13 replies to this topic

[jpc]

Hey,
I was just wondering if anyone could tell me if the older A series dished pistons will fit into my 86 mayfair, whick i think has flat tops, and if they can be easily sourced.

Edited by jpc, 27 April 2011 - 09:10 PM.

[Ethel]

Yes they can, don't think an 86 Mayfair will have flat tops though - look up your engine number http://www.guess-wor...Tech/engine.htm

[Wil_h]

I think in '86 there was a low and high compression engine, all the '86 engines I have had had flattops.

There is no consistance in A+ 998 engines, with different pistons and rods seemingly randomly used, I also have at least 3 (maybe 4) different castings, some linered.

The dished 20773 piston is still available from Minispares. do you have interference or push-fit conrods?

[jpc]

I havnt a clue to tell u the truth because the engine is still in one piece, ill be headin home the weekend so ill get the engine number and try the site ethel left a link to. Any ideas what cc the dish is on the minispares pistons are and if they r the same as the origional mini ones.

[Wil_h]

the 20773 has a 5.6cc dish iirc.

If I were you I'd use flat tops and keep the CR at 10:1, it will make a fare more efficient engine, with no downsides.

[Ethel]

I'd assumed hew particularly wanted dished pistons for a reason

I think Mayfairs used used the low compression engine, the high compression one was in the City E.

[jpc]

I probly should have explained my situation a bit better. I am looking to get more power out of my 998 and was thinking of going down the turbo route but not too sure yet , i am suposed to look at a 12g295 head this week but the fela that owns it doesnt know if it was skimmed or not so im wonderin if it is skimmed could i use dished pistons from an older mini to bring the CR down.(thats asuming theres flat tops in my 998). Any opinions on a turbo 998- good idea/bad idea. Want to keep it as reliable as i can.

[Wil_h]

Now I can give you a propper answer. Depending on how skimmed the head is will decide what pistons to use.

To get a CR of a little over 9:1 with flat tops you need 28cc in the head. So for every cc you lose in the head (owing to skimming) you'll need to find in the piston.

On one of my 998 turbos (daily drive) I have a 295 with 26cc chambers, I then bough some minispares mega pistons (flat top) and had a 2cc dish machined in them. I have had up to 6cc taken out of these pistons, but the smaller the dish the better.

If the head has really been skimmed, you could use the 20773 piston, they are strong enough for decent power, but a bit old and heavy, I would be looking for a more suitable head, rather than use the 20773.

As for reliability, as long as you are sensible with boost and have good fuelling and ingnition control, they are as good as any mini. I have had mine as a daily for 12 months and even went to Le Mans in it, still going strong.

[jpc]

So what your saying is to pray that the head isnt skimmed. Will the standard flat tops do and can they be machines a couple of cc. I dont plan on going mad with boost but dont want the engine to go boom either.

The man who owns the 295 doesnt know much about it as he bought it and never used it. Its meant to be pressure tested by the previous owner buts thats about all i know. Im assuming it doesnt have the hardned valve seats and by the sounds of it was never rebult either but the owner says its ready to bolt on. I dont know much about heads so any tips on how to check the condition of it .If its in good enough condition would it be suitable for a turbo build without a rebuild

[Wil_h]

Yes you could always get a few cc machined in the standard flattops, I've never done it so check how thick the crowns are first. With 9:1 CR 12 psi should be no problem.

I would always use decent stainless exhaust valves in a turbo as they do get hot. I'm no head expert though and sen my heads to those who know better than me, turbo phil off here knows his onions regarding what valves to use in the 295 head.

All my 998 heads (weather 295 of 940) have 33 inlet and 29 exhaust valves, this seems optimal. My current 998 turbo was on the rollers last week and made 178bhp, so there is no need to go bigger.

[jpc]

I gave a look at that link Ethel left but i doest seem to tell me what pistons i have. This is what i got.

Source Vehicle : Mini
Engine type & CC : A+ 998 cc Transverse mounted
Fueling : HS4 FZX1415 with Tamperproof Capstat, AAC Needle, Red Spring
Engine features :
1st Alternative compression ratio
Pre-engaged starter
ADO 20 Design variants
Gearbox features :
3.105:1 Final Drive

I also went to look at the 12g295 and ended up buying it cause they are like hens teeth round me and seems in good condition but have to inspect it another bit. Not too sure if all the valves were changed but the exhaust valves seem to be sitting a bit higher than the inlet, is this the way they are meant to be, also 3 of them have an N on them but the other doesnt so im assuming some were changed . i tried to check the cc of the chamber but seem to be getting inconsistent results so i must get a better thing for measuring the liquid but it seems to be around the 28cc mark.

[jpc]

Anyone know how i can find out what pistons i have in my 998 engine (dished or flat tops)

[lrostoke]

Long shot thinking out loud, take a plug out, bring piston up to top and shine a light in. maybe able to make out if flat or dished

[jpc]

Ha, I never thought of doing that. i wont be home until the weekend so cant check it til then but ill give it a shot. Cheers