17 replies to this topic

[Stiggytoo]

So, I thought it would be a good idea to start this topic; in order to get a bit of advice, but also to share the information with anyone who is thinking of doing something like this in the near future.
 
I recently decided I wanted a bit more power from my 998 Mini City. Obviously a 1275 is the way to go, but I wanted to keep the charm and originality of a 998 (plus 1275s are expensive) so that's the direction I took. After finding three 12G940 heads kicking about at my dads' lock-up, I saw this as a subtle sign that the A-series Gods wanted me to use one...
 
Now, I have read that that to be on the safe side I would either need to recess the valves or pocket the block in order to get the necessary clearance on the exhaust valves, especially if I wanted to use a higher lift cam. Somewhere or other I read that recessing the valves could help to improve efficiency or power/torque, by essentially making it more difficult for intake gases to escape into the exhaust port during valve overlap, as it would have to travel further. Is there any truth to this? Not important really as I purchased a pocketed engine with gearbox for £50.      
 
A few weeks ago I picked up my newly machined A+ block from Roland Alsop just down the road in Chobham.
Here's what he's done:

• Re-bored to +20thou
• Refaced block
• Camshaft bearings
• Freshly re-profiled the original camshaft to early MG Metro spec.
• Opened up pockets as they were done on the piss (Must have been done either under the influence of alcohol or upside down - who knows.
• Ground crankshaft.

Also got a few bits from him including flat top pistons, bearings and cam followers etc.

 

Additional things that I have accumulated include a custom made exhaust manifold with twin box rc40 system, inlet manifold, oil pump etc.

 

 

Now my thoughts have turned to which head I should use, and what (if any) modifications I should do to them. Two of the heads have 33mm ish inlet valves. The other has 36mm ish inlets. Possibly a turbo head, but I will have to check. My first thought is that on a road going engine are these larger valves just going to lower the torque output of the engine? Would I be better off with the smaller valve head?  

Secondly, what kind of compression ratio should I be aiming for? Somewhere around 10:1?

Would you recommend porting it? I will be reconditioning whichever head I use.

 

At a later date I will also need to consider what carb to use. Would an Hs4 be too small for this application. I think I have an Hif 44 laying about.  

 

 

Here are a few pics of the engine. Needs a lick of paint, best colour suggestion gets a free thumbs up!

 

 DSCF1486.JPG   32.17K   26 downloads

 DSCF1481.JPG   24.04K   16 downloads

Attached Files

•  2013-07-01 23.47.11.jpg   71.3K   12 downloads

Edited by Stiggytoo, 12 September 2013 - 07:02 PM.

[IainStallard]

Coat the whole block in oil to protect it whilst it's not built up, otherwise it may start to surface rust

[Turbo Phil]

Go with the smaller valved {33-29} head. Don't enlarge the inlet ports to any great degree, just a simple clean up, though the ex ports can benefit from a little enlarging. Remove the guide boss on both the inlet & exhaust ports. 3 angle the seats, back cut the valves to narrow the seat width to suit & bulletnose the guides. The chambers can be cleaned up & cut back a little to deshroud the valves,the small valves don't require to much. 
This will give you a decent working head for your spec.

Phil.

Edited by Turbo Phil, 12 September 2013 - 10:26 PM.

[IainStallard]

Oh and red paint always gives more power!

Don't forget to check the gearbox over and maybe upgrade to a x-pin diff for strength and reliability

[Stiggytoo]

Oh and red paint always gives more power!

Don't forget to check the gearbox over and maybe upgrade to a x-pin diff for strength and reliability

Checked over and replaced a few bits in the gearbox. Happens to have a central pickup.

[Stiggytoo]

I've already made a wanted ad for these, but I need some press fit 998 conrods as I have been supplied with the wrong pistons. Machining has been done now so don't really want to send them back. If you have, or know of someone who is likely to have some, please let me know! Thanks.

[Cooperman]

Around 10:1 C.R. is a good figure to set it to. That way you can use 97 Oct petrol without the need for an octane booster, although if the seats are for leaded fuel, a dose of Castrol Valvemaster or similar at each re-fuelling will help to preserve the valve seats.
The HS4 carb will be good on a decent alloy inlet manifold. The HIF44 will probably be too large for a road 998 and would only work at mega-high revs.
You might want to consider a better distributor with a more optimum advance curve. Aldon Automotive could help you there.

[Stiggytoo]

Block painted and crank in. Is 7thou end float out of the question?

[albo]

0.0015" to 0.002

yours may be ok if you have a long left leg

[ACDodd]

Hehehe I too am just starting a build for a1014cc fast road engine using a 12G206 head ported and using 1275 valves. Can't beat the small ports to make the most of a hot 998 based engine.

Ac

[Stiggytoo]

Am I right in thinking the crankshaft end float should be between 0.001 - 0.005 in? Or is this the pre a+ measurement? Whilst I'm at it, should the main bearing bolts be 63lb ft?

I've seen people put grease behind the thrusts, does this in any way affect the end float?

Finally, are +0.003 oversize thrusts +0.003 overall or each?

 

Thanks guys.

[ACDodd]

If you fit a set of 4 +0.003" thrusts it will take up 0.006" of clearance. Aim for 0.002" to 0.003" of running clearance.
People grease them to hold them in. I use engine oil during assembly. When all bolted up rap the crank at each end sharply with a bronze mallet. This will 'seat' the new thrusts and then you take your readings.

Ac

Edited by ACDodd, 05 October 2013 - 09:29 AM.

[Stiggytoo]

I've spoken to the engine builder who's been on the scene for a while. He said that anything upto 12 thou would realistically be ok. Not sure if he's going senile though.

[Matt1293]

Might be worth you assembling the whole thing and turning it over by hand to check clearances first.

[Cooperman]

I've spoken to the engine builder who's been on the scene for a while. He said that anything upto 12 thou would realistically be ok. Not sure if he's going senile though.

12 thou is most definitely not OK - far from it!

Most engine builders will look for 0.0015" to 0.004" from new and will re-build if it gets to 0.007".

It can be difficult to get the clutch to operate at large end float figures, apart from anything else.