Race Engine Forum UK

Am I right to believe that before I send my crankcase to the engineers to have the cylinders re-bored and honed I first have to find out it's current bore.

Then order the pistons BEFORE getting it bored and honed?
So that the engineers (or me) can make sure that it's all done within tolerances.

Yes, your reasoning is correct. Measure the bore to be sure the pistons you order are larger.
Then take the block to be bored along with the pistons, so they can measure them.

Csaba

Thankyou, guess the block shall wait till I order my pistons

Pistons are marketed as being for say 84.00mm bore and the pistons themselves will be some amount smaller across the gauge diameter, say 83.95mm with 0.05 clearance (cold) often marked on the piston crown (SP 0.05 in German = Spiel = free play).

Because measurements made with bore gauges depend so much on calibration accuracy and room temperature it is a good idea if requested by the reboring firm to supply the pistons so they can check in the bores with a feeler gauge.

However. Reboring is done in stages:
1. Bore to within a few thou" of required diameter
2. Rigid hone with stones to true diameter
3. Plateau-hone for good oil retention and correct surface 'roughness' (finish) with reborer's preferred medium, could be Flexhone (though for some reason not popular among reconditioners in the UK) or diamond impregnated cork-stones etc. This op does not alter the diameter to an extent that you could measure it with normal shop gauges.

(all naturally followed by the most diligent cleaning regime with hot water and powerful detergent containing corrosion inhibitor).

The problem one faces when ordering either the rebore or the pistons is one's never sure what size the bores will clean-up at during St 1 above. It is better to find out the aval sizes of pistons before buying and get the reconditioner to size cheeck the bores before starting and do a trial bore on the one most worn. If the bores have to be sleeved to save a valuable block (where they will not clean even at the largest oversize) then my advice is get all of them sleeved because pressing the liner in can cause adjacent bores to bulge. In other words sleeve all first then rebore.

G

Thanks Guy, the cylinders have very light surface rust (I covered the cylinder bores in axle grease while laying up) so a re-bore might not be necessary.

That's me working out a worst case scenario and instead of re-boring, installing liners.

So my best option would be to clean the block, take to my engineers and check the bores for pitting etc and have them honed to see how they come up. Then measure the bore to get an accurate bore size to then contact my piston supplier and order accordingly.

First tool on the list, a cylinder bore gauge, checked pricing and surprisingly cheaper then expected.

Beware of honing by conventional parallel stone method to clean up an old bore.

A worn bore will not be round and trying to hone it so by that means (and I mean trying..) always leads to a big increase in bore size and massive increase in ring end gaps.

If you want to re-ring and just 'freshen up' the bores to prep them so they bed in - first smooth off the wear ridge at the top of the bores (where the top ring changes direction) by hand with 80 and then 120 abrasive tape and get yourself a Flexhone of correct grade for top ring type. I can advise.

Read the thread by Brit01 in "Readers' Cars" about this before you do anything please.

G

Thanks Guy for referring to my thread. It has been quite a learning curve and still learning.

Page #3 of my thread shows the results of the flex-hone I used. 180 grit S/C (for chromed rings) as advised by Guy. Very easy to use. I honed in both directions with a cordless drill.
As Guy mentioned smooth off the wear ridge with 80 grit.

I have some horizontal scouring on the bores unfortunately but could not afford the time or money to have the block taken to a workshop for a re-bore to fix this.

I will monitor oil consumption in the next few months but I expect it to be small and within limits.

Regards,

Chris

Thankyou kindly for your responses, I have in fact been following your Alfa 33 thread.

180 Grit Silicon Carbide flexhone you used, I am guessing I'll have to find out the construction of the rings used in the pistons I'll be getting. Got my eyes on a set of JE pistons which use Pro Seal piston rings.

JE's Pro-Seal rigs need 240 grade silicone carbide medium duty Flexhone.

I recently fitted a set in a VW Golf GTi 1800 fast-road Jetta. The hot-washed block shows the 30 seconds @ 30 deg honing with 45 deg overlay for 10 seconds with 240 Flexhone, following Stages 1,2 outlined earlier

GC

Perfect answer, thankyou I was struggling to find the material used on these rings.

So it's 240 Grade SC flexhone required. I'll get one ordered later today.

I see several different types of Pro Seal rings.

Do you have the DMB type (Moly)?

Guy: Is that why you suggested the 240 grit as he will be supplied with the Moly top rings?

Top Ring:
CUR - Carbon Steel, Chrome, Barrel Face, Back Cut, Torsional
CNS - Carbon Steel, Nitride
DMB - Ductile Iron, Plasma Moly...

2nd Ring:
IPQ - Iron, Phosphate, Tapered Under Cut, Neutral
IPA - ...
...

Sorry I might have jumped the gun there.

The only rings you want to hone with the coarser 180 grade S/C are chrome plated/faced.

Cast-iron, moly coated/faced/PVD coated all get 240 grade.

G

I've not ordered the pistons yet, I'll be getting them within a month or so while my finances settle after this dry spell at work.

I'll contact my supplier for more info on the pistons to make sure what rings they use, they are only a 10 minute drive away from here.

EDIT, I see Guy's new post, so 240 is still most likely, but I will confirm the ring details first.

But remember as Guy said 180 for chromed rings.

He mentioned to me before that if you use 240 for chromed rings then they may never bed in correctly and end up burning oil.

Let us know what type you're getting.

Good luck

Chris