Offset Crank Grinding.

[warpspeed]

From what I hear on the Forums, most people that fit the Getrag boxes eliminate the PPF and use the original B M W rear gearbox rubber mount, bolted to a simple home brew crossmember up under the tunnel.

I believe what happens to the dual mass flywheels is that the spring slowly sags over time and then you start getting clunks and bangs during gear changes. Its possible to move the two flywheels with respect to each other, with just the strength of your hands about four or five ring gear teeth, which is normal. If there is a great deal of free play, say twelve to fifteen ring gear teeth, it all goes into the dumpster. About eight or nine hundred dollars for a brand new OEM one I believe.

Conversion should not be too difficult, plenty to do, but all small jobs:

Block off three of the oil return holes in the top of the block with screwed plugs (1/4 and 3/8 BSP taps suit the existing holes pretty well).
Block off the oil feed hole in the top of the block, and the oil feed hole in the head.
Bolt the head onto the block using B M W bolts and B M W head gasket.
Run an oil line from the oil pressure switch to the cylinder head.

Fit an M40 oil pump to the front of the block after cutting away the original water pump.
Bolt a thick aluminium plate over the giant 80mm water pump hole at the front of the block.
Drill and tap two 10mm holes in that plate for the two cam belt idlers, plus fit a Miata thermostat water neck to get water into the front of the block.
Fit the two cam belt idlers suitably spaced to line up with the cam belt.

Bore out a BP or B6 23 tooth crank pulley to 32mm (original bore 22mm or 27mm) re cut the 5mm keyway back to correct depth (2.5mm)
The two 46 tooth camshaft pulleys need to be moved forward to line up with the toothed crank pulley.
Fit a 147 tooth Gates T197 timing belt (two teeth longer than original BP belt)
An electric water pump and its controller will be required.

There are several different sumps, but the M44 "rear" sump needs to be cut off at the back 130mm below the pan rail and a 3mm aluminium plate TIG welded onto the bottom. Oil pickup needs to be shortend slightly, very easy to do.
Two engine mounts need to be fabricated one either side of the block.
And that is it in total.

Use B M W starter, alternator, power steering pump, and original spring loaded belt tensioner, all dead stock.
B M W air conditioning compressor fits on the cold side with its own belt if required.
Getrag gearbox and clutch slave all stock B M W. I believe a smaller bore clutch master cylinder is required, not sure.

Gear linkage pretty easy, single rod and fork, with ball mounted to top of tunnel.
Custom tailshaft required obviously.

Induction and exhaust all original Miata.
Various crank and cam sensors used over the years on different B M W engines, but aftermarket engine management should be able to accommodate whatever you end up with.

Only tricky bits are machining the three toothed cam belt pulleys, and fabricating and balancing a tailshaft and welding the sump.
Everything else really just needs normal workshop hand tools, which most of us will have.

[Vindi (Russell)]

Just done sone catching up on this thread ... love the idea of cheap BM based stroker, this might be something I can mess with especially as I don't have any interest in my Mk2 1.8 engine!

ECU ... what are the options?

Russell

[warpspeed]

I stumbled onto all this quite by accident only a few weeks ago.
As far as I know this BP/B M W mix of parts has never been done before.

The B M W guys fit the 88mm stroke cast  crank from the diesel version of this engine, when going for max capacity.
Max safe bore size I believe is 87mm.

That will get you out to 2.092 Litres.

Once you go aftermarket with engine management, its up to you how much you wish to spend.

But probably the most important thing is finding an engine tuner close by that is not only extremely familiar with the system you plan to get, but loves the things.

If you have a local engine tuner nearby you know and trust, ask him what he recommends.

[dickie]

Hi Folks,

Just to refresh this and bring it back to the original idea,

I have sourced a VVT engine locally for not much money, I am waiting for it to be removed from the shell.
I will be stripping it down and sending the crank away to have a copy of it made with reduced stroke. I have already had a quote but will have to renew the quotation.
I will have a look to see if the big end diameter can be increased without getting the big ends to close to the crankcase.

The block will be bored to take 85mm Motorbike performance Pistons ( These might have to come in from the USA ).
Once I know the piston deck height I will be able to spec some rods for it.
I already have new stud sets and Boundary oil pump as well as an ATI Damper ( in use on current engine )
The pistons should have the small end closer to the ring pack and oversize valve cut outs which along with the short stroke crank should give a useful increase in rod length.
This in turn will improve the rod ratio and change the piston dwell at TDC and BDC.
My intention is to reduce the capacity by around 50cc while having the ability to rev to over 9000 RPM
The engine will become an Oversquare configuration with a higher compression ratio.
I will have to sort out something for the ancillarys as the water pump is likely to cavitate at high revs so it will have to go electric.
The power steering pump might not cope and the alternator pulley ratio will need altering.
I will be reworking the head once the block is built up.
More port work and bigger inlet valves,
I expect that the exhaust ports will be enlarged too.
I will have a complete 2 1/2" exhaust system on the car by the time the new engine is ready, I already have a Simpson Racing 2 1/2" manifold.
This will not be rapid build as I have to limit my monthly spending on it.

