To be clear, I’m not a mechanic/vehicle technician/engine builder, I’m a dental technician to trade. I’ve built more than a few engines, mostly Suzuki, and watched, usually in horror as people have destroyed perfectly functional engines by “rebuilding” them.
First off, BMW engines aren’t made very well. The cranks, rods and pistons are very poorly balanced, more on that later.
Second, why do you want to rebuild it? It’s utterly pointless replacing everything if it doesn’t need it. Check your oil pressure, that can tell you if the big ends ‘May’ need replacing, though it may be the pump or the relief valve. Check your compression, and do it properly, with and without oil. Do a vacuum test, that gives you more info on rings and valves. See google and YouTube, or I’ll be writing for days. It’s all Information you do BEFORE stripping the engine. Mileage can be irrelevant if the oil has been changed regularly. I built several Swift GTi engines with over 100K where I didn’t need to touch the short motor other than balancing, all the spec’s were in tolerance for a new engine.
Use an engineering company you TRUST, you may be spending a great deal of money with them. I only use West of Scotland Engineering in Glasgow, www.wose.co.uk, have done for nearly 40 years, tried a couple others more local to where I lived, learned expensive lessons. Happy to make the 120 mile round trip.
Have a Plan. Do know what you want to do. Keep It Simple Stupid. KISS
Perfect Planning Prevents Poor Performance. 5P
It’s all the same, don’t just blunder ahead, that burns money, and blows engines.
Which brings us to the most important thing; THE INTERNET IS FULL OF BULLSHIT PASSED OFF AS ‘GOSPEL’.
Z3 ‘gospel’ examples, the steel frame seat will protect you in a rollover, 4 cyl engines don’t like a lightened flywheel, both hilarious nonsense.
That takes you back up to, do know what you want to do. There are various piston, crank and rod combo’s for these engines, lots of the information is spread from people who haven’t actually combined some of these and is wrong/incomplete.
BMW cranks aren’t perfectly balanced, the rods can vary up to 3g, the pistons the same. Doesn’t sound much but it produces vibration. 6 cyl owners smugly thinking “just fit a 6cyl”, yours aren’t built any better, you just have six events per rotation and an even heavier flywheel combo to damp them.
To give you an idea I could balance a 50p coin on a Suzuki rocker cover (2g spread over both rod/pistons instead of 6g). It would fall over about 15-1600 rpm, balanced to 0.00g it fell over 5-5500rpm. You can’t balance clutch discs and gearbox internals, well, you can, but that’s into stupidly serious levels of spending.
I have a 76,000 engine I’m wanting more power from.
I’m not going into stripping it down, covered in manuals, do buy one, if only for the torque schedule. Be very aware, there’s a bolt at the top of the rear timing case that’s in a dirt trap. Find and remove it. Otherwise the 2 part timing case turns into a three part case when you try to shock it loose with a plastic hammer as I did.
Measure the end float of the crank, the thrust washers are incorporated into the center main shells, not the greatest design.
Buy yourself a pack of red plastigauge. www.plastigauge.co.uk Read the Bloody Instructions!!
Wear clean gloves, remove each main bearing one by one, check for scoring, use plastigauge to measure each clearance and write them down. Make sure everything is clean, oil the journals and reassemble. Do the same with the big ends, though at this stage I remove each rod/pistons combo after measuring. Mark both so you can reassemble correctly.
If the journals aren’t scored, the shells don’t have any untoward marks (see google images), and the bearing clearances are in spec, you’ve won a watch, no regrinding needed. Happens more often that not if the oil has been changed regularly.
In the image above you can quite clearly see which were the big end bottom shells, they have the slight scuff. You really don’t want to see much more than that. You definitely do not want to see the mark extend completely across, or continue up the side of the shell.
Top one you can see a line, you would check there isn’t damage to the journal, there wasn’t in this case. If reusing the rods and shells, store them carefully, it’s easy to score a shell, see second bottom, I did that to show what can happen.
I’m using a lightweight flywheel for a 6 cylinder, bought from the States. Takes a 240mm M3 clutch cover, which is the same as a 7 series and much cheaper, I’ll add the number. Had it a while, if I were doing it now I’d buy one of the spring center Z4 multiplate flywheel combo’s from eBay, only solid ones were available when I bought the flywheel.
The crank, damper (front pulley), flywheel and clutch cover were all balanced at West, I confess I forgot to ask Ross, who did the balancing, to note how far out it was. The crank needed balancing slightly, the front damper adjusted, the flywheel was out slightly in relation to the crank. As expected, being for a six, but the cover needed no adjustment. Though it was all marked so it could be reassembled as balanced.
