23 replies to this topic

[79Clubman]

Hi, All

My plans to use a 12g295 were just snookered, as it has some cracks, so it's becoming a future project.

What do I need to know about fitting a 12g940 to the 1098?

Cheers

Edit: it does have the following cam-

Type 510
@015º 257/270º
@050º 205/216º
Cam lift 250/265”
Valve lift 325/344”

Or, should I stick with the 12g202?

Edited by 79Clubman, 09 December 2011 - 11:03 PM.

[andymini12]

To put the 240 on a 998 you need to get the block pocketed, this is where they cut 'pockets' in the block for the exhaust valve so it does not hit the block.
EDIT: i put 998 but the 1098 block is the same.

Edited by andymini12, 09 December 2011 - 11:47 PM.

[bmcecosse]

You don't need pockets - this has been covered over and over again. You measure the distance from valve top to head face - if it's less than 320 thou you simply sink the valves into the head until you get that 320 thou. Often the head will already have sufficient recession to give the necessary clearance without you doing anything! It's a great conversion - FAR better than the 295.........

[79Clubman]

@andymini12

Thanks, I'll see how I go with @bmcecosse's measurements (from that I can at least see what clearance I need, either way).

@bmcecosse

I've seen the 320" mentioned, but I wasn't sure how my cam would affect that (valve lift- as above- 325/344”)... any advice?

Cheers

[Brams96]

You measure the distance from valve top to head face - if it's less than 320 thou you simply sink the valves into the head until you get that 320 thou.

bmcecosse - Sorry to be a pain but do you mean with the valve fully open, could you please illustrate where to measure on a pic or diagram?

[Cooperman]

From the valve fully closed you need to establish that the amount the cam opens the valve at full lift is less then the dimension from the valve face when fully closed to the level of the face of the head. Just put a straight edge across the combustion chamber and measure (accurately) how far it is to the valve face(s). If it is slightly less than maximum valve lift with your cam you are OK. If it is no more than 0.040" greater you can recess the valve seats into the head a bit more. Any more than 0.040" and you will need to pocket the block.
This allows the gasket thickness as a tolerance for safety.
By the way, the 510 is a great cam in a 1098. It was originally a Cooper 'S' production cam.

[Cooperman]

You'll need to open up the chambers if fitting to a 1098, standard 940 chamber is 21.4cc's. In a 1098 your gonna end up with a compression ratio a little under 11:1.

Depends on which pistons you fit, flat-top or dished. You really need to measure and calculate, then adapt to give what you want.

[Cooperman]

Be quite high with flat-tops and block decked to bring pistons right to the top!

[Cooperman]

With any non-standard engine build or modification, if a head change or piston change is carried it is always necessary to measure and calculate in respect of compression ratio. With respect, if you can't do this you really should not do the modifications without first learning what is involved, by asking on here how to do it or reading up on it off the web or from the right books, which is exactly what the OP is doing with this thread.
There should really be a comp ratio calculation process as a 'sticky' on here.

[wile e coyote]

With any non-standard engine build or modification, if a head change or piston change is carried it is always necessary to measure and calculate in respect of compression ratio. With respect, if you can't do this you really should not do the modifications without first learning what is involved, by asking on here how to do it or reading up on it off the web or from the right books, which is exactly what the OP is doing with this thread.
There should really be a comp ratio calculation process as a 'sticky' on here.

This do? - best one I've found...
http://www.rbracing-...staticcalc.html

[79Clubman]

Thank you for the comments.

Pistons are dished, bore is 60" oversized.

While I'm prepared to pocket the block, I'm not so sure that modifying the combustion chamber is a good idea.

It therefore sounds like I should stick with the 202, especially as I was aiming for about 9.1:1 CR.

I have access to (and assistance available with) proffessional cc'ing equipment.

Sounds like the 202 will have potentially less problems- comment?

Cheers

[Brams96]

Isn't the 202 a leaded head? or have you converted it?

[bmcecosse]

940 head is a FAR FAR better head - go for it - sinking the exhaust valves into the head by say 40 thou will drop the CR nicely.

[79Clubman]

@brams96 and @bmcecosse

The 202's I have are older and need hardened exhaust inserts to run unleaded (which is all we get in New Zealand and I'm not keen on additives all the time).

@bmcecosse

With putting inserts in the 940, I'm not sure that there's sufficient "meat" to recess the valves by about 40". Correct me if I'm wrong, but, with the 940, I'll have to measure the valve lift, recess/pocket to get the required clearance and the results to get my desired cc? With the info I've provided, any ideas on the most likely outcome?

Thank you all once again.

Edited by 79Clubman, 11 December 2011 - 09:15 PM.

[Brams96]

79Clubman - Where in NZ are you from? I'm originally a 'Naki boy now residing in Canterbury, UK. There are a few clubs out there so maybe get in touch with them for some local knowledge. The 940 shouldn't need inserts as it should be an unleaded head already. You could always keep your eye on ebay for a 12G940 from over here & get it shipped out, I've looked on TreadMe & there doesn't seem to be very many classic mini parts at all.