970s piston height

[Gray]

How close should the flat top piston crowns be to the deck on a 970s. Just had mine to the rolling road to try to get it running better after build up and am told there is lack of compression, I took the head off and they are approx 2.8mm from the deck at tdc?

[Spider]

It almost falls in to the proverbial 'how long is a piece of string' as the rods may have been closed and honed a few times which shortens them, how many times has the block been decked and then there can be some variation in crown heights between manufacturers.

One thing that does sorta poke out though, being a 970 is did you fit 970 rods? They are - from memory - 1/8" longer (3.2 mm). Being a short stroke, low capacity engine, it would be fairly important to try to get the pistons level with or even slightly proud of the deck at TDC.

[ivor badger]

2.8mm seems a bit large. Iirc, 1mm/0.040" is more normal in std form. So as pointed out, 1275 rods in a decked block.

[guru_1071]

how on earth did the engine builder not pick up on this when it was built?

[mk1]

2.8mm is a massive gap.

Whenever I build small capacity engines I am always looking to get the gap between the top of the pistyon & the deck down to about 10 thou.

I can't think of a way you are going to get a respectable C/R without taking the engine out & getting the block decked by about 2.6mm

M

[Gray]

My thought was that I had given Bryan Slark the wrong rods to put in, I was sure that I hadn't mixed them up and as mentioned how come it wasn't picked up. I had already consigned myself to pulling the lump back out, not that worried on that issue it is more to the point working out the issue but may be the age old problem with both 970 & 1275 having the same part numbers but different lengths.

[Gray]

Oh and the pistons are std nos bmc.

[251 ENG]

Something,s wrong there

We normally set the pistons , 0.010" ( 0.25mm ) down the bore on 970 , 1071 and 1275 road engines . This involves -

Measure chamber cc in head
Dummy assembly crank in block with assembly shells
Fit piston to rod with undersize pin with a very light press fit
Fit rod and piston in bore and to crank with assembly shells
Measure position of piston at TDC

If piston is out of top of block , machine piston crown down in lathe.

Calculate compession ratio

Machine , head face , block face , piston dish as required

Fit pistons to rods

This may sound like a lot of messing about BUT this is what you have to do to get it right . It called engine building
Don,t assume anything , the older the bits you are using the more hassle it will probably be.

[KA2S4]

Gray;

Don't worry about it, just fit a turbo!

J

[Spider]

how on earth did the engine builder not pick up on this when it was built?

That did go through my head but I didn't want to be so forward in actually writing it here, but yes, it also makes me wonder what else has been overlooked.

[Gray]

Gray;

Don't worry about it, just fit a turbo!

J

[ivor badger]

Gray;

Don't worry about it, just fit a turbo!

J

the perfect setup. A peaky low torque 999 with turbo lag.

[KA2S4]

Gray;

Don't worry about it, just fit a turbo!

J

the perfect setup. A peaky low torque 999 with turbo lag.

Can't argue with 146mph

[Vegard]

A 970 with a small turbo would be a fantastic little package!!!

[KA2S4]

A 970 with a small turbo would be a fantastic little package!!!

My thoughts exactly Verg, nice small one to plump up the torque and reduce the lag...

Might talk to the Project 64 boys about doing me a road version of their Bonneville motor...

J

[Gray]

Well all is back in with the right length rods now but I cannot for the life of me get the thing to start! Good spark and fuel checked the dizzy is not 180 degrees out. Checked compression tonight and cylinders 1 & 4 are 70psi and 2& 3 are 150psi !? Getting a bit hacked off with it now.

[Andrew1967]

Is it making any attempt to fire Gray?

I'm not surprised it won't start on 70 psi but it ought to at least try to fire on the two that are at good compression.

If you're that hacked off with it, I'll swap you for a made-up S that runs very well

[ricardo]

Gray,

the other day I took a couple of hours to get a 998 engine to start too. Turns out me and my friend spent the day dealing with other minor issues and then, tired, fitted the dizzy 90º out and although it was right in front of us we weren't able to figure it out until the next day.

So, go step by step...

1) Check if all the rockers are correctly adjusted

2) Remove all sparks and rotate the engine until piston 1 (near water pump) is at TDC with both rockers loose (both valves closed). Then remove the dizzy cap and check where the rotor is pointing, it should be somewhere around 1 to 3 o'clock. If not, then something is out. If it is pointing to 7 - 9 o'clock you probably have the dizzy shaft 180º out. Other than that, should be the dizzy. Undo the dizzy clamp and correct it to bring the rotor pointing near to the no. 1 lead on the cap. If the shaft is 180 out you can use a gearbox stud to take the shaft out without letting it fall into the engine.

3) Adjust the points, better to err on the wider side of the recommended gap at this point than the tighter. 0.016'' IIRC. Don't forget to put the rotor back in.

4) Grab a 12V lamp (from a speedo or something) and connect one wire from the bulb holder to the coil (the same side of the coil where the dizzy wire is connected, look for a black/white wire I think) and the other wire to the engine / shell. This should give 12V to the lamp when the spark fires and should work for a positive or negative earth car.

5) Through the inspection hole in the flywheel cover (or other marks you've made in the pulley) slowly rotate the engine until it is around 10º BTDC. This should be just a tiny amount of rotation as you already have piston 1 at TDC.

6) Now turn on the ignition key and rotate back or forwards the dizzy just until the lamp comes on. Back it off a bit to the point when it turns on. This is just a generic approach to get your engine running. Tight the dizzy clamp. If the lamp is always on, re-check the connections and points gap.

7) Check the plug gaps, 0.025'' for points. Test spark in all plugs.

8) Try to start it again...

I think 70psi is too low, especially when you have 150 in the middle cylinders. I have 210psi in my engine, testing WOT, so you have an idea. If the engine doesn't start, maybe a leak down test should be done...

[Spider]

I'd check all the stuff the other guys have mentioned first, however, I'll just add that I had one the other day, fuel, spark (dissy hadn't been touched), wouldn't start. Turned out that the needle and seat had jammed open and so it was flooded.

Did you re-ring it?

[Gray]

Not even a cough from it, I have sprayed a bit of easy start to double check if that made a difference but nothing, tried a bit of fuel down the bore and nothing.
I have double checked timing and checked against #01 being tdc with both rockers rocking and the rotor pointing to lead #01.
The engine has new pistons and rings the dizzy had just been rebuilt by the Dizzer Doctor and ran fine when the wrong rods were installed along with the carbs having been overhauled and they too ran well.
Thanks for the hints guys will have another dig at it at the weekend.