998cc Short Engine Build

[Monty-4]

As hinted at in my post in the wanted section I've decided to assemble the various spare parts I rediscovered during a recent garage tidy into a short engine. When I built the engine that's in the car now I did it in a spare block so quite a few spares remain.

Given I've never tried it I thought I'd have some fun and build a 998 using the 1098 block I have and sourcing a 948 crank, given the standard one in there is pretty knackered. I've read what I can find on the subject in searches here and on 'theminiforum'. A few queries though...

1. Will the 948 crank really just "drop in" to the 1098, I thought the main bearings were different?.
2. Are press fit or circlip fit pins preferable in the little ends, can I fit either to any rods?
3. 998cc pistons seem to come with the big old 6.9cc dish or as flat tops. This will land me with very low or a quite high effective/dynamic compression ratio of 8.37:1 with my current head and cam. Is that grenade territory?
4. What is an acceptable cost for balancing the lot? Should it include the (dampened) crank pulley, the pistons and the clutch assembly?

[philthehill]

The crankshaft bearings are the same size @ 1.75" main, 1.625" crank pin.
The stroke is different though - 3.296" 1098cc, 3.00" 948cc.
The 998cc Mini has a stroke of 3.00" and a bore of 2.543" The 948cc 'A' Series has a stroke of 3.00" with a bore 2.28".
Whilst the crankshaft may be interchangeable the stroke may be what lets you down. You may end up with too much space above the piston which will result in a huge combustion chamber.
The bore size is different - 2.543" 1098cc, 2.478" 948cc.
The 948cc rods have a bolt retained gudgen pin, the 1098cc gudgen pin is retained circlips, the 998cc piston gudgen pins are retained buy either circlips or press fit ('A' Plus engine). With the 'A' Plus engine you need the rods to match the pistons/gudgen pin.
I would fit flat top pistons with 'A' Plus rods but if you have the 998cc/1098cc the rods (you can use 'A' Plus rod bolts) with circlips they are perfectly acceptable.
Compression ratio will be determined by what pistons you use, how much you have skimmed off the head and how much you have or can skim off the top of the block.
Ball park cost for balancing all reciprocating parts around £200.
Best to fit the crankshaft damper.
Try it by all means (and it does seem possible) but personally I would not even go there. If you want a 998cc engine use the 948cc block as the basis as it is a well trod and proven route.
What you will gain is the white metal bearings in the camshaft bearings when using the 1098cc block.

[Monty-4]

Thanks phil!

The information I've found suggests that with the 998cc rods and pistons they will have the correct throw and height to be flush with the deck. We shall see shortly and my experience may be illustrative!

Seems circlips will be the easier route given most pistons available are at least marketed for this application. Flat tops will give me a static comp of 10.3:1 or dynamic comp of 8.37:1 on the calculator. That's 24.5cc chambers, intake valve timing 14-46, +.060" overbore (the topic should perhaps be 1048cc).

I've been quoted £135+VAT for balancing by Meridian in Cambridge so will go with them I think. Assuming it all fits when mocked up of course.

[philthehill]

Please keep us informed on how things pan out with lots of photos as well.
Good luck with the project.
Phil

[liammonty]

I think that compression may be borderline - it will certainly be a case of using high octane fuel I reckon. I had a static CR of 10.3:1 with an SW5-07 cam (short duration) - I'm not sure exactly what dynamic ratio it gave - but it was too much and I ended up having to enlarge the combustion chambers to get it to run safely.

I don't know what head you are running, but if it's a 12G202 or similar, there is always scope to fit a 295 or 206 head to reduce CR, as the combustion chambers are massive compared to the 'standard' heads.

[Monty-4]

The SW5-07 is very similar to my current cam - the ACD RT - so I would guess your dynamic compression would have been very similar to mine unless your chamber volume was smaller. Original casting before porting, larger valves etc was a later CAM4810.

Is it possible to take a little off the top of flat-tops to create the equivalent of about a 3cc dish? That would pull it back down to 9.4 & 7.7 static/dynamic. If only Sprite pistons came in a 998 flavour as I believe the high-comp 1098 pistons have that exact dish.

[liammonty]

You'll be very happy with Adrian Dodd's RT cam - it's what I eventually replaced the SW5-07 with (I had terrible trouble with the cam drive gear on the Swiftune cam). I've been really happy with the AC Dodd RT. It's fitted to my 948 which is now 1030 (bored to 998 + 40 thou) together with a 295 head, HIF 38 and Maniflow LCB and exhaust, and together it makes a lovely engine.

