36 replies to this topic

[Echan42]

 

 

My head will be able to take un-leaded fuel fine when I install these valve guides, right?

 

 

You'll need new valve seats if it hasn't got hardened ones already.

 

You¡re absolutely right but I can't seem to find them on minispares, seems like the reference number is TAM2152

[MiniLuke]

 

 

 

My head will be able to take un-leaded fuel fine when I install these valve guides, right?

 

 

You'll need new valve seats if it hasn't got hardened ones already.

 

You¡re absolutely right but I can't seem to find them on minispares, seems like the reference number is TAM2152

 

I used Rimflow valves on mine and increased size from what I remember..... whilst cutting new seats you may as well

[Echan42]

I used Rimflow valves on mine and increased size from what I remember..... whilst cutting new seats you may as well

 

Thanks, I might as well consider Rimflow but I don't know if I'll increse the size sinde the 12g295 already has increased size from what my stock head had and I don't want to get too amicious! Thanks for pointing out the hardened seat missing from my shopping basket, will report when I find them!

[Cooperman]

To get a combustion chamber volume before the valves are lapped in, use some bathroom sealer lightly around the valve seats. Then measure using water once the sealant has gone off. In fact, you can do this even without the valve guides fitted.

[Echan42]

To get a combustion chamber volume before the valves are lapped in, use some bathroom sealer lightly around the valve seats. Then measure using water once the sealant has gone off. In fact, you can do this even without the valve guides fitted.

Yep. I was going to do this with mounting grease and hoping it holds the valves in place, thanks for the tip

[Turbo Phil]

The valve guides have nothing to do with being able to run the engine on unleaded fuel. You’ll require hardened exhaust seat inserts fitting to eliminate valve seat recession.
The trouble with the 295 heads is the valve seats are stupidly narrow from the factory and wear terribly. Add to the mix the age of these castings and you usually find they are pretty much knackered unless they’ve previously been overhauled.
Have you taken the valves out yet to inspect the seats ? Unless you’re lucky you’ll most likely find you’ll need to use oversize valves in order to be able to cut a decent seat, or have inserts fitted.

Phil.

[Echan42]

The valve guides have nothing to do with being able to run the engine on unleaded fuel. You’ll require hardened exhaust seat inserts fitting to eliminate valve seat recession.
The trouble with the 295 heads is the valve seats are stupidly narrow from the factory and wear terribly. Add to the mix the age of these castings and you usually find they are pretty much knackered unless they’ve previously been overhauled.
Have you taken the valves out yet to inspect the seats ? Unless you’re lucky you’ll most likely find you’ll need to use oversize valves in order to be able to cut a decent seat, or have inserts fitted.

Phil.

Thanks Phil, this is exactly the part of the build I'm stuck at right now, I was planning on buying valve seats and dropping them off at the machine shop.

Can valve seats be fitted independently of how knackered the head may be? Does their size need to be matched to the valve size?

I've narrowed these ones down that are for small bore heads and they claim to be compatible with cooper heads

http://www.minispare...|Back to search

 

Is it as easy as having the machine shop fit this "ring" and carry on with my build?

[hhhh]

I didn't see any mention of porting in this thread. The exhaust guide bosses especially are very restrictive in these heads and their removal will result in more power and better economy. For optimum economy, it's recommended to leave the inlet ports alone.

[Echan42]

Sorry for the lack of updates, my package got lost in Samoa...

Anyway while I'm waiting for that to come thru I read up on Tuning the A Series and I think I've decided on a 9,6:1 compression ratio since I'd worry about my engine overheating with a CR anymore close to 10. I don't know first hand how much of a factor overheating becomes with such little increments but I don't want another overheating cooper S lookalike mini, feel free to correct me.

 

Anyway, I used the calculator and got the following results

I'm assuming my piston deck height to be 10 since to my understanding the only way to measure it is removing the head and measuring with a digital dial, is there a standard measurement for stock 998s or does this value need to be measured due to factory differences and recess over time?

In that case I guess it's time I cracked that engine open, can I get an accurate mesurament with a caliper?

 

Once I get the piston deck height confirmed I can confirm if the final chamber volume of the 12g295 has to be 19ccs.

Cheers! And can't wait for that packet to arrive!

Edited by Echan42, 28 August 2019 - 09:28 PM.

[Echan42]

Also while where at it, the general assumption or recomendation when fitting a 12g295 to a standard 998 engine is to skim 80thou to achieve 10:1 compression.

I suspect this to not be the case since to my understanding every 40 thou you remove 2.92cc of chamber, so 80thou would remove 5.84cc, taking into account the standard chamber ccs for a 12g295 is 28.3cc, that comes out at 22.46ccs wich is just 2cc less than the standard 998 head.

 

For the sake of calculating let's assume the piston deck clearance is 0cc and Vizzard states the piston dish is 6cc, that would bring our compression ratio to 8.91, wich is definetly an upgrade, but far from 10:1, of course this changes when flat top pistons are involved,

So 80 thou seems to be the best amount to skim, but will not achieve 10:1 on a standard 998.

Edited by Echan42, 29 August 2019 - 10:38 AM.

[luismx123]

Also while where at it, the general assumption or recomendation when fitting a 12g295 to a standard 998 engine is to skim 80thou to achieve 10:1 compression.

I suspect this to not be the case since to my understanding every 40 thou you remove 2.92cc of chamber, so 80thou would remove 5.84cc, taking into account the standard chamber ccs for a 12g295 is 28.3cc, that comes out at 22.46ccs wich is just 2cc less than the standard 998 head.

 

For the sake of calculating let's assume the piston deck clearance is 0cc and Vizzard states the piston dish is 6cc, that would bring our compression ratio to 8.91, wich is definetly an upgrade, but far from 10:1, of course this changes when flat top pistons are involved,

So 80 thou seems to be the best amount to skim, but will not achieve 10:1 on a standard 998.

 

please, please stop assuming things. Have you ever done any work on a mini? no one side is the same as the other, no mini the same as another. You need to measure youre chambers, measure the piston to deck clearances and then recalculate. Doesn't help to assume anything.... im sure this has been said multiple times above too

[gazza82]

And who says that head hasn't already been skimmed?

[Echan42]

And who says that head hasn't already been skimmed?

Edited by Echan42, 26 October 2020 - 08:17 PM.

[Echan42]

please, please stop assuming things. Have you ever done any work on a mini? no one side is the same as the other, no mini the same as another. You need to measure youre chambers, measure the piston to deck clearances and then recalculate. Doesn't help to assume anything.... im sure this has been said multiple times above too

 

We're on the same team here, I was adressing everybody that has previously suggested skimming 80thou because "that's what my mate dave did and he runs 10:1 and gained 60hp" by crunching my own numbers and proving that even with a hypothetical standard measurement those numbers don't check out.

Also, multiple times above too, me taking measurements and crunching the my own numbers.

[Dusky]

How did you calculate the 40 thou 2.92cc measurement?
I skimmed a 295 40thou from stock to get 10.1 worh flattops. ( 42 thou actually but hey)