MiniSpeed web site and looked up pistons. They appear to sell the AE P21253 pistons that MiniSpares sells. I didn't see the same price you did though. I found them for 155 GBP +VAT. They did have the P21251 pistons for about 85 GPB +VAT... are those the ones you were looking at?

Piston prices go up as you make the transition between cast, pressure cast, and forged. The P21253 pistons are pressure cast and I believe the P21251 pistons are simply cast. The pressure cast pistons are suitable for a mild performance build (I use them). The P21251 pistons are strictly for the street. I would not use them if you do start adding performance bits to your plans. The P21253s would be a better choice but they would not be suitable for a really high performance A-series engine. For that you'd want the much more expensive forged pistons.

I hope that helps a bit.

Just to clarify, the 21251 pistons and 21253's are both cast in EXACTLY the same fashion, using the same material. The manufacturing cost is very similar between the two. The mark-up is down to the distributors making the most of the the "better" 21253 piston

[dklawson]

Interesting.

I was advised to go for the 21253 pistons over the 21251s because they were processed/manufactured differently. I was specifically told the 21251 pistons were best suited to a street engine rebuild while the 21253 pistons were an entry level piston for slightly higher performance.

Were it only one distributor who was selling the 21253 pistons for a higher price I wouldn't question your statement but the fact that they all seem to sell the 21253 for more than the 21251 implies that the mark-up is from the manufacturer, not from the retailer.

[dklawson]

CORRECTION:

I inquired about the 21251 and 21253 pistons on another board I frequent. I received both catalog information and user testimonials.

My statement about different production materials was incorrect, there is no mention that the two piston types are anything other than cast (and they are probably cast with the same materials). They are both dished with an 8.4cc dish volume. What is different is the design of the two piston types.

The 21251 piston is designed to produce a compression ratio of up to about 8.8:1. They use a 4mm oil control ring and have a wrist pin height of 1.469".

The 21253 piston is designed to produce a compression ratio of up to about 10:1. They have a wrist pin height of 1.528". They use 3mm oil control rings and different drain-back slots design to reduce breakage.

I stand by my earlier comment that the 21253 is a more performance oriented piston than the 21251 but I apologize for implying that the two used different materials and methods of manufacture.

EDIT:
I got the following reply to my query on another message board. It made me chuckle. Have a good weekend guys.
" lots of knowledgeable Mini race engine builders believe that the 21253 is suitable for vintage racing at modest compression ratios and RPMs. Quite a number of vintage Mini racers have used the 21253 at one time or another. The 21253 should be more than enough for most street motors. But then, some people can break a hammer. "

Edited by dklawson, 02 November 2007 - 07:52 PM.

[Flat6]

Interesting.

I was advised to go for the 21253 pistons over the 21251s because they were processed/manufactured differently. I was specifically told the 21251 pistons were best suited to a street engine rebuild while the 21253 pistons were an entry level piston for slightly higher performance.

Were it only one distributor who was selling the 21253 pistons for a higher price I wouldn't question your statement but the fact that they all seem to sell the 21253 for more than the 21251 implies that the mark-up is from the manufacturer, not from the retailer.

I know what the manufacturing costs are, but see from your recent post you've spotted the design difference between the two. You're quite right in saying the 21253 is far superior, and a much better piston than a lot of people give it credit for. If you need any more info on these drop me a line, I've got access to all the drawings.