2.5 not turning over when hot

[TimW]

Freshly built 2.5. Late pi recessed block +30thou, new pistons and rings, Mk1 2.5pi rebuilt head with very light skim, ground crank with new big ends and mains. 

Starts very well when cold, won't turn over when hot. Sounds like the battery is flat but it obviously isn't. Was fitted with a saloon pre-engauged starter now replaced with a brand new inertia starter but still same symptoms. Any ideas? 

Compression test done and varies across cylinders between 185psi and 200psi

[glang]

To discount a mechanical problem you could try turning it over by hand when hot and compare the tightness when cold?

[TimW]

1 minute ago, glang said:

To discount a mechanical problem you could try turning it over by hand when hot and compare the tightness when cold?

I'm asking for a friend. I should have said.

I've made that suggestion to him. It turns over fine on the starter when hot with no plugs 😂

[glang]

thats good but really need to compare hot and cold while keeping everything else the same ie. plugs in. Not easy to compare I suppose but if there is a detectable increase in tightness hot that would be unusual (most engines spin easier when hot) and might explain the problem...

[Clive]

Pistons expand a smidge when hot, and that can make the engine hard to start when hot. I had that when I built a 2.5 about 20 years ago, took a while to loosen up/bed in.

A high torque starter may be the answer in the meantime.

[TimW]

9 minutes ago, Clive said:

Pistons expand a smidge when hot, and that can make the engine hard to start when hot. I had that when I built a 2.5 about 20 years ago, took a while to loosen up/bed in.

A high torque starter may be the answer in the meantime.

That's pretty much as we thought Clive. Doing some research though a hot engine compression test shouldn't be more than 10% higher than a cold one so I thought it wouldn't make so much difference? 

[Clive]

I only do observations....

But think once the rings etc all bed in the problem seems to go away. At least that is what happened to mine.

[yorkshire_spam]

Fingers crossed it's just a bit tight with the rebuild and just needs to bed in.

But if I were a pessimist I'd check rockers/valve gear make sure nothing is too tight and binding up when warm. 

Were the ring gaps checked before fitting? Worst case scenario - rings are too long/insufficient gaps and expansion is causing them to grab the bore.

(My money is on #1 though... just needs to bed in)

 

Edited June 30, 2021 by yorkshire_spam

[RobPearce]

11 hours ago, TimW said:

a hot engine compression test shouldn't be more than 10% higher than a cold one

That's because the pistons should be sized such that the rings form a good seal when cold without binding when hot. It's the piston that expands, not the rings, as they're made of different metals. (Actually the rings do expand, and so does the block, but the pistons expand more). The expansion is nowhere near 10% but if the engine is tight then even a tenth of a percent will make a big difference to how tight.

[TimW]

Quick update

Owner fitted a WOSP high torque starter motor and its fixed his hot starting issues. Looks like his engine is just a little tight following the rebuild.