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 Posted: Aug 27, 2018 06:56AM
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GB
I've been running my 1107 68mm bored 998 for nearly a year now and it's awesome.

Offset bored .025" and .015" as per Vizard to avoid the oilway, running a custom CST larger valved 12G295 to avoid the faff of pocketing the block and having ports that are way too big (a 295 isn't as good as a 202 on port size, but is still way better than a 940), head & block modified for a 1275 head gasket for better fire ring seal. Probably slightly undercammed with an MED RS cam which isn't quite the duration of the Piper 270 I wanted (part of a parts for car deal with them) which is a shame as the bottom end is built for 7500 rpm. 60-odd horsies and torques at the wheels is entertaining.

Next build could well be a 1098 out to 1215 with a Russell RE83 cam - torque monster.

 Posted: Aug 25, 2018 06:57PM
 Edited:  Aug 27, 2018 09:13PM
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A friend and I recently built a 1098 block with Russell's 68mm pistons, offset bores and a 998 crank. As the pistons are made for a 1098's stroke Graham Russell  decked the block 3.75mm. Displacement is 1108cc. It is in a road car.

We used a Payen BK450 (1275) gasket because a stock one overhung the offset bores. The head is a ported 12G202 with 998 Cooper intakes and 27mm exhaust valves.
A few water holes had to be moved and one plugged to use this head and gasket combination.
Camshaft is an RE266SS, rockers are stock 998/1098 pressed steel, carb is a DHLA40 Dellorto with 34mm chokes. It is very smooth and revs hard to 7000+. 
I expect the 998 crank will be much harder to break than a 1098 one (with 1.75" mains).
[Edit] This motor was slated to be a 1215, but the 1098 crank we acquired had 2 cracks in it.... so a 998 stroke it is!

Kevin G

1360 power- Morris 1300 auto block, S crank & rods, Russell Engineering RE282 sprint cam, over 125HP at crank, 86.6HP at the wheels @7000+.

 Posted: Aug 21, 2018 03:28PM
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If you are considering a K100 head, make sure you get the pistons too, the stock K100 pistons are 67mm with 18mm pin.  Mod the 998 rods for the 18mm pins, deck the block.

 Posted: Aug 16, 2018 12:27PM
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US
It's not necessary to look for Imp pistons.  Russel offers a modern 3 ring flat top 68mm for the 1098 that works in the 998 with the MUCH preferred short stroke (but still very 'oversquare' at 2.675" x 3.00").  I absolutely would not bother with the absurdly long stroke 1098, by far the worst choice for a performance build, particularly if you have the old 1.75" mains! That crank is as stiff as a noodle!
(PLEASE note I said "I" would not bother... yes I know racers have spun them up and countless people have had great 1098 cars, but all things equal, that's my opinion!)

The nice short piston requires quite a bit chopped off the block of the 998. The standard pistons resemble a beer can to reach the deck with the short stroke.... a whole bunch of piston is above the pin... not very modern!  I took .145" off the block to bring the pistons to about .015" below the deck.  I offset the .134" overbore on all cylinders after sonic checking the wall thicknesses of the 998 A+ block... I moved the outer bores .025 outward, and the inners .025 towards the center.  I did have to sleeve the oil bore on number 4, but well below the rings.

I'm using a mildly cleaned up 12G1316 North American 1275 head (so called 'smog' head, but really the best casting) with the standard valve sizes. I was ALMOST able to avoid having to notch the block for the exhaust valves... I could get away with only notching cylinders 1 and 3.  I do have a very standard 12G295 head, but elected to go with the 1275 head, as a proof of concept...

Tony has built one of these big-small-bores using a good small bore head right here at Mini Mania. It's too bad they no longer have anyone that can write articles, as Tony really does some cool stuff!  I don't think he had to offset the bores, but otherwise the same idea and pistons.  It was set to be dyno-tested a week or two ago, so it will be very interesting to see how much power we can make with these two combinations.  Mine is going onto a straight cut box with low gears and light flywheel to try to minimize internal drag that the now 1100 cc engine needs to overcome.

 Posted: Sep 24, 2013 07:59PM
mur
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Small bore engines have rods that dismantle at an angle, so that they can pass through the small bore.  Board member Workinprogress was for a while racing a 1098 MG midget, and he was using Pauter rods.  This makes for a crazy engine assembly process because the rod has to go in first from below, as it is like a Cooper S rod, and dismantles in a horizontal manner.  Rod comes up from below, piston goes in, the wrist pin gets snuck into place, buttons are snuck in, piston is popped up through top of block and then the rings are added, piston is then fitted to the block.

This is fabulously indulgent stuff.  I wouldn't do this for a street engine but the allure of lighter internal engine components is, well, alluring.  I suppose you could use 1300 MG midget connecting rods and lighten them a bunch to get the same rod strength improvement, however these are normally press fit, and you would need to fit a bushing to do a sneaky wrist pin install, and then all of a sudden simply buying Pauter Rods is just a call and they do that for you.

Any repeater with a copy of Vizard's first book will talk about how the spring clips that hold the wrist pin will probably fail.  Indeed, I have seen many cases of 998 Cooper pistons re-used in other blocks where the clips fail.  Later 998 rods use a press fit wrist pin.  One of the piles of engine parts that I expect to assemble soon is a 998 that will use these later rods, but I have not had a problem with the earlier system.

