688 Machine Work to Convert to 812?

[monoposto]

I have a spare 688 lying around and would like to make use of it as a potential back up engine.

Theres a lot of knowledgeable folks here that know the necessary modifications needed for this. But I would like a more comprehensive list so that I can take it to a machinist and let them know exactly what is necessary for these mods.

Off the top of my head:
1. Liner bores need to be bigger but how much?

2. Liner bores need to move some distance but I don't recall how much?

3. Push rod lifter bores need to go from 19mm to 22mm I think?

4. ?

5. ? I'm sure there is more but if anyone wants to fill in the blanks and has some solid numbers I would really appreciate it!

 

[manicgt]

I have a spare 688 lying around and would like to make use of it as a potential back up engine.

Theres a lot of knowledgeable folks here that know the necessary modifications needed for this. But I would like a more comprehensive list so that I can take it to a machinist and let them know exactly what is necessary for these mods.

Off the top of my head:
1. Liner bores need to be bigger but how much?

2. Liner bores need to move some distance but I don't recall how much?

3. Push rod lifter bores need to go from 19mm to 22mm I think?

4. ?

5. ? I'm sure there is more but if anyone wants to fill in the blanks and has some solid numbers I would really appreciate it!

There was a block change in 1970 that changed the

I have a spare 688 lying around and would like to make use of it as a potential back up engine.

Theres a lot of knowledgeable folks here that know the necessary modifications needed for this. But I would like a more comprehensive list so that I can take it to a machinist and let them know exactly what is necessary for these mods.

Off the top of my head:
1. Liner bores need to be bigger but how much?

2. Liner bores need to move some distance but I don't recall how much?

3. Push rod lifter bores need to go from 19mm to 22mm I think?

4. ?

5. ? I'm sure there is more but if anyone wants to fill in the blanks and has some solid numbers I would really appreciate it!

In 1970 the replacement blocks the bores where equally spaced meaning the center 2 cylinders where brought closer together by 2mm . I think fab 688-02 # 721-071 and the 688-06 # 28-676
The older blocks also had different side mounting brackets but the original 812 blocks used the older mounting brackets .
The newer 1.4 cross flow engines run a 19mm lifter same as the 810 blocks .
The cam race of the 812 blocks also had bearing inserts where the others ran in the stock cast iron bores .
The actual 812 cams also ran a wider cam lobe for the 22mm lifters .
Again depending on the vintage of the block the thrust washers thickness could be larger than the earlier blocks and also the bearing caps .
Just a few things to look out for !
hope this is helpful and again this from memory so may have forgotten some other things .

 

[Frans]

I think the cam bearings which are white metal bushes are there for the greater valve spring pressures that are pushing down harder onto the cam. They can be replaced if worn and I have replaced a couple with wear on them. They need to have the correct tolerances or else you will lose oil pressure at these points. You need special tools that I made myself and you need to open some oil galleries to drill through for the new bushes. The special tools's dimensions are in the factory Gordini manual.

After the replacement, they need to be line bored to the cam bearing dimensions.

At the back is a frost plug that needs to come out and be replaced. I plugged the open oil galleries by tapping them and screwed some plugs in with LockTite.

So if it is going to be a stand-by engine I would think twice before putting bushes in there unless you have the equipment or mates that can do it.

Regards, Frans.