12 replies to this topic

[GreaseMonkey]

Hi,

I'm considering using +20 21253 pistons for a 1275 rebuild with a SW5 cam and a Minispares road weight flywheel (slightly lightened I think) Or may stick with a verto clutch?

I've heard good things about these pistons however I've also heard that the ring gaps can be large and they dont seal as well as the Mega pistons?

The head to go onto the engine once the rebuild is done is a normal 1275 head, from what I can see, it has been ported very slightly I think but dont think any work has been done to the chambers.

Would I be able to build the engine up and just put the head on and run the engine or do I have to check the compression ratios and so on? I know the 21253 is a high compression piston.

Can anyone rate the pistons? Or the flywheel?

Sorry for the vague questions and understanding as I'm not too clear on compression ratios and so on

Many thanks

[bmcecosse]

It will be fine - just use them! Check on the head - all heads start off as 3.750" thick. If it's any less - it has been skimmed!

[Cooperman]

A strictly personal view, but the 21253 seems to be a really excellent value piston. I have built two 1275 engines for the Endurance Rally Series with these. Both these engines gave excellent performance with minimal oil consumption. In fact, on the one I used a journalist asked me how the car was going and I said that after about 800 miles I was worried about the new engine as it had used no oil to speak of!
The ring gaps usually come up at about 0.012", which is fine. If the gap is larger than this, then suspect that the bore is slightly oversize.
All-in-all, the 21253 is an excellent value piston, capable of high revs and low oil burn. In a full race or rally spec at C.R.'s of over 11:1 and with over 110 bhp, then I would be less confident at sustained revs of over 6500, but that's an extreme and to do this regularly would mean the need for a top-grade piston like a Karl Schmidt, at over 3 times the price of the 21253.
With regard to C.R. calcs, you really should always do this in case the CR is too low and you end up disappointed.
At +0.020" overbore, if the pistons come right to the top of the block, or within about 0.005" of the top, then the CR with the 21253 would be in the region of 9.5:1 with a standard 21.4 cc chamber head, which is a bit low if top performance is wanted. It's easy to check CR (PM me if you want more info on this) and it's cheap to have the head skimmed to the right amount.

Peter

[GreaseMonkey]

Many thanks, really informative

The engine has yet to be stripped as I'm still wondering whether to rebuild the 1275 or stick another engine in

But I know the pistons are standard as it has the 21962 marking which comes up as a standard metro piston so I assume the engine is standard and the block has the A stamp in it so it hasnt been skimmed or refaced.

When taking the engine for a rebore, should I ask to have the block refaced by a minimal value?

I think I will go for the 21253 if I decide to rebuild this engine as I've heard nothing but good about it

Edited by GreaseMonkey, 25 August 2009 - 10:01 PM.

[Cooperman]

What you do is to strip the engine and have it bored to the next size up which, as you say, will be +0.020" making it 1293 cc. When the old pistons are pressed off the rods, ask for the old gudgeon pins to be saved. Have them linished down by about 0.0005" so that they go through the new pistons with a very light push. After the boring is done together with any crank re-grinding, do a 'trial build' of the engine by using the linished down pins. Don't tighten the mains or big-ends right up, just lightly 'nip' them. Bring the trial engine to TDC and measure, with a straight edge and feelers the amount the top of the pistons are down the bore from the blolck deck. In fact, it's best to carefully remove the piston rings whilst doing this, and you can 'gap' the rings at the same time. The rings must go back in the correct positions and, where applicable (definately ring no. 2) the right way up. Keep the rings stored as sets for each piston.
Strip the engine and have the block 'decked' so that the pistons come right to the top of the bores. Then the CR can be calculated and the head skimmed to suit what you want. A good comp ratio with a 21253 to give good power and torque is about 10.4:1. At this time you can also check the crankshaft float when new thrust bearings are trial fitted. If the float with standard thrusts is too great, fit a set of +0.003" thrusts on one side, or on both sides if the float is 0.008" or more. The ideal (sorry 'bmcecosse', I know you differ on this ) is 0.002" to 0.007"
Finally clean the block, making sure to remove the oil way plugs to ensue the oil ways are clean and free of machining swarf.
Assemble the block, crank & pistons, with a new Metro Turbo oil pump (they're the best now) and new timing chain, etc, and 'jobsagudun'. It takes time, but if you then complete the build by timing in the cam accurately you'll have a super engine which will go well not use much oil and be reliable.
Save the linished gudgeon pins for the next time!

