Piston Scratches Your Opinion Please

[Lakeland997]

Rebuilding my rally engine before the new season.
The engine has done a couple of seasons and this is the first time I’ve pulled the pistons out
They are Omega forged type.
All four are displaying scuffs to the sides and the cylinder bores show corresponding marks.

What do you guys think? Time to re-bore and change or good to go another season?

[bwaminispeed]

Well, being the cheap Git that I am, and, with a couple of points, I'd run them again..........

Point one: Is there any ridge at the top of the bore, if yes, re-bore..........

Point two: How are the ring lands on the pistons, if OK, and, point one is OK, go for it, re-use them......

If it passes points one and two, I'd hone the bejeebers out of the cylinders, lightly smooth the scratches on the pistons (I use brown scrubby pads for that)......re-assemble, and, go racing......

New rings, you're call, if it wasn't smoking and the rings don't look too trashed I'd re-use them too (cheap git, remember)

The wear looks very even all round.......

[bwaminispeed]

Although, with that much scratching in two seasons, it almost looks like you are running without air filters........

[Lakeland997]

Although, with that much scratching in two seasons, it almost looks like you are running without air filters........

That's correct, and now you've said that it all makes sense.
No room to fit them because the (daft) regs won't let us modify the bulkhead.

[Oneball]

What carb are you running. We aren’t allowed to modify the bulkhead either and I’ve had air filters on SUs and a Weber.

[Lakeland997]

Split Webers

[Oneball]

I guess the left one is tricky to get a filter on. My single Weber has filters in the speedo hole and a box on the inside of the car

[Lakeland997]

As you can see my speedo hole is 'enlarged' slightly but it has been accepted so far.

Andy

[Lakeland997]

Actually guys, thinking about it, this engine has only run unfiltered in 5 events
Before that it ran filtered HS4’s
A rally at Blyton, MK1 day 2019 and three days of hill climbing in the IOM.
None of the events were particularly dusty.
Seems like a lot of scoring over just a few events.

Andy

[Spider]

Are the scratched all the way in the bores to the top ring ?

[Lakeland997]

Are the scratched all the way in the bores to the top ring ?

Yes Spider, they are evident all the way up to the top ring
Here’s a better image of that.

[Oneball]

Did you check piston clearance when you built it?

I’d go with crud getting in there as others have suggested, I’d expect poor clearance to be more worn and less grooved but may be worth checking.

[Spider]

Are the scratched all the way in the bores to the top ring ?

Yes Spider, they are evident all the way up to the top ring
Here’s a better image of that.

Ah, OK, cheers.

I'd agree with the other guys - Air Filtration. Pity it's too tight to fit something there.

[Hipwell]

Classic no air filtration damage.

Check how slack the rings are in the grooves, if not too bad I would just reuse with new rings and a kiss hone. Give the piston skirts a clean with a scotch brute pad and check the is nothing embedded that will cause further bore damage.

[Lakeland997]

I’ve just re-checked my photo records and found that this engine didn’t even do the Blyton rally - I was using my other engine.
So that means it has only run unfiltered The MK1 day last year and the three days hillclimbing at the Manx Classic where it rained most of the time (as Mark will testify) so I only did about 9 runs over the three days. Prior to that it did a season on the filtered HS4s
Could so much damage be caused in such a limited time?
It’s like someone has thrown sand in there.

[bwaminispeed]

Yep, the damage done when not using filters can be quite substantial in a very short time..........

Especially in tweaked high reving engines, the more you rev them, the quicker the wear takes place.....

[Lakeland997]

I have to admit that I’m shocked.

think I need to experiment with filter socks.

[wantafaster1]

Hello Andy
Socks wouldn't work for me even on a run to the mot centre. Choked all the air off.

I had some of Calvers pistons in 3 or 4 years before I changed to omega and they weren't as bad as that. I'm wondering oil contamination.

[bwaminispeed]

Pretty sure if it was oil contamination that bad, there would be no bearings left in that motor at all.........

[smithyrc30]

Check the ring to piston clearances, if they are good check the bores for ridges and very deep scores (a bore mirror is very useful here) then measure the bores and the pistons. If the bores are round and not tapered, ball hone them. If the piston to bore clearance is within your acceptable clearance range, LIGHTLY clean the skirts and put them back in.

I personally never use abrasives on pistons, too much danger of embedding a piece of abrasive into the aluminum and then having the same issue you are trying to correct.

Always use a file, a wide fine one and just let the weight of the file apply the pressure, it will remove any high spots which is all you would need to do, there is no need to put any pressure on the file.

A bore mirror does not need to be expensive or complicated. I use an old 2.5" hard drive disk pulled from a drive with an old Jag V12 throttle rod screwed into the centre with some washers. If it gets chipped or scratched, I just get a new failed drive.

You will be surprised what you can see that you could not see before, even without a light source.