36 replies to this topic

[Echan42]

 12G202

 

Edited by Echan42, 26 October 2020 - 08:15 PM.

[Its a min]

I asked the same question here.

 

http://www.theminifo...pression-ratio/

[Its a min]

9.6:1 compression ratio appeared to be the recommendation unless it's highly tuned.

[Echan42]

9.6:1 compression ratio appeared to be the recommendation unless it's highly tuned.

Nice, I think I'll try and run those numbers, how did your 295 head go? Any words of advice?

[Its a min]

 

9.6:1 compression ratio appeared to be the recommendation unless it's highly tuned.

Nice, I think I'll try and run those numbers, how did your 295 head go? Any words of advice?

 

Hi Echan42,

 

Unfortunately, I've not had time to do anything with it except give it a good clean and keep it oiled up. I'm hoping to get it sorted in the near future but keep me informed how your's goes.

[Echan42]

Where can I find reliable mesuraments for the 998? I'm trying to run the numbers myself and not rely on someone else to run the numbers?

I'm trying to achieve 9.5CR on a stock 998 with dished pistons, I'm guessing 60thou or close.

I'd like to measure TDC and my engine's bore but my engine is currently mounted, can someone confirm if Vizzard's book would have the internal measurements for the 998 + dished pisons so I can order it and re run my calculations?

 

Edit:

https://www.mmoc.org...pic.php?t=47667

I'm seeing other people trim them down 60thou so I'm feeling pretty confident.

Edited by Echan42, 12 August 2019 - 11:47 AM.

[Retroman]

You can't acheive what compression you want by guessing or assuming anything

 

The only reliable place for measurements is your own engine

 

...The 295 will probably have been skimmed and might have chamber work / recessed valves, anything is possible

 

Get new seats / valves / guides...then measure cc's....crunch numbers and skimming is the final job, not the first

 

https://jscalc.io/ca...RNEglVkKLQir11T

[Turbo Phil]

Completely agree with the above. The factory stated chamber volume is only a nominal value and you’ll find the seat heights can vary quite a lot which can affect volume considerably. On a used head this can be even greater.
As suggested above, measure, measure, measure !

Phil.

[DeadSquare]

I recently picked up a 12G295 real cheap on my local Craiglist alternative so I guess it's time to get to work!

I read up on previous threads that either never followed up or where simply missing the information I need.

I'm currently making my shopping list before I visit the machine shop, and I'm almost ready to add all this stuff to the basket.

• New studs (maybe ARP?)
• Small bore copper gasket
• Stainless steam thermo and hear gaskets
• Regular studs for exhaust and intake
• Lead free valve seat inserts
• Valve guides (not sure wich ones)
• Valves
• Valve stem seals
• And just for the sake of doing the job right a new set of springs

 

 

 

I'm pretty excited to catalog my head restoration and can't wait to update this thread with the process!

 

You are not going racing;  Standard head studs will do just fine, in fact, I'd say that your studs will do OK.  They don't wear out.

 

Strip and examine the rocker gear, this does wear.

[Retroman]

'They don't wear out' 

 

No but they can and do stretch every time they are torqued and can do go like egg timers

[Dusky]

'They don't wear out'

No but they can and do stretch every time they are torqued and can do go like egg timers

Should be elastic deformation, not plastic. A torque wrench helps here ?

[Cooperman]

As above, the only way to do this job properly is by accurate measuring followed by calculations, more measuring, check calculations and finally machining correctly. That is what proper engine building is all about and is also why a commercially done engine build is never cheap.

 

With regard to compression ratio, the old A-Series does like a decent compression ratio and so long as 97/98 RON petrol is used with a correctly-curved distributor a ratio of around 10:1 has always given good results. For CR's higher than about 10.5:1 an octane booster is usually needed as well as 98 RON petrol. I know my Cooper 'S' with 11.1:1 CR needs booster/lead-additive in order to run properly. (An alternative to 98 RON with booster/lead-additive is 100LL aviation fuel, but that is not cheap, although I do sometimes use this).

 

Head studs - fit new standard ones from Mini Spares. They are not expensive, but they do stretch over time.

[Retroman]

 

'They don't wear out'

No but they can and do stretch every time they are torqued and can do go like egg timers

Should be elastic deformation, not plastic. A torque wrench helps here

 

 

Yes it should be elastic but they are stretched every time and the more times they have been torqued and the more heat cycles the more they will have stretched. If someone has over torqued them this will be worse, some can be horrendus, on the verge of stripping and will just continue to stretch when torqued. ARP conrod bolts should be measured for stretch, but very few do.

 

It even happens on main bearing bolts / studs....you can just see the 'egg timer'  on the back main stud in the photo

Dread to think what would have happened if I had missed that

[Echan42]

Updates!

 

Gave it a nice clean and it's looking quite nice

 

Machine shops in my area insisted they can't help me in any way except milling, measuring and calculating combustion chambers is not a service they provide.

Tried to take my own measurements, results inconclusive, I'll have to wait for my mini spares order and my new valves since any liquid I poured in any chamber didn't hold and just leaked out the exhaust...

A bourette and a clear piece of plastic is the best way to measure cc's in this case, right?

 

Also my order is nearly ready, can someone proof read my shopping basket and confirm I have everything I need? I uprated the springs since the upgrade was only 4GBP will I need new or upgraded pushrods? The ones I pulled from the head look good and are not warped or even slightly bent, I intend on re-using the valve cotters.

Also I'll be adding engine studs to my order after reviewing these last few messages, since I've never opened this engine and I don't what condition they will be in.

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

 

Also please don't missinterpret my excitement with this project for willingness to cut corners, I fully intend to measure twice cut once!

Hopefully I'll finish the order asap so I can measure my head's cc's  with the new valves fitted.

 

Edit: forgot to mention I have the head gasket and stems from a gasket kit I bought previously

Edited by Echan42, 15 August 2019 - 12:47 PM.

[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.