Rebore, or just a hone?

Post Sun Sep 30, 2012 9:35 pm

I'm currently building an M20B25 into an LPG-powered track tool.
The starting point is an engine which span a big end shell four years ago and has been standing ever since.
Well, today I finally got around to pulling it apart; the crank has been scuffed and is probably junk, the conrod looks similar. Not an issue, I have two spare cranks and the rods are being replaced anyway.
What's worrying me is the state of the bores, there's some wear there. Honing marks are gone and there's a detectable lip near the top.
In attempt to measure the wear, I put a piston ring into the bore and measured the end gap at the top and about halfway down. The gap differed by less than 0.05mm, suggesting <0.02mm of bore wear.
Can these bores be recovered with a hone, or do they need a full rebore? This would probably scrap the block as I can't afford new oversize pistons, rings etc.

Post Sun Sep 30, 2012 10:01 pm

Make sure the lip at the top is not carbon build up. If your measurments are correct then the block may well be still within tolerance, but only just, depends on the actual measurment of the bore. Ideal would be get a 2.0 rebored.

I have good blocks that I can rehone for you, cheap. Need them out the way. Also have 2.0 blocks. Give me a call if you are stuck.

Post Sun Sep 30, 2012 10:08 pm

Just hone it with some nice/rough stones

Post Sun Sep 30, 2012 10:10 pm

DanThe wrote:Just hone it with some nice/rough stones

That's what I had done on mine.

Turned out like this.

Post Sun Sep 30, 2012 10:17 pm

That corrosion is creeping dangerously close to the edge of that bore Rav, you having a bit off the top?

Post Sun Sep 30, 2012 10:22 pm

johnl320 wrote:That corrosion is creeping dangerously close to the edge of that bore Rav, you having a bit off the top?

Yes John, it'll be a clean face before it goes back together.

Post Sun Sep 30, 2012 10:24 pm

Sweet

Post Wed Oct 03, 2012 7:13 pm

Cheers all, will get honing

Post Fri Oct 05, 2012 7:31 pm

If you want to measure the wear properly then you will need a dial bore gauge, these can be picked up quite cheep now, otherwise take it to a machine shop and pay them £10 to inspect it.

Rule of thumb i was told once was anything less that 7 thou (0.007") will be ok, if you can't hook a finger nail on the ridge then the ring is unlikely to hook on it.

Post Sat Oct 06, 2012 11:58 am

What I can't measure using a piston ring is ovality, which worries me a little, but the total wear is so low it shouldn't be an issue.
Bore wear of 7 thou (0.18mm) would open the ring end gap by 0.56mm, mine is about a tenth of that, should be OK.

Incidentally, can I re-use piston rings and swap them between pistons (after thorough cleaning). I have nearly three sets of pistons and want to select for end gap.

Oh, and can we please get with the bloody metric system here? The only things measured in inches are wheels, willys and pizzas!

Post Thu Oct 11, 2012 8:45 pm

Aye agreed on the metric front, trouble is a lot of the engine builders are American or old so only deal in inches.

What i ment with the 7thou is for a lip at the top of the bore.

If you can't get a dial bore gauge i would strongly suggest taking it to someone who can measure it properly.
The trouble with using a ring to do it is you might find it sits on the high spots and gives a false reading.
http://www.max-t.co.uk/measuringEquipme ... auges.html
For £30 its not worth risking an engine build.

Rings can be reused but its very subjective, not something to consider doing without very careful measurements of the rings or on a disposable engine where your expecting something else to give up first.