26 replies to this topic

[JonnyAlpha]

You are not going circuit racing.

 

The reason to have extra clearance over the manufacturer's bore diameter is that there is a little bit less friction, and when competing, every little bit might counts.

 

A club racer's engine might do practice and race twice on 30 week ends a year, at the most 2,000 miles

 

Boring a road engine with .002" extra clearance just save the pistons having to wear that much more metal away before the engine needs a rebore.

 

Agreed, I am not going circuit racing

 

Sorry a little confused by "extra clearance", is the manufacturers clearance 0.03mm (0.001") - as stamped on top of the pistons, but the recommended clearance is 0.0025" - 0.003" which is 1.5 to 2 thou more than the manufacturers recommended clearance? 

 

Again a little confused by your line "Boring a road engine with .002" extra clearance just save the pistons having to wear that much more metal away before the engine needs a rebore." 

 

The 0.002" we have measured is the actual clearance not any extra?

[JonnyAlpha]

for a standard(ish) build i'd go with the 2 thou unless it was in a hot climate, if its performance I'd look at getthing them to run the hone through it to take it out a bit.

 

This will be a fast ish road car with hopefully some great pulling power - that's HP and Torque not ladies!!

 

I am planning on fitting an AC RS Cam of which AC quotes:

 

"In a typical example build (1293 to 1330cc) this cam will deliver 73 to 75bhp with a stock head and rockers and 81 to 83lbft of torque with a single HIF44 SU and a  typical stage 1 kit fitted. Power would rise to a typical 86 to 88bhp and 92lbft of torque with the addition of a gas flowed head and 1.3 rockers. Add 1.5 rockers on the same engine, and power climbs to 97 to 100bhp and 95 to 97lbft of torque."

 

So would the 2 thou clearance be acceptable?

[JonnyAlpha]

Get the rod small ends checked for interference fit ( should be no more than 0.8112"), they can be too loose then the rods are scrap. Also, there is no harm in swapping the rods around, the factory machining tolerances mean that some rods are longer than others.

 

Just saw this, so now the old pistons are off I need to get the small ends measured?

 

And yes I have been advised about maybe needing to swap the pistons / rods around when working out the deck height, problem is at the mo I don't have any linished down pins, could I get my old ones linished down? 

Edited by JonnyAlpha, 10 September 2019 - 06:57 PM.

[DeadSquare]

 

You are not going circuit racing.

 

The reason to have extra clearance over the manufacturer's bore diameter is that there is a little bit less friction, and when competing, every little bit might counts.

 

A club racer's engine might do practice and race twice on 30 week ends a year, at the most 2,000 miles

 

Boring a road engine with .002" extra clearance just save the pistons having to wear that much more metal away before the engine needs a rebore.

 

Agreed, I am not going circuit racing

 

Sorry a little confused by "extra clearance", is the manufacturers clearance 0.03mm (0.001") - as stamped on top of the pistons, but the recommended clearance is 0.0025" - 0.003" which is 1.5 to 2 thou more than the manufacturers recommended clearance? 

 

Again a little confused by your line "Boring a road engine with .002" extra clearance just save the pistons having to wear that much more metal away before the engine needs a rebore." 

 

The 0.002" we have measured is the actual clearance not any extra?

 

 

I don't know anything concerning your pistons, but when I was young, if an engine had a bore of say 3", the pistons were made to fit in a 3" bore.

 

When the bore had worn a couple of thou, it could have the glaze honed and the pistons could be re ringed to extend the engine life.

 

To take up any oval wear, the replacement rings had more spring in them which wore the bore more quickly so that in an equivalent time the bores may have worn 5 or 6 thou.

 

Now, the only way of continuing to use the engine was to have it re bored, usually 10 thou, so the pistons were made to fit in a 3.010" bore.

 

This way, everybody knew exactly where they were, so to my mind, if the block of a road going engine is bored to 3,0125", the bore is half way to 3.015" when it needs to be re bored to 3.020".

 

As far as I know. a fast road going car called a Mini Cooper S had pistons that fitted the bore, not a special bore that fitted the pistons.

 

Our racing Downton Mini se7en engine was bored plus .040", as permitted by the regs, and I know that the Dykes top ring racing pistons fitted that bore, because when I mentioned to Downton that I had measured the bore at .0415" , Downton told me to bore another block plus .040" and fit new rings.

Edited by DeadSquare, 10 September 2019 - 08:22 PM.

[Earwax]

Usually the machine shop would want the pistons on hand to bore, but perhaps your guy has seen lots of Nurals and his experience is pretty spot on.  His recommendation on tolerance  is on the tight side, but valid. As far as pistons and measuring go, I think OMEGA fit the piston and ring to the actual size.... ( so if you like the total measure adds to plus 40 neat,,, so the piston is just undersize)  , but other makers make the piston to spec size.  There are quite a few people on this forum who have lots of experience with Nural pistons , so wait for them to chime in.

Edited by Earwax, 10 September 2019 - 09:19 PM.

[Rorf]

The 0.0025 to 0.003 inch piston to cylinder clearance you have been given is dead right

[JonnyAlpha]

The 0.0025 to 0.003 inch piston to cylinder clearance you have been given is dead right

Whilst several folks have suggested that 2 thou would be fine, I am going with the initial advice and the majority and going to ask for the bores to be honed another 1/2 thou.
This would give me the 2.5 thou recommended clearance and the safety factor if running a better head at some point.

Edited by JonnyAlpha, 11 September 2019 - 04:27 PM.

[Rorf]

Just remember that this clearance figure is for the piston skirt diameter to cylinder wall. 

[JonnyAlpha]

So for peace of mind I had the bores and pistons measured by another machine shop, they measured top, middle and bottom of each bore and measured all of the pistons.

 

When I picked up the block and pistons they had marked three of the pistons as 2.818" and one as 2.8175". 

 

Across all four cylinders, at the tops of the bores I have between 2 and 3 thou gap, at the middle I have between 2.5 and 3 thou gap and at the bottom between 2 and 3.5 thou.

 

So I am going with this.

Edited by JonnyAlpha, 13 September 2019 - 04:48 PM.

[Rorf]

After all your hard work just make sure you gap the piston rings correctly; that will be another huge discussion point

[JonnyAlpha]

After all your hard work just make sure you gap the piston rings correctly; that will be another huge discussion point

Interestingly the guy in the Engine shop when I picked my stuff up said. “Don’t bother messing around with piston ring gaps,”The pistons and bores as set perfectly and you’ll only go and break a ring!” ?
But I will check them of course ?

[mini13]

Sounds like a great engine shop....