Bottom end rebuild question

[tomson]

Im about ready to go buy the bits from BMW but was just checking ive missed nothing obvious.......

What would you guys renew when rebuilding a bottom end??

[Ant]

change the main cap bolts, con rod big end bolts, renew all the seals, get a new oil pump and drive too.

new core plugs for the block are also a very good idea dude.

[tomson]

Cheers Ant, i didnt realise you should change the bolts too, i just thought the bearings were enough.
More to go on the list then!

Are the core plugs easy to change then?

Tom

[Ant]

rod bolts are stretch so must be replaced, Mains are "recommmended by bmw" TBH I'd not take the risk dude.

core plugs are easy enough, punch a hole in them and pop them out, make sure the holes are mint and then pop the new ones in with a socket and a small hammer for persuasion.

Sealant is optional, shouldn't really be needed unless there's some corrosion.

what block you using ?

[tomson]

Its M20 B25, basically my painfully slow 2.7 build!

I have a 325i block now stripped down and an eta crank and rods with early 2.5 pistons to go into it.

I wanna do this right and renew everything that should be....

Am i right in saying that when i dummy build to take a measure of how much the block needs decking i should do this with all my new components?

Also do i need to fit and tighten all 6 pistons and measure them all or is 1 ok?

Tom

[Turbo-Brown]

Main bolts are about £12 a set from the dealer so you might as well.

[tomson]

Main bolts are about £12 a set from the dealer so you might as well.

Cool, they are now on the list

[Ant]

things to watch for.........

check the bores on the B25 block, they will have no wear at the lower parts of the cylinders, but the upper section will be worn by the rocking motion of the pistons on the thrust side of the crank.

now, using the B25 crank again there are no issues, but the 81 mm throw ETA unit will mean the pistons will run 3mm further into the bores, so the oil ring will be moving into the unworn area of the block, and over the "step" as a result

this could cause some preamture wear dude so measure it all carefully, or just get the cylinders honed out to be paralell again and use some oversized piston rings for the build(recommended).

[tomson]

i was gonna get the block honed and all degreased first actually.

Where might i get some oversized piston rings from Ant?

Tom

[Ant]

chemical clean is a damn fine idea dude !

the rings you want are thicker in section rather than bigger in diameter, any decent engine rebuild shop should be able to supply these dude

alternative is use a B20 block and have that bored out +4mm to suit the 83.98mm pistons, not the cheap option but would give perfect bores and an as new engine for your troubles.

good idea to lighten the crank and rods/pistons while you're there

the 9.75:1 pistons need the skirts removed too dude !!!

[Turbo-Brown]

Thought these engines were pretty wear resistant myself. My B25 had absolutely no wear whatsoever to the bores after 200k.

Tis a good point though, you don't want piston rings running past any ridges in the cylinder walls or the walls will wear out very rapidly

[tomson]

Yeah, a mechanic friend of mine had a look at the bores on friday and said they were fine but a nice hone out is always a good idea......

[Ant]

Indeed Alex, using the 2.5 crank again in the bores is OK normally, I've seen a few heavily scored bores though, have 3 B25 blocks in the scrap pil outside the unit, always cyl 3 too, weird

My concern is using the B25 block with the longer throw crank, with those pistons speeds it would not take long to be burning some oil

[DanThe]

Will a hone be enough to completely remove the ridge though? Cant see it myself

[Ant]

you'd not remove it with a glazebuster thats for sure

proper diamond hone @ the machine shop is the way to go, or use a cylinder reamer to even it out then hone.

[DanThe]

Quick search found this, reasonable price id say

[Ant]

Very not bad.

[Turbo-Brown]

Scary stuff, think I prefer the idea of having a smaller block bored out

[DanThe]

Scary stuff, think I prefer the idea of having a smaller block bored out

Its got me weighing up my options again

[tomson]

Scary stuff, think I prefer the idea of having a smaller block bored out

Its got me weighing up my options again

Me too!!!

[exup]

Yep
think I agree 100%

keep your std 2l block and have bored out so you get parallel bores , then fit total seal rings , 10.9 bolts and ring the block / weld the head

thats my plan for the 2.7 (t build) ,

[WillG]

you really dont need to ring the block for just a 2.7 build it going to be around £100 for that and the same with welding head its a waste of money if you arent turboing

[Simon13]

welding the head is not a waste of money imo by the waterways is where to do it.

Grifften motorsport thought so too. If it's going to be a big power lump thats going to get abused then i think it should be done. Better safe than sorry, especially if you end up paying loads of money on headwork and it cracks later on......

[tomson]

welding the head is not a waste of money imo by the waterways is where to do it.

Grifften motorsport thought so too. If it's going to be a big power lump thats going to get abused then i think it should be done. Better safe than sorry, especially if you end up paying loads of money on headwork and it cracks later on......

hmmm good point as ive been quoted 750+vat for a dogs bollox head work job by bexleys.

Im gonna call ant on monday about the whole build to see what he suggests to go for. (ive been putting off calling incase that alpina turd didnt go for silly money!!)

[WillG]

welding on my head was £100 from QEP, but main reason its done is to stop gasket being pushed out into water channel if this happens the compression and heat from cylinder pressurise water and thats what cracks head. It probly will strngthen head but if you still manage to overheat or pressurise it will still probly crack the head. Reason I think its not worth being done on a 2.7 NA build is becuase you havent got that much more pressure then std, i mean a 2.7 makes just over 200 hp and people have been fine without welded heads with over 300. Also ive heard from swedes about people doing DIY head welding jobs and had them crack becuase of that, its not that easy to get right and should be done by a welding engineer who will probly pre/post heat the head to avoid any residual stresses

[DanThe]

No reaming for me, I rubbed the ridge off my cylinders with my finger! Nothing but carbon there

The original honing pattern is still very much there too

[DanThe]

Just got a couple of quotes off the dealers for my M50 bits

Main bearing - approx £7.20 per shell + VAT x 12
Guide bearing - approx £12.50 per shell + VAT x 2
Main bearing cap bolts - 14p each + VAT x 14

Big end bearing - approx £7.20 per shell + VAT x 12
Big end cap bolt - £2.17 each + VAT x 12

Head gasket - £71 + VAT
Head bolts - £20 + VAT

Grand total = £360 ish

[Bailey]

Call me stupid, but whats all this welding of the head about then?

[Turbo-Brown]

You're missing a main bearing there as there are 14 in the engine!

[DanThe]

Main bearing - approx £7.20 per shell + VAT x 12
Guide bearing - approx £12.50 per shell + VAT x 2
Main bearing cap bolts - 14p each + VAT x 14

Anybody know any other (cheaper) suppliers for bearing shells?

[Turbo-Brown]

Go and find a good old fashioned machine shoppe, can't remember exactly what mine cost, but they weren't anywhere near that much!

Are those steel-backed-aluminium or copper-cast-lead? The copper ones are more expensive and seemingly totally unnecessary.