M42 piston rings?

Post Sun Oct 05, 2008 9:08 pm

well i am re-conditioning my 318is have got the whole engine in bits.. just a bit confused on what size piston rings i should use.
I measured the pistons and piston chambers.
Piston 1 83.16mm, Chamber 1 83.86mm
Piston 2 83.14mm, Chamber 2 83.20mm
Piston 3 83.10mm, Chamber 3 83.64mm
Piston 4 83.32mm Chamber 4 83.94mm

So to the Question Should i use original sized rings or over sized rings ie + 0.50mm.

Post Mon Oct 06, 2008 7:50 am

i don't think you've measured the bores very accurately there, those figures vary far too much across cyls. also at a guess you're not measuring the pistons at the skirt, this is the correct way to find the dia of the piston.

Post Mon Oct 06, 2008 5:06 pm

The skirt of the pistons are

P1 = 83.34mm
P2 = 83.34mm
P3 = 83.34mm
P4 = 83.34mm

I got all the measurements using a venier caliper so they shold be accurate i think?
unless i am measuring the clys in the wrong place..?

Post Mon Oct 06, 2008 6:35 pm

Which way did youmeasure the bores? left to right or front to back?

I would have though front to back would be best, as then your not measuring the potetionly "oval" bit were the piston pushes hard against the side of the block on its power stroke.

Might be more consistant.

Did you use a digital caliper or one of the proper internal bore measuring calipers that are like a "T"?

Post Mon Oct 06, 2008 7:03 pm

When measuring the bores make sure you are measuring the cylinder and not the carbon build up or lip at the top

Post Mon Oct 06, 2008 7:05 pm

When a company machines a block they take measurment's along and across the block and at 3 heights top, middle and bottom using a dial type bore gauge zeroed at the ideal measurment on a external micrometer.

Well thats how they did it where i worked.

Post Mon Oct 06, 2008 7:26 pm

Vernier caliper is no good for measuring bore diameters, ok for outside diameters. You need to use a decent bore gauge in conjunction with the vernier calipers, or best still use a decent internal micrometer.

Post Mon Oct 06, 2008 9:11 pm

Ok well think i will have to find someone with a bore gauge so i can get a better measurement then..
unless someone round the west midlands has one??

Post Sat Oct 11, 2008 10:21 am

sorry to be completely useless - thought i could get my hands on one and I'll be in shropshire later but no luck unfortunately.

Post Sat Oct 11, 2008 12:19 pm

No worries mate thanks though..

Post Sat Oct 11, 2008 12:34 pm

Hens teeth comes to mind, when i needed one i drew a blank everywhere. The only internal one i came across was a digital set on ebey for £400, stuff that.
I tried bodging it with a set of these but didn't trust the resuts i got. http://www.tooled-up.com/Product.asp?PID=16548

In the end i took the block down to a machine shop and had to twist the guys arm to measure it, the boss was against it because he new i would't have any work done so he couldn't then charge me at some extortionate rate.
Needless to say i don't use the place anymore.