If you do find a 1098 crank, you will want to use a new or rebuilt dampner and not just a steel pulley like the 998s come with.

 Posted: Sep 24, 2013 01:09PM
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A series 998 and 1098 conrods are the same parts.
The 7.5mm extra stroke of a 1098 is catered for by the 1098's piston pin to crown height being 3.75mm less than a 998 is.

Kevin G

1360 power- Morris 1300 auto block, S crank & rods, Russell Engineering RE282 sprint cam, over 125HP at crank, 86.6HP at the wheels @7000+.

 Posted: Sep 24, 2013 06:42AM
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US


Great Info!

I didn't know a 1098 crank could be swapped.

What Conn rods are the 998's up to the task?

 

Mitch

 Posted: Sep 23, 2013 05:54AM
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US

The 998 standard pistons are something like sixty five thou over for the 948..  I bought a set from MM for my 948.  The engine has been at the machine shop for some time, but no questions or problems have come up.  I'm using the 10.3CR Cooper pistons with a few other tweaks in my A40 Farina.

 

 Posted: Sep 22, 2013 10:25PM
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Quote:
Originally Posted by DRMINI

Std size Imp pistons are 68mm, about .134" over stock 998/1098 bore.
However they are very hard to find now, like most Imp parts.

....

These days as you can get +.120" flat top pistons for a 998, it's hardly worth messing with Imp pistons.

I wasn't recommending it either, just pointing out that the block can handle some pretty ridiculous overboring. I'm looking at overboring my 948, and it's much the same story: people routinely put .060 over 998 pistions into the 948, and some even manage .100 over 998 pistons. That's about .170 over.

 Posted: Sep 21, 2013 11:50PM
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GB

The 998 in Betty is +.080 and is a very nice smooth motor, even though it hasn't been on the rollers yet.

The 1100 in Tubbs is +.-60 at the moment, but I have a set of the GR/KC +.100 pistons to go in a new engine.

 Posted: Sep 21, 2013 04:20AM
 Edited:  Sep 21, 2013 01:52PM
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Std size Imp pistons are 68mm, about .134" over stock 998/1098 bore.
However they are very hard to find now, like most Imp parts.

I have built 3 motors way back when using these pistons with a 1098 crank. The pistons are short, so you need to mill a big chunk off the block. I doubt you could run them with a 998 crank you would need another .148" off the deck!
Also note the oil pump's outlet drilling is VERY close to #4 bore when you go this big, even with the offset bore. On one of mine it broke thru and we had to sleeve the oilway.

These days as you can get +.120" flat top pistons for a 998, it's hardly worth messing with Imp pistons.
[edit] Russell Engineering sells flat top 1098 pistons +.100", this gives about 1188cc. These are a 12% silicon alloy (good stuff).

Kevin G

1360 power- Morris 1300 auto block, S crank & rods, Russell Engineering RE282 sprint cam, over 125HP at crank, 86.6HP at the wheels @7000+.

 Posted: Sep 20, 2013 09:20AM
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Check your block carefully, but even Imp pistons have been known to work: those are about .120 over.

 

There are rumors of early 998s and 1100s that were really overbored 850s, and those won't go out that far. Check your castings, not your current displacement.


There's a picture in Vizard page 404 that shows the offset bore pattern to maximize success with 998s and 1100s.

 Posted: Sep 20, 2013 08:30AM
 Edited:  Sep 20, 2013 08:56AM
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CA

Edit needed...post went up without typing a word...

Everything MUR said, having been a Graham Russell house guest and ridden with his son Matt in his Mini with well used 998 with the RE treatment that still went like a 1275 Cooper S...read Graham's articles in the Technical Section here.

Of course riding with Graham in his Clubman LS with 146 hp from his massaged 1275 was another thing.

 Posted: Sep 20, 2013 08:20AM
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US
My 998 is bored 100 over, did this to remove the factory sleeves and get a little extra hp. Also has a cooper head the mur mentions.

 Posted: Sep 20, 2013 07:59AM
mur
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You can go much more than that, however, you should also consider increasing the stroke with a 1098 crankshaft.  Graham Russel makes awesome pistons, however their web site has been under construction forever.  Our host do carry them:

//www.minimania.com/part/RE-A1119-60/Russell-Engineering-1098cc-060-Piston-Set

It should not be hard to find a Y drilled, similar to being cross drilled, but that is how they were made, crank out of an MG 1100.  Use a later thick flange 998 block, my personal choice would be one of the last Canadian mini 1000 blocks as they have the timing chain tensioner but are not A+.

Then consider using a sorted out small bore head.  Easily found in the MG midget world, or from the transverse 1100 you find the crank in.  Just don't go looking for a 998 Cooper head because then all of a sudden there aren't any left...

This would make an 1150cc engine, however using Imp pistons you could get another 100cc  --  these pistons would require different rods to work in a 998, but you could bore the block that big.

 Posted: Sep 20, 2013 07:06AM
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How much is the max a 998 block should be safely bored?  Is .060 the highest?

Any problems with head gaskets blowing out with a 1275 head?

 

Thanks,

Mitch