998 engine 1275 head
Discussion
It's strange really. In the 60's, when I first started building engines, flat-top pistons with solid skirts were regarded as the d-b's. Now, of course, it has been demonstrated that a 'bowl-in' design gives the better burn characteristics.
With the 998 you have a bit of a problem with heads, as has been stated. The 295 head is undoubtedly the best option as it's easier to get a decent chamber shape and still get the right c.r. The 998 Coopers had 'D' top pistons, but these are unavailable and that is why it can pay to allow the piston to come proud of the block deck by a few thou.
Dished pistons are best, but be prepared to have to do the bit of welding on the head if the oil way breaks through. I would only recommend these for full race units.
The question for a road engine is always whether to trade off c.r. for gas flow, especially if you can't obtain a 295 and have to use the standard basic casting. After you grind out the standard 998 head casting the c.r becomes very low and lots needs to be skimmed off. Making sure the pistons come right to the top or slightly proud is an essential thing.
With race engines it's a differnt matter as long-term reliability and low end torque as not an issue.
Boring out to +0.060 is a help in getting the c.r. up, so long as any competition regs allow this.
I have a spare 295 casting which I recently bought. I intended to gas flow it, fit some bigger valves, etc, for fitting to an historic rally Sprite with a 1098 cc engine. Alternatively I might just sell it as a bare casting.
With the 998 you have a bit of a problem with heads, as has been stated. The 295 head is undoubtedly the best option as it's easier to get a decent chamber shape and still get the right c.r. The 998 Coopers had 'D' top pistons, but these are unavailable and that is why it can pay to allow the piston to come proud of the block deck by a few thou.
Dished pistons are best, but be prepared to have to do the bit of welding on the head if the oil way breaks through. I would only recommend these for full race units.
The question for a road engine is always whether to trade off c.r. for gas flow, especially if you can't obtain a 295 and have to use the standard basic casting. After you grind out the standard 998 head casting the c.r becomes very low and lots needs to be skimmed off. Making sure the pistons come right to the top or slightly proud is an essential thing.
With race engines it's a differnt matter as long-term reliability and low end torque as not an issue.
Boring out to +0.060 is a help in getting the c.r. up, so long as any competition regs allow this.
I have a spare 295 casting which I recently bought. I intended to gas flow it, fit some bigger valves, etc, for fitting to an historic rally Sprite with a 1098 cc engine. Alternatively I might just sell it as a bare casting.
Cooperman said:
It's strange really. In the 60's, when I first started building engines, flat-top pistons with solid skirts were regarded as the d-b's. Now, of course, it has been demonstrated that a 'bowl-in' design gives the better burn characteristics.
With the 998 you have a bit of a problem with heads, as has been stated. The 295 head is undoubtedly the best option as it's easier to get a decent chamber shape and still get the right c.r. The 998 Coopers had 'D' top pistons, but these are unavailable and that is why it can pay to allow the piston to come proud of the block deck by a few thou.
Dished pistons are best, but be prepared to have to do the bit of welding on the head if the oil way breaks through. I would only recommend these for full race units.
The question for a road engine is always whether to trade off c.r. for gas flow, especially if you can't obtain a 295 and have to use the standard basic casting. After you grind out the standard 998 head casting the c.r becomes very low and lots needs to be skimmed off. Making sure the pistons come right to the top or slightly proud is an essential thing.
With race engines it's a differnt matter as long-term reliability and low end torque as not an issue.
Boring out to +0.060 is a help in getting the c.r. up, so long as any competition regs allow this.
I have a spare 295 casting which I recently bought. I intended to gas flow it, fit some bigger valves, etc, for fitting to an historic rally Sprite with a 1098 cc engine. Alternatively I might just sell it as a bare casting.
I think you've got the wrong idea from my message cooperman regarding flow/cr etc. However without doubt, for originality the 12G295 casting is the head to fit, but when looking for the best in performance then the 1300 type casting has to be used in order to fir a 1.406" (or larger)inlet valve and a 1.215" exhaust. If you choose to let the pistons out of the block be carefull to make sure you allow .030" between head and piston if you intend running the motor beyond 7400 rpm ( dont forget to allow for the odd missed gear
P.S. sorry about the imperial sizes cannot get my head around metric valve sizes
Kind Regards
Rob
Hi Rob,
I do take your point about how far to allow the piston crowns to project above the block. I really would never like to see much over about 0.004". What bI am really saying is that it always surprises me that some people can actually build an engine with a skimmed head, all nicely opened up and ported, with good quality pistons, and then when I look at it, for whatever reason, the piston crowns are sitting about 0.020" down from the deck at TDC. I've seen this many times, not just the odd once or twice. Maybe you've had the same experience.
I also do take your point about the need for a 940 casting in order to get the best valve sizes for higher revving 998's. My experience is mainly with rally engines, as that's my sport and has been for 45 years (oh, dear, that's showing my age again!), so driveability is an issue as is durability. I did move away from 998's a long time ago and all my recent builds have been on 'S' and 1275 blocks.