[dickie]

I collect the donor engine on Saturday morning.
I had better clear a space in the garage !!!

[dickie]

Plans brought forward.
I now have a Twin Engined Volvo V70
I was able to collect the engine after work today, It will have to stay in the Volvo till the weekend so that my wife does not see it.
I am about to be more naughty as Bofi are doing a deal on M2 motorsport exhausts. So I will have a 2 1/2" system to connect to the Simpson Racing header.
A small amount of welding or a slip joint will be required to connect it all.
I will be adding a motorsport cat into the mix at some point.

[dickie]

I have just raided the squirrels ££££££ store and placed the order for the exhaust !!!

Early Christmas present to me !!

[dickie]

Hi folks,
The donor engine is now on the work bench and I have started to strip it down.
Manifolds are off along with the crank damper and the cam belt covers.
The cam belt has never been changed and is covered in oil, it was slack enough to pull off the crank pulley !!
I have managed to undo the crank bolt by locking the flywheel with a big screwdriver and heating the front end with a hot air gun then flogging a ring spanner with a big hammer.

The donor car had caught fire at the rear end and had got hot enough to just start to melt the temp sensor plug on the back of the head.
No heat damage beyond the one plug.

[dickie]

Hi Folks,
Not much more progress to report.
The block is now almost completely stripped.
All the main bearing caps and big ends are loose but still in place.
I have been doing more research work and have decided that I do not need custon pistons. I will use 85mm Wossners 12.1 cr. This might mean a thicker cometic head gasket as I have a 60 thou head shave already.

Using a shorter stroke crankshaft will mean custom rods but I will be able to retain the stock size bearings.
The combination of shorter stroke with longer rods puts the rod ratio at 1.75 which if my calcs are correct which should work well with an oversquare engine.
The capacity drop will be about 46 cc.
I am hoping that with bigger inlet valves some exhaust porting the modifications should yield some decent gains. ( And a very empty wallet )

[dickie]

Hi Folks,
After writing the above I took a walk out to the playpen and took the rear main oil seal plate off and undid all the mains / big ends then took the crank out.
The crank is now on a shelf next to a couple of Austin Seven cranks.
I will have to clean the cat litter from the workbench top ( Oil Absorbent ) take the pistons and rods out after that it will be removing the oil squirters.
The block will be ready to take for a rebore afterwards. I will probably get the engine workshop to supply the pistons for me.
It will make a change from building pintos for rally cars for him.
I might even get him to source the crankshaft and rods !!

[dickie]

Hi Folks,
I now have a Naked block on the workbench.
Time to take it for a trip to SRD engines soon for an 85mm rebore in the new year.

[dickie]

Hi Folks,
Long time with no update.
We have just sold our 20" motorboat so I now have funds for the Crankshaft Project.
I have been in contact with Phoenix Crankshafts and now have taken the donor crankshaft into work to make a wooden box to ship it to them.
They are a bit more expensive than the previous quote but have a better lead time and require a smaller deposit.
I have a set of 85mm Wossner high compression pistons which I bought when I was sent to Help Uncle Sam out with a small problem.
So I will have to have some custom length rods made to suit. I am going to carefully measure the block height and commpression height of the pistons to see how long a rod I can get away with.
I should be able to get the 1840cc down to 1796cc So only a 44cc capacity reduction.
The engine will become Oversquare and gain a more favourable rod ratio
From my research the engine should be more balanced in piston motion, the piston speed should be reduced which will allow more revs and there should be more piston dwell around TDC & BDC as well as a slight reduction in side thrust against the cylinder walls.
An improvement in breathing is to be expected due to the increased dwell.
I will need a Cometic head gasket for the 85mm pistons and to stop the compression ratio going too high and the valves getting to close to the pistons.
Flywheel and clutch to be arranged later in the project.
This lot will make for an expensive engine build !!!!
At least with the boat sold that means a bit more to spend on things with wheels per month.


Lucky I do all my own spannering

[wannabe]

I'm guessing that measuring the exact length of rods that you need is going to mean being, er, 'very careful'... lol

[dickie]

The rod length should be just plain maths. !!??!
I will check the block height to see if I can get the pistons right to the top of the block rather than decking the block.
I might be able to get a bit more rod length by getting the pistons right to the top of the bores.
I will have to take the block into work to get it measured up.

[dickie]

Hi Folks,
I have sent the Mk2.5 Crankshaft and an aftermarket billet rod off to the Crankshaft Manufacturers.
It should be with them by this Thursday so project short stroke is moving slowly forward.
The car will become even more of a Mongrel than it already is.
Once the new Crank and Rods are sorted its time to send the block off for its 85mm re bore.
I still need to buy a full set of bearings and a full gasket set.
I expect to have to use a Cometic Head Gasket with the Wossner 12.1 pistons and a head that has already had 60 or 80 thou taken off.
The shorter stroke will drop the 12.1 down a bit with the reduction in swept volume.
The head will be getting big inlet valves and a possible spring upgrade to cope with more than 8000 RPM ( Current Supertech springs are fine with 8000 RPM )

What could Possibly go wrong ?