11 replies to this topic

[gaspen]

Hello

 

I am rebuild a pretty standard 850 engine. Before dismantling it I measured the compression and it showed poor values ( between 70 and 110 PSI)

 

Then I dismantled the engine and measured all the bores and pistons. All mesurements looks good to me : ovality and taper is no more than 0,02

 

As far as I know all pistons have their own spec for the clearance but I don't find anything about them.

 

Are these numbers and letters say anything about the piston ? Should I go with the Haynes parameters ?

 

I have a new set of Hepolite R8576 ring. Do you know the clearance spec for this set ?

 

 20231116_171236.jpg   50.92K   2 downloads

[Bjarnetv]

The large number stamped in the middle is the factory sizing number that is matched with the marking on the block.

according too google its spaced in 0.002" steps, so the letter 2 should be 0.004 oversized. (meaning pretty close to STD size)

 

the rest of the numbers are piston manufacturing numbers and i dont think there is any lists of what they mean specifically. (except the FRONT, which you want facing the fan side when mounting the pistons)

AC Dodd has a video with some more stuff to measure if you want to reuse the pistons.

Edited by Bjarnetv, 27 December 2023 - 11:47 AM.

[Bjarnetv]

As for the specs, it is safe to follow the ones in the Haynes manual as far as i know 

[gaspen]

Haynes says the STD bore diameter of the 850 engine is 62.94 mm, but I actually measure the smaller values : -0,02 / -0,03 in all cylinder, all direction and depth.

 

The worst bore-to-piston clearance is 0,35 at the very top of the piston/bore.

[ACDodd]

If you have 0.35mm at the top of the cylinder it needs a Rebore.

Ac

[gaspen]

If you have 0.35mm at the top of the cylinder it needs a Rebore.

Ac

 

The problem is that the block is sleeved

 

 

 20231114_190131.jpg   30.24K   1 downloads

[gaspen]

Well I found the mistake : Haynes manual is wrong

 

 pistons.png   116.01K   2 downloads

[gaspen]

I guess my pistons collapsed during the years. The liners looks pretty thick, maybe they have some "spare". 

 

Is it possible to re-bore a sleeved cylinder ? What is the limit ? 

 

OR

 

Should I fit a STD set os pistons ?

 

Somerford sells it but it has 5 rings.

 

https://www.somerfor...-850cc-std-size

Edited by gaspen, 28 December 2023 - 11:42 AM.

[ACDodd]

Rebore the cylinder or replace the liners. +0.020” will likely clean that up unless the lip is mostly on one side.

Ac

Edited by ACDodd, 28 December 2023 - 02:43 PM.

[ACDodd]

Done a few 850 liners……

Attached Files

•  IMG_5297.jpeg   60.41K   3 downloads
•  IMG_5298.jpeg   67.83K   8 downloads

[gaspen]

Done a few 850 liners……

 

Thanks AC

 

Is that piston the same that Somerford sells or Hepolite 15037 ?

[ACDodd]

Most likely.

Ac