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Suggestions For A 998 Block Build

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#16 ACDodd

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Posted 12 October 2020 - 09:24 PM

Use these,

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

Ac

#17 Minigman

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Posted 12 October 2020 - 11:17 PM

Over the years I’ve had many minis with engines from 850 to 1330. The sweetest of them all was the 850, it purred! but had no power or torque. I’ve had well specced 998s with twin SUs, modified heads and updated cams, and they went very nicely. However until about 3 years ago I’d never driven a 1098 engined mini and so I put one in my mk1. It was standard and very torquey, accelerated well and effortlessly got me up the steep hills where I live on Exmoor. Being honest with myself I rarely rev the engine above 5500rpm so the fragility of 1098 crankshaft is not an issue. So in the last couple of months I’ve set about building a good 1098. An overbore of +0.080 sees it up to 1167cc with Calver ST (hypatec) pistons, piper 255 cam and a 29cc Calver ST005 head it should go very nicely for a small bore. ACD knows his stuff with 1100s as does Keith Calver. I know Keith has a stash of past articles he’s written on them and he’s very helpful too. I’d go the 1098 route every time if sticking with a small bore.

Edited by Minigman, 12 October 2020 - 11:18 PM.


#18 Minigman

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Posted 12 October 2020 - 11:22 PM

I’ve got some 1098 +0.040” Karl Schmidt pistons (7cc dish) complete with rings with approx 2000 miles on them from new if you want them. Just PM me if interested.

#19 chocobochan

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Posted 13 October 2020 - 09:43 AM

I’ve got some 1098 +0.040” Karl Schmidt pistons (7cc dish) complete with rings with approx 2000 miles on them from new if you want them. Just PM me if interested.

I wouldn't mind going +0.040" instead of +0.060" if I'm adding all that torque to my setup with the crank, I'll PM you for more details on the pistons, thanks!
Chosing these pistons + the 1098 crank would bring my CR down to a tamer 9.69:1 wich is in the range of what I intentionally considered, not 10:5:1, I gotta consider that.
 

 

I considered those but I'm worried about this David Vizzard quote referring to 4 ring pistons contributing measurably to increased internal engine friction, I'll look into it.

 

I’d go the 1098 route every time if sticking with a small bore.

Words of wisdom, this is exactly why I opened this thread, I'm still at the stage where I can consider alternatives and draw up a new engine spec! Thanks for the input!
Just so I'm clear on this, 1098 is the same block as 998 but with a longer stroke, so any 998 piston and rods should work fine on a 1098 crank? Or are the pistons shorter to compensate for the travel distance?
I'm assuming the longer the stroke the more important it is to balance the rotating assembly, so I'll probbly pick up a MED harmonic balancer.



#20 Minigman

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Posted 13 October 2020 - 11:14 AM

Your on the ball. The crank has a longer stroke and hence the pistons have to be shorter from pin to crown height. So long as you don’t rev the crank for too long above 6000rpm it will be fine. Worth getting all rotating parts balanced also and add a crank damper to the pulley.

#21 chocobochan

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Posted 14 October 2020 - 05:18 PM

So you may want to do a rethink on your piston/head choice.

Phil.

Hey Phil, I'm having trouble sourcing the 33mm & 27mm valves you used in the 12g295 videos, correct me if I'm wrong but these heads only take short stem wide groove valves right? That limits my options.

 

After seeing that graph after you changed the valves I'm considering going up to bigger ones when I drop the head off at the machinist for porting.


Edited by chocobochan, 14 October 2020 - 05:44 PM.






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