Technical New build 1.6 16v has low compression on two cylinders

[Clintmac83]

Hi guys, I rebuilt a 1.6 16v strada motor. The cylinder head was completely redone and skimmed then the block was also skimmed and cylinders honed. I fitted new rings and bearings. I now have full compression on cylinder 2 and 3 then number 1 and 4 have only 4 bar compression. I have triple checked the timing.

Originally the motor had 2 bar compression on cylinders 2 and 3 now after the rebuild it swapped around.

 

[Clintmac]

Here is the video

 

[bugsymike]

Just to throw another spanner in the works, at # 15 you mention putting extra gaskets in as it was leaving the exhaust valves open, surely if both cams timed correctly then when doing a leak test on a cylinder with piston on combustion stroke at TDC then all valves on that cylinder must be closed before locking engine up to prevent it turning when compressed air is introduced as if piston moves then valves will open anyway.

 

[Clintmac]

Just to throw another spanner in the works, at # 15 you mention putting extra gaskets in as it was leaving the exhaust valves open, surely if both cams timed correctly then when doing a leak test on a cylinder with piston on combustion stroke at TDC then all valves on that cylinder must be closed before locking engine up to prevent it turning when compressed air is introduced as if piston moves then valves will open anywahave have

Just to throw another spanner in the works, at # 15 you mention putting extra gaskets in as it was leaving the exhaust valves open, surely if both cams timed correctly then when doing a leak test on a cylinder with piston on combustion stroke at TDC then all valves on that cylinder must be closed before locking engine up to prevent it turning when compressed air is introduced as if piston moves then valves will open anyway.

I have two head block and sumps but only one set of cam boxes. It looks like the cambox was skimmed at some stage. On the original block it seals but on this block when the valves should be closed they are a half a mm open. Then if I loosen the cam box bolts just alittle then the valves seal. This only happens on the exhaust valves.

I did the leak down test with both camboxes removed and only moved the pistons to the top. All the other cylinders hold pressure except cylinder 1.

 

[Mike1alike]

Sounds like you need to set the valves clearances.

 

[bugsymike]

I have two head block and sumps but only one set of cam boxes. It looks like the cambox was skimmed at some stage. On the original block it seals but on this block when the valves should be closed they are a half a mm open. Then if I loosen the cam box bolts just alittle then the valves seal. This only happens on the exhaust valves.

I did the leak down test with both camboxes removed and only moved the pistons to the top. All the other cylinders hold pressure except cylinder 1.

You do need to lock the crank when doing the leak test. If you set it at TDC and put air in the plug hole you will see the flywheel move, I have.
However you do it, the test only works if the valves are shut on the cylinder you are testing locked at TDC and the same on each other cylinder you are testing.

 

[Pugglt Auld Jock]

You do need to lock the crank when doing the leak test. If you set it at TDC and put air in the plug hole you will see the flywheel move, I have.
However you do it, the test only works if the valves are shut on the cylinder you are testing locked at TDC and the same on each other cylinder you are testing.

Absolutely. It's sometimes difficult to install a flywheel lock but you'll find that if you've got the pistons very near to TDC then you can easily hold the crank at TDC with a powerbar and socket on the crankshaft bolt. Because there is almost no angle on the crank big end relative to the con rod you can easily resist turning just with your hand on the end of the power bar. Make sure it is at, or VERY NEAR TDC though. If it's appreciably off it'll whip the power bar out of your hand no matter how strong your grip. Maybe cause damage and/or loosen the crank pulley bolt. However, get it right and it's a very easy way to stop the crankshaft rotating.

One thing that does occur to me is, are you absolutely sure this head is a perfect match to the one you took off. What's going through my mind is that often manufacturers make small subtle changes to castings during a production run and sometimes this makes parts incompatible?

 

[bugsymike]

One thing that does occur to me is, are you absolutely sure this head is a perfect match to the one you took off. What's going through my mind is that often manufacturers make small subtle changes to castings during a production run and sometimes this makes parts incompatible?

Very true, I once modified a Mk1 VW Golf 1.6 GTI cylinder head from a burnt out car to fit a 1500 Golf automatic I was repairing, it involved aluminium blanks fitted in two holes in the cylinder head used for oil returns and then redrilling them to align or the gasket would have missed the head. It sounds like a lot of work but at the time it was the cheapest option and the car ran well although it wasn't done for a performance gain. I recall using coins and seals to blank the injector holes in the head.

 

[Clintmac]

This motor has hydraulic lifters

 

[Clintmac]

Absolutely. It's sometimes difficult to install a flywheel lock but you'll find that if you've got the pistons very near to TDC then you can easily hold the crank at TDC with a powerbar and socket on the crankshaft bolt. Because there is almost no angle on the crank big end relative to the con rod you can easily resist turning just with your hand on the end of the power bar. Make sure it is at, or VERY NEAR TDC though. If it's appreciably off it'll whip the power bar out of your hand no matter how strong your grip. Maybe cause damage and/or loosen the crank pulley bolt. However, get it right and it's a very easy way to stop the crankshaft rotating.

One thing that does occur to me is, are you absolutely sure this head is a perfect match to the one you took off. What's going through my mind is that often manufacturers make small subtle changes to castings during a production run and sometimes this makes parts incompatible?

Hi, I did hold the crank in place with a socket and longbar while testing. The two cylinder head looks 100% identical but I will go double check myself again. Thanks

 

[Pugglt Auld Jock]

Hi, I did hold the crank in place with a socket and longbar while testing. The two cylinder head looks 100% identical but I will go double check myself again. Thanks

Good luck.