.030 Pistons

[Joesprocket]

Heard from my machinist this morning and he's telling me based on mic readings of the bore I should be looking for:

 

.030 pistons

.010 rods

 

Bathtub heads. These would be mated to 121TI head from an early 69.

 

thanks!

 

[tech71]

4 hours ago, Joesprocket said:

.030 pistons

So 0.030 = 0.762mm oversize... thats a toughie, as far as standard bores go you are looking at oversizes available in .25mm-.5mm and 1mm. Never seen oversize in .75mm😉

Boring up to 1mm and getting a set of 10:1 Mahles still floating around may be the best solution provided your ok with that level of compression.

[Joesprocket]

Ha! That was my first thought when I received his text. I didn't recall that being an option for standard oversizes in the manual. But then again you never know what people have collecting dust on their shelf

 

So, tell me if i'm wrong but I suppose I could custom forge from Ireland Engineering or JE Pistons? Some folks seem to be happy with that.

 

Or I found these Mahles on ebay and thought of just going with the 1mm over. Maybe dumb question but should I be concerned with 10:1 compression on a bone stock early 69 set up? We're talkin' 4 spd, single solex, OG cam, 6 bolt fly. 
 

i do have a line on some dual 40 solex carbs and maybe this is tipping point to grab those and lift the cam to 292? I’ve heard no one complain about that setup matched to that compression. 

 

Appreciate the guidance

 

Set of 4 Piston (90.015 mm, 1.00 mm Over Size) MAHLE 0819203AM for BMW 2002tii

WWW.EBAY.COM

---

1971 - 1974 2002tii. Set of 4 Piston (90.015 mm, 1.00 mm Over Size). (Only for cars with 121 Head).

 

 

Edited February 29 by Joesprocket

[bergie33]

See what WN has. I got Mahles from them, maybe they have something in that range for your setup. 

[tech71]

I dont want to hijack your WTB post but what size piston was installed? From what I gather the bores have some corrosion?

[Joesprocket]

7 hours ago, tech71 said:

I dont want to hijack your WTB post but what size piston was installed? From what I gather the bores have some corrosion?

 

No worries, wanna get this right. 

Machinist informing that block was previously rebored; .020 (1st over)

However, tops of each piston are marked 89.46. (for 2nd over)

 

That math doesn't work out for me. Am i missing something?

 

When I bought the car the pistons were frozen to the cylinder walls from sitting. So yes we are looking at getting through some corrosion to a fresh surface.

Edited February 29 by Joesprocket

[bergie33]

2 hours ago, Joesprocket said:

 

No worries, wanna get this right. 

Machinist informing that block was previously rebored; .020 (1st over)

However, tops of each piston are marked 89.46. (for 2nd over)

 

That math doesn't work out for me. Am i missing something?

 

When I bought the car the pistons were frozen to the cylinder walls from sitting. So yes we are looking at getting through some corrosion to a fresh surface.

 

0.02" is about 0.5mm.  For the M10 that is 2nd oversize.  If the machinist isn't super familiar with M10 engines, they may not know that.  The first oversize is 0.25mm.  See below from the factory shop manual.

 

When I rebuilt my engine a couple years ago I was faced with the same issue.  I was already at 89.47, needed to rebore to 89.97.  I couldn't find any piano top Tii pistons in that size and after researching, I wasn't comfortable going with custom forged pistons.  I decided that while M10 engine blocks were still plentiful (and in my case I already had one that was still standard size), that I was better off starting with a standard size block and boring it to 89.47 where more options were available.  That way, down the road a bit if I needed another rebore, there was still enough meat to bore and I could go custom forged if it ever came to that.  Oh, another easy factor in my case was that my block already was a factory replacement, so VIN didn't match.

 

Just my thoughts. 

 

 

Just my thoughts.

 

 

Edited February 29 by bergie33

[Joesprocket]

Karl - thanks for the response and helping confirm that the math works out (.02 rebore = 89.46 pistons)

It also makes sense with Tech71 comment above that .03, as requested from machinist, is 89.762 and standard sizes are only .25mm-.5mm and 1mm. Never .75mm.

 

I'd like to keep the original block even if it means going 1mm and 10:1 compression. My thought is that's a big leap from a full stock setup on only 1 dimension. To enjoy the car as a fun street driver I feel like I need to also consider my carb, cam and dizzy set up.

Do you agree?

[tech71]

11 minutes ago, Joesprocket said:

'd like to keep the original block even if it means going 1mm and 10:1 compression.

Or installing sleeves, expensive but gives you  a few more options piston wise

[Son of Marty]

Max has these on sale for 759.00 a set for a 90mm bore.

[tech71]

Dont see why you couldn't mill off a few thou from the bathtub to drop compression a bit.

!0.1 with a single barrel carb is probably overkill.

How is this 69 121ti head? has it been determined to be serviceable yet? If not, this conversation is moot.

Edited March 1 by tech71