It’s a good idea to pop to Metric Mechanic for a read of their 4cyl builds
First off, BMW engines aren’t made very well. The cranks, rods and pistons are very poorly balanced, more on that later.
Second, why do you want to rebuild it? It’s utterly pointless replacing everything if it doesn’t need it. Check your oil pressure, that can tell you if the big ends ‘May’ need replacing, though it may be the pump or the relief valve. Check your compression, and do it properly, with and without oil. Do a vacuum test, that gives you more info on rings and valves. See google and YouTube, or I’ll be writing for days. It’s all Information you do BEFORE stripping the engine. Mileage can be irrelevant if the oil has been changed regularly. I built several Swift GTi engines with over 100K where I didn’t need to touch the short motor other than balancing, all the spec’s were in tolerance for a new engine.
Use an engineering company you TRUST, you may be spending a great deal of money with them. I only use West of Scotland Engineering in Glasgow, www.wose.co.uk, have done for nearly 40 years, tried a couple others more local to where I lived, learned expensive lessons. Happy to make the 120 mile round trip.
Have a Plan. Do know what you want to do. Keep It Simple Stupid. KISS
Perfect Planning Prevents Poor Performance. 5P
It’s all the same, don’t just blunder ahead, that burns money, and blows engines.
Which brings us to the most important thing; THE INTERNET IS FULL OF BULLSHIT PASSED OFF AS ‘GOSPEL’.
Z3 ‘gospel’ examples, the steel frame seat will protect you in a rollover, 4 cyl engines don’t like a lightened flywheel, both hilarious nonsense.
That takes you back up to, do know what you want to do. There are various piston, crank and rod combo’s for these engines, lots of the information is spread from people who haven’t actually combined some of these and is wrong/incomplete.
BMW cranks aren’t perfectly balanced, the rods can vary up to 3g, the pistons the same. Doesn’t sound much but it produces vibration. 6 cyl owners smugly thinking “just fit a 6cyl”, yours aren’t built any better, you just have six events per rotation and an even heavier flywheel combo to damp them.
To give you an idea I could balance a 50p coin on a Suzuki rocker cover (2g spread over both rod/pistons instead of 6g). It would fall over about 15-1600 rpm, balanced to 0.00g it fell over 5-5500rpm. You can’t balance clutch discs and gearbox internals, well, you can, but that’s into stupidly serious levels of spending.
I have a 76,000 engine I’m wanting more power from.
I’m not going into stripping it down, covered in manuals, do buy one, if only for the torque schedule. Be very aware, there’s a bolt at the top of the rear timing case that’s in a dirt trap. Find and remove it. Otherwise the 2 part timing case turns into a three part case when you try to shock it loose with a plastic hammer as I did.
Measure the end float of the crank, the thrust washers are incorporated into the center main shells, not the greatest design.
Buy yourself a pack of red plastigauge. www.plastigauge.co.uk Read the Bloody Instructions!!
Wear clean gloves, remove each main bearing one by one, check for scoring, use plastigauge to measure each clearance and write them down. Make sure everything is clean, oil the journals and reassemble. Do the same with the big ends, though at this stage I remove each rod/pistons combo after measuring. Mark both so you can reassemble correctly.
If the journals aren’t scored, the shells don’t have any untoward marks (see google images), and the bearing clearances are in spec, you’ve won a watch, no regrinding needed. Happens more often that not if the oil has been changed regularly.
In the image above you can quite clearly see which were the big end bottom shells, they have the slight scuff. You really don’t want to see much more than that. You definitely do not want to see the mark extend completely across, or continue up the side of the shell.
Top one you can see a line, you would check there isn’t damage to the journal, there wasn’t in this case. If reusing the rods and shells, store them carefully, it’s easy to score a shell, see second bottom, I did that to show what can happen.
I’m using a lightweight flywheel for a 6 cylinder, bought from the States. Takes a 240mm M3 clutch cover, which is the same as a 7 series and much cheaper, I’ll add the number. Had it a while, if I were doing it now I’d buy one of the spring center Z4 multiplate flywheel combo’s from eBay, only solid ones were available when I bought the flywheel.
The crank, damper (front pulley), flywheel and clutch cover were all balanced at West, I confess I forgot to ask Ross, who did the balancing, to note how far out it was. The crank needed balancing slightly, the front damper adjusted, the flywheel was out slightly in relation to the crank. As expected, being for a six, but the cover needed no adjustment. Though it was all marked so it could be reassembled as balanced.
It’s a good idea to pop to Metric Mechanic for a read of their 4cyl builds
Last edited:
