14 replies to this topic

[amsgw]

Hi all, currently carrying on with a 1275 rebuild and have come across a difference in my dry build piston deck heights. All 4 cylinders were brought up to TDC using a DTI and measured at 4 X points on the crown each, and then taking the average for each cylinder - each measurement checked 3 times!!

 

Cyl 1 - 27 thou

2 - 23.5 thou

3 - 18.5 thou

4 - 28 thou

Does this seem like a normal variance? It seems big to me

 

The cyls have been bored out +20 thou by the machine shop who had the pistons to use, the crank has also been ground, and all new main and big end bearings were provided by the machine shop. I've also checked the crank to big end which is within tolerance at 1.5 thou each, and the mains which are all 1.5 too

 

Another question - if I take the average of all 4 cyls to give the average deck height, this does actually magically bring the correct CR I was after (10:1 so can't really go any higher as this is a fast-ish road build). Is it advisable to deck the block anyway and then find the volume elsewhere? The head has already been skimmed previous to the engine rebuild so is fresh. Or can it just go back on as is once the machine shop checks it is flat?

 

Thank you :) 

[DeadSquare]

Now rotate the crank and measure how far down each piston goes.

[KTS]

personally, for a road car i wouldn't be too worried about those numbers; just use the average of all of them for your CR calculations.

 

other than throwing a load of money at the issue, you could measure the deck height of each piston in every bore to find the combination that gives the best figures

[nicklouse]

Swap them about in the block and remeasure. If it repeats try different pistons on different rods.

[Spider]

The variations will most likely be from the the Rods and possibly the Pistons depending on who's they are.

It could also be in the stroke of the Crank, depending on how it was re-ground.

There's almost 1 CC of difference there between highest and lowest.

[amsgw]

Thank you, will try swapping the pistons around in the bores! The rods are used, the pistons are new, they are evos from minispares.

[Icey]

I had this a couple of months ago when dry building the latest 998. Started with a ~0.006” difference, but with some swapping about got it down to ~0.002”.

[Bobbins]

Before you do anything else take the measurements suggested by DeadSquare in post #2, it’ll enable you to eliminate a crank problem.

[Cooperman]

I would suspect that the crankshaft throws are the cause.
I have seen crankshafts with quite a bit of difference between journals. Since most crank re-grinding machines simply use the existing journal centres, re-grinding does not correct this.
However, it's possible to get the crank stroke corrected by accurate grinding to the next undersize and angular correction (indexing) can also be done at the same time. It's not a cheap process.

[amsgw]

Now rotate the crank and measure how far down each piston goes.

Do you mean spin the crank over to see if the piston height changes? Sorry, not sure I’m following you

I have realised that whilst the rods are not my original rods, the engine rebuild was because of failed big ends and the crank journal that is the highest in the bore is the cylinder with the heaviest damage- starting to think Cooperman might be onto something.

What kinds of issues would I be looking at really for a fast ish road engine if it kept this cc difference? I’m assuming dizzy timing and detonation problems, but how severe?

[sonscar]

Rotate the crank 180degrees and measure down the bottom of the bore.If the pins are offset the highest piston will probably be the lowest,Steve..

[Cooperman]

Manufacturing tolerances on piston dimensions including gudgeon pin centre-line to piston deck are very accurate these days with CNC machining techniques.

It is alomst certainly the crankshaft journals which are not all at the same 'throw' and this is not uncommon.

For a road car engine it is probably not a major issue as the slight difference in CR for each cylinder won't show up in the running.

Whn I build a rally car engine I get the crank throws measured and the crank stroked and indexed. On a 1071 'S' crank for  race/rally engine, which was brand new I had to have a 10 thou undersize machined to correct the original manufacturing tolerances both radial and angular. It was a very smooth engine once done and balanced and would rev to 7500 rpm+.

You have two choices. To live with the dimensional differences or to get the crank stroked by a capable machine shop. A normal machine shop doing crank regrinds may not be able to do this and a more specialised machinist may be needed.

[DeadSquare]

Manufacturing tolerances on piston dimensions including gudgeon pin centre-line to piston deck are very accurate these days with CNC machining techniques.

It is alomst certainly the crankshaft journals which are not all at the same 'throw' and this is not uncommon.

For a road car engine it is probably not a major issue as the slight difference in CR for each cylinder won't show up in the running.

Whn I build a rally car engine I get the crank throws measured and the crank stroked and indexed. On a 1071 'S' crank for  race/rally engine, which was brand new I had to have a 10 thou undersize machined to correct the original manufacturing tolerances both radial and angular. It was a very smooth engine once done and balanced and would rev to 7500 rpm+.

You have two choices. To live with the dimensional differences or to get the crank stroked by a capable machine shop. A normal machine shop doing crank regrinds may not be able to do this and a more specialised machinist may be needed.

Where did you get a brand new 1071 S crank  ????

[Cooperman]

 

Manufacturing tolerances on piston dimensions including gudgeon pin centre-line to piston deck are very accurate these days with CNC machining techniques.

It is alomst certainly the crankshaft journals which are not all at the same 'throw' and this is not uncommon.

For a road car engine it is probably not a major issue as the slight difference in CR for each cylinder won't show up in the running.

Whn I build a rally car engine I get the crank throws measured and the crank stroked and indexed. On a 1071 'S' crank for  race/rally engine, which was brand new I had to have a 10 thou undersize machined to correct the original manufacturing tolerances both radial and angular. It was a very smooth engine once done and balanced and would rev to 7500 rpm+.

You have two choices. To live with the dimensional differences or to get the crank stroked by a capable machine shop. A normal machine shop doing crank regrinds may not be able to do this and a more specialised machinist may be needed.

Where did you get a brand new 1071 S crank  ????

 

It was in 1989 and I was building an FIA Group 2 1071 'S' rally car for the 'up to 1200 cc' class. I got the crank from an old friend, who had it on a shelf and I did not have to pay a lot for it. My other, now deceased friend who had a great machine shop, measured it and re-stroked and indexed it. Again I didn't have to pay a lot since I put a lot of business his way. I was surprised at how far out it was from new, but a 10 thou undersize corrected it. The car was fantastic too.

[amsgw]

Manufacturing tolerances on piston dimensions including gudgeon pin centre-line to piston deck are very accurate these days with CNC machining techniques.
It is alomst certainly the crankshaft journals which are not all at the same 'throw' and this is not uncommon.
For a road car engine it is probably not a major issue as the slight difference in CR for each cylinder won't show up in the running.
Whn I build a rally car engine I get the crank throws measured and the crank stroked and indexed. On a 1071 'S' crank for race/rally engine, which was brand new I had to have a 10 thou undersize machined to correct the original manufacturing tolerances both radial and angular. It was a very smooth engine once done and balanced and would rev to 7500 rpm+.
You have two choices. To live with the dimensional differences or to get the crank stroked by a capable machine shop. A normal machine shop doing crank regrinds may not be able to do this and a more specialised machinist may be needed.

Thank you Cooperman, mine isn’t going to be a rally car or close to it (though my partner might argue otherwise) so I think in this case I will make sure I can’t get the difference down by swapping them around, and then live with the different CR

Thank you all for your help!!