I hope this helps,

Peter

Edited by Cooperman, 25 August 2009 - 10:26 PM.

[bmcecosse]

I certainly think 2 thou is FAR too little - but 5 to 7 thou is fine !

[Cooperman]

The BMC Workshop Manual says, End Float: 0.001" to 0.005"
I guess all engine builders work to their own developed figures and what works for them is usually fine.

[mini7boy]

The BMC Workshop Manual says, End Float: 0.001" to 0.005"
I guess all engine builders work to their own developed figures and what works for them is usually fine.

the 21253 are good pistons for road us and some people race them. If racing them, the RPMs need to be limited to about 7000-7500 and the compression to about 12:1

As far as finding out your compression ratio is concerned, you really should do the math and use a burette to accurately measure the chamber volume.
By measuring everything, you'll find out exactly what YOUR engine has for a compression ratio. This is not a good idea to use the guesstimates of others
as it's just too important in terms of engine performance and longevity.

Compression ratio is important and you need to match the compression ratio with the cam profile being run.

Turbo oil pumps are overkill and end up pumping a lot of oil right back into the sump. If you have one, use it. But otherwise buy a standard pump.

Here are some words in a reply by AC Dodd on a forum to a reader asking about calculating the comp-ratio for an sw5 in a 1275.
This reply was found on the forum "Ask Doddy" and the thread's title is "CR calculation for SW5 cam".

Dodd said:

Re: CR calculation for SW5 cam

(22/01/07 22:10)

OK Yes I changed my calcs and have got the same figure.

So enlarge the chambers to 26.4cc to get 9.75:1.

Anyway running 10.9:1 CR with this cam can mean melted pistons and a wrecked engine. Basically you are in to detonation territory!!!

People and some engine builders still run into the mistake and use too much CR. I have found with the pump fuels we have now the secret to the best performance is to use a lower CR coupled with more ignition advance.

'An engine that has a little too much CR, and has the ignition timing optimised to run with this, will run worse than an engine that perhaps has had the CR set little too low and optimised ignition timing'.

Using a thick head gasket has a detrimental effect on squish. Basically increasing the unswept space above the piston does increase the chance of knock. Use a standard head gasket and open the chamber.

AC

PS I have not built a road engine with over 10:1 CR for about 4 years!!!

[Mart8V]

I've just run in my 1330 with 21253s. I did not take a particularly scientific approach and am not an experienced engine builder but have been very happy with the result so far. The engine burns virtually no oil, is producing 73bhp at the fly with a pretty much standard set up, and most importantly the pistons were good value!

Just for info, I am running an MG Cam and a Verto Turbo clutch- seems to handle the power perfectly well. With an SW5 and lightened fly you should get a higher BHP figure but if on a budget the Turbo clutch seems to do the job nicely. (And is very cheap!!)

Good luck!

[GreaseMonkey]

Thats great thanks Mart, I was thinking of sticking with the Verto clutch and putting a turbo clutch plate in but I'll see, the road weight flywheel from minispares is lightened by 8lb and is is the 3 piece diaphragm type

"Therefore by lightening the flywheel by 8 lb, the engine sees a total reduction of the cars accelerative weight of 89.68 lb"

Not sure how much of a difference that would make to performance? I may stick with a verto setup with a turbo clutch plate...

Edited by GreaseMonkey, 26 August 2009 - 02:37 PM.

[Mart8V]

As another option, Guessworks offered to lighten a Verto fly for me at £50 plus postage a while back. I was strapped so did not take him up on the offer but may well go with it in future. No shortage of useable flywheels to choose from! That way you can stick with the Verto set up and the Turbo plate.

Just a thought......

[Turbo Nick]

i run 21253's in my motor with an 11cc dish macined into them giving me 8.7:1cr, they've stood up to 163hp so far over 30k miles with no issues at all. going to give them a try at ~200hp in the next couple of weeks.

They get my vote as a good budget piston.

[Cooperman]

We all seem to agree that the 21253 is an excellent piston and good value compasred to other pistons.