I'm not sure regarding skimming the pistons and will have to defer to someone else. If I'm not mistaken, though, the CAM 4810 is a closed chamber small bore head from a Mini? If that's the case, if you need to lower the CR, it would make sense to me to open up the head to do so as you'll get a decent increase in power. I think the inlet valves are even smaller than on a standard 1098 12G202 head - you would be better off even using one of them if you are on a budget, as the chamber size is bigger at 26.1 cc, so you'd potentially cure the CR issue as well as gaining a little performance.

[philthehill]

Here is a set of 998cc L/C pistons suitable for your engine build.


https://www.ebay.co.uk/itm/174744818209 ... %3A2334524

Make sure that they are the circlip type.

As regards skimming the top of the pistons - it can be done but you have to make sure that there is enough metal to be able to do so.

[Monty-4]

Nice find but I think anything less than +60" thou pistons would require sleeving. Doable but I may end up paying full price for new +60s instead, we'll see!

I have just recieved some late push-fit rods and some standard 998 flat-top pistons for dead cheap and will use these to see if this is going to all fit. Now to source an engine stand to save my desk and back...

[Monty-4]

Please keep us informed on how things pan out with lots of photos as well.
Good luck with the project.
Phil

Test fit in old spare 1098 block; the 948 crank with 998 rods and pistons.

Only concern is I couldn't get the first rod and piston I picked to up fit at the fly/flange end, the rod cap fouled on the crank arm (web?). It went in fine at the other end and the second one I picked from the box went in fine.

Piston tops are damn near flush with the block deck, a touch under. With new appropriately sized bearings and torqued up, it may be perfect.

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[Monty-4]

Any opinions on lightening the flywheel, while it's out?

Sure it reduces the power required to spin up but also presumably lowers the inertia so I'd suffer when taking off from a standstill and encountering inclines?

Fly is currently a standard 1098 drilled for the Ford clutch.

[philthehill]

I would lighten the flywheel.
Much better pickup and very little if any loss of inertia or lumpiness.
I had a lightened flywheel on my 998cc and never had a problem.

[Monty-4]

Today concludes a couple of days off work; yesterday I got the Type 9 to a specialist to stop it whining, today I stripped the current engine which has perhaps done approx 10-15,000 miles. All the parts to be balanced and lightened are in a box, ready to go.

Oil pump looks good, as does the water pump. I already have a new timing chain to go on. Something that I wasn't expecting from a relatively young engine was marks in the cylinders. The circle around cylinder #2 looks unsightly but I couldn't feel anything when running my finger over it, might need a re-hone?

Note the apparent marks in cylinder #1 are WD40 + camera flash.

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[Monty-4]

Doh!

Subtracting the diameter of my block cylinders from the piston diameter gives 0.005-0.006" clearance rather than 0.0025-0.003". That is a little excessive.

I'm aware the usual order is to bore the block after measuring the pistons, but I already had a good block at +0.060" that barely even needs a glaze-bust or re-hone.

Hmm!

[philthehill]

Personally I would not worry about the piston to bore measurement. More important is the piston ring gap. If the ring gap is correct fit and forget.
Phil

[Monty-4]

I'm sure you're right phil although the little slip of paper that came with pistons does specify 0.0025-0.003". First step will be returning the piston set and trying some from Minispares instead. If they're still out then I'll check the ring gap and discuss it with the machine shop. Worst case I end up refurbishing my other block (cam bearings & cleaning) and boring to match the pistons.

A race to get it done in time for the MMOC picnic though!

[philthehill]

You could get all four bores re-bored to one specified size and still end up having four different sizers of bore. That is why BMC had graded pistons to suit the actual bores.
0.0025" - 0.003" is going to make very little difference to performance, oil consumption and/or create piston slap..

[kevin s]

5 to 6 thou seems quite a big piston to bore clearance for a small bore like this, I know Lotus Twin cams which have a much bigger bore (83mm) run fine but suffer piston slap when cold if run with this sort of clearance.

[Bowie69]

When the pistons is canted over it won't help with the ring seal, so oil consumption and power will be affected.

[philthehill]

The factory piston to bore wall is specified as 0.0021"- 0.0037" at the top of the skirt.

The additional 0.0025" - 0.0030" whilst not perfect will still work.
The piston will not cant over sufficiently to effect oil consumption and effect performance.

Unfortunately you cannot have it all ways - you either stick as is with new rings (which would be my preferred option) or fit the correct size/grade pistons to a dedicated bore size or re-bore with the correct size/grade pistons.