All the best,
Peter
I do take your point about how far to allow the piston crowns to project above the block. I really would never like to see much over about 0.004". What bI am really saying is that it always surprises me that some people can actually build an engine with a skimmed head, all nicely opened up and ported, with good quality pistons, and then when I look at it, for whatever reason, the piston crowns are sitting about 0.020" down from the deck at TDC. I've seen this many times, not just the odd once or twice. Maybe you've had the same experience.
I also do take your point about the need for a 940 casting in order to get the best valve sizes for higher revving 998's. My experience is mainly with rally engines, as that's my sport and has been for 45 years (oh, dear, that's showing my age again!), so driveability is an issue as is durability. I did move away from 998's a long time ago and all my recent builds have been on 'S' and 1275 blocks.
All the best,
Peter
Hi Peter,
As the Americans say there is no substitute for cubes! If you can use up to 1300cc it must be the best choice, and an S block assembley in standard form is almost bullet proof, so I can see why you prefer it for rallying. We have had our own flow bench and dyno for 20 years and still like messing about with Mini engines we are currently using 5mm stem valves with excellent results, it must be the best fun car ever made
As the Americans say there is no substitute for cubes! If you can use up to 1300cc it must be the best choice, and an S block assembley in standard form is almost bullet proof, so I can see why you prefer it for rallying. We have had our own flow bench and dyno for 20 years and still like messing about with Mini engines we are currently using 5mm stem valves with excellent results, it must be the best fun car ever made
selbymsport said:
Hi Peter,
As the Americans say there is no substitute for cubes! If you can use up to 1300cc it must be the best choice, and an S block assembley in standard form is almost bullet proof, so I can see why you prefer it for rallying. We have had our own flow bench and dyno for 20 years and still like messing about with Mini engines we are currently using 5mm stem valves with excellent results, it must be the best fun car ever made
You are quite right Rob, within reason, the bigger the better. I envy you your dyno and flow bench. I only build engines as a hobby, although I do build some for my friends as well. With 2 rally Minis there is always something to do on them.
My historic car is 1293 with about 112 bhp and it is one of the quickest historic Minis out there. The other one conforms to the new Enduro 1400 formula, which mandates a standard engine and box. I 'blueprinted' that one and made all of the permitted changes such as bored +0.060", Aldon dizzy, air filter, exhaust, etc, and the net result was measured by MASS Motorsport as 84 bhp at the flywheel. Up from a quoted Rover figure of 67 I thought that not too bad. It's still very hard to be on the pace against Corsa 1.4's, Pug 106 Rallye's, etc., but we try!
When next I change some valves and guides, maybe I'll try the 5mm stem type. Do these come ex-stock in a variety of diameters ar have to be specially machined?
You're right, Minis are still fun.
MR2Mike,Cooperman Hi, the valves are made in small batches 50-100 off as are the guides, the demand for specialised items is very small the maintenance frequency for them is high, ok for race engines. I use a transition fit lash cap on top of the stem as the rocker footprint is over 5mm. I'm just making another small mod to them for the next batch. Its amazing, the origin for this engine goes back to the early 50's at least, and we are all still finding ways of improving it!! I also have some modified 1300 cranks that lower the couple unbalance error and improve piston seal.
Regards
Rob
Regards
Rob
Rob, in your expert opinion, would these valves be suitable for a rally car, bearing in mind the need for more running time at max performance between rebuilds. I strip and do a 'freshen-up' after between 6 and 8 events, but as I use max size valves I don't want to have to re-cut and re-lap each time I rebuild.
Very kind of you to call me an expert Peter I hope my advice lives up to your expectations!
Unfortunatly the 5mm valves are not suitable for long service intervals. The race heads need to be serviced every 2 meetings or 1.5 hours runnning time to keep the performance at maximum. The trouble is with one make formula every last ounce of torque/power counts in order to be competitve. Our 9/32 waisted valves are the best for long service intervals with maximum power. The design has been in use since 1995 and so far no failures. For rally engines these are ideal and a direct replacement. We also have titanium retainers for 9/32 valves, the material spec is carefully chosen for maximum strength/durability although not easy to machine.
Best Regards
Rob
Unfortunatly the 5mm valves are not suitable for long service intervals. The race heads need to be serviced every 2 meetings or 1.5 hours runnning time to keep the performance at maximum. The trouble is with one make formula every last ounce of torque/power counts in order to be competitve. Our 9/32 waisted valves are the best for long service intervals with maximum power. The design has been in use since 1995 and so far no failures. For rally engines these are ideal and a direct replacement. We also have titanium retainers for 9/32 valves, the material spec is carefully chosen for maximum strength/durability although not easy to machine.
Best Regards
Rob
jeffriesmullet said:
yer hes changing the cam 2 a 286 or sumthing i think a road rally 1
Car definitely won't pull until around 3500 rpm. I have the Piper 285 on a fully prepped anc customised 1040 and it doesn't go until 3500. You'll alos probably need to change the FD to get the full benefits of the cam.
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