46 replies to this topic

[minivanman8]

Evening all,

 

I have a 1293 supercharged A+ which is burning lots of oil (about 50-100 miles between max and minimum on dipstick!)

 

Lift dipstick out and oil spits out of the dipstick hole.

 

I stripped the engine down today to see the cause and this is what I found:

 

 

 

Bores look ok. There are some slight vertical scores, but nothing too drastic. Pistons seem good too. Again, some vertical marks but nothing too major. Theres no ridges on pistons or bores.

 

The only thing that there was an issue with was ring gap - 0.026" half way down bore. Seems a bit on the big side to me? Top of piston to bore measurement was 0.017" and 0.008 bottom of bore to skirt so nothing too untoward there either.

 

Would piston ring gap of 0.026 cause very excessive oil consumption?

Edited by minivanman8, 21 February 2016 - 09:32 PM.

[Cooperman]

Quite possibly, but not necessarily. It may be that the bores are worn slightly oval. That is a very high oil useage.

Rather than remove the dipstick, remove the oil filler cap & run at tick-over. If it 'puffs' smoke, then there is a crankcase pressurisation issue and this is usually a bore wear issue.

[Spider]

IMO - 0.026" is on the big side, Huge in fact. 0.003 to 0.005" per inch of bore diameter, however yours being blown, I'd set them at the bigger end, certainly no more than 0.019".

 

It does sound like it's suffering blow-by, may not be either. The A+ doesn't use a dip stick tube on the block and so do have a tenancy to throw oil up the Dip Stick hole.

 

Could you also measure the piston to bore clearance 1/2' up from the bottom of the piston skirt, at 90⁰ to the Pin line?

 

The big ring gap may not on it's own be the cause of the possible blow by and high oil consumption, but is possibly telling of a combination of issues.

[minivanman8]

I think it may have some iffy valve stem seals, so I plan on having them changed with seals held down by the valves themselves while its all out and apart.

 

How can I check for ovality of bore? I guess a dial bore gauge?

[ACDodd]

Hang on did you say 0.008 at the bottom and 0.026" in the middle?

That's actually 0.006" in bore wear or taper! I'd look at this and bore wash!

Ac

[minivanman8]

0.008 bottom of bore to skirt of piston
0.025 top piston ring gap at mid bore

[MRA]

Hang on did you say 0.008 at the bottom and 0.026" in the middle?

That's actually 0.006" in bore wear or taper! I'd look at this and bore wash!

Ac

Are you sure it's only 0.008" ? 

[MRA]

I think it may have some iffy valve stem seals, so I plan on having them changed with seals held down by the valves themselves while its all out and apart.

 

How can I check for ovality of bore? I guess a dial bore gauge?

 

You will need to measure with a bore gauge or a odd 3 legged micrometer .... or if really lucky a CMM

[MRA]

How many miles has this engine covered ?

Is the oil leaking out or burning off ?  maybe both ?

 

it is not uncommon for bores to wear unevenly but not without cause, ie high miles or lack of oil, high boost but this is not common on cast iron blocks.

 

Also you mention valve guides, these are often reamed instead of the correct method of honing, reaming will give you more "clearance" than honing so more leakage.

[Spider]

0.008 bottom of bore to skirt of piston
0.025 top piston ring gap at mid bore

 

Sorry, I missed that in your earlier post.

 

0.008" Piston to Bore Clearance - I think that's your problem right there.

 

 

 

 

Also you mention valve guides, these are often reamed instead of the correct method of honing, reaming will give you more "clearance" than honing so more leakage.

 

Actually, IMO, both methods, Honing and Reaming are correct.  Reaming is perfectly normal and accepted practice, however Honing does generally give a better result, though more time consuming, therefore usually more expensive, hence why it's not always done. It's also not always the best long term way to go either. Bronze Guides respond better to Honing than Iron.

 

For 99% of the road going 'stuff' out there, Reaming is quite OK, however for top end 'stuff' and when looking for every 0.1% advantage the Honing may be the way to go. Horses for courses really.

 

One potential issue thought with Honing guides is that they may hold no oil what so ever, which will lead to more rapid initial wear of the Guide and Stem than with Reamed Guides, which will always hold some Oil, which will aid with Guide-to-Stem Sealing.

[MRA]

 

0.008 bottom of bore to skirt of piston
0.025 top piston ring gap at mid bore

 

Sorry, I missed that in your earlier post.

 

0.008" Piston to Bore Clearance - I think that's your problem right there.

 

 

 

 

Also you mention valve guides, these are often reamed instead of the correct method of honing, reaming will give you more "clearance" than honing so more leakage.

 

Actually, IMO, both methods, Honing and Reaming are correct.  Reaming is perfectly normal and accepted practice, however Honing does generally give a better result, though more time consuming, therefore usually more expensive, hence why it's not always done. It's also not always the best long term way to go either. Bronze Guides respond better to Honing than Iron.

 

For 99% of the road going 'stuff' out there, Reaming is quite OK, however for top end 'stuff' and when looking for every 0.1% advantage the Honing may be the way to go. Horses for courses really.

 

One potential issue thought with Honing guides is that they may hold no oil what so ever, which will lead to more rapid initial wear of the Guide and Stem than with Reamed Guides, which will always hold some Oil, which will aid with Guide-to-Stem Sealing.

 

 

Yes of course, because everyone reams their blocks to fit pistons !!!  lol  reaming guides is a bodge,

 

Can you add your proof ?   as it is a fact well known within the industry that honing retains oil .... think engine block !

 

Honing retains oil, reaming does not !

Honing is much more accurate so has better valve cooling properties than reaming.

Reaming valve guides is nothing more than a bodge in after market applications.

 

In production, reaming is done to under size guides because the correct amount of material is then removed, after market guides are already to size and reaming takes them oversize.

[minivanman8]

Engine has done about 2000 miles.

I'm not sure how guides were done. Machine shop did them. I guess they took too much meat off the bores when going 1293 if 0.008 is too much. I spoke to MED today and they said I should have 0.002-0.003 clearance at bottom of bore?

Looks like I need a new block!

[Cooperman]

_{^{Not a new block, just go to 1310 cc, that's +0.040" and fit new Omega pistons.}}

[minivanman8]

I could, but the +20's are 18cc dishes for the supercharger and they don't do the dished ones in any other size.

Edited by minivanman8, 22 February 2016 - 08:11 PM.

[minivanman8]

And a set of new Pistons is £££££!