JP piston for a VB

[bevanclark]

Hi folks,
I have read many posts regarding the excessive weight of JP pistons and was wondering if anyone has a view on whether a +60 JP piston would be likely to cause undue excess vibration in a cast iron head VB.
Cheers,
Bevan

[Dave.Barkshire]

You will hear many opinions about these pistons.

Does the club not have pistons at the moment? The club piston that went into my VH is just about perfect.

[Roger Gwynn]

I have just fitted one of these to my SB and was surprised to find it lighter than the Covmo piston that I took out. Covmo were piston maker like Hepolite. Old piston weighed 703 gms, new JP is 683 but reduced to 661 when used with Covmo pin

[Pete.Silson]

The three VB pistons I have weigh in at (with rings circlips and tapered pin):

• Split skirt (Heplex 11327), STD, 572g
• Early solid skirt (Hepolite 5631), +020, 619g
• Solid skirt (No maker 11327), +060, 625g

[bevanclark]

Very interesting info.
Thanks everyone.

[Roger.Baker]

I had a JP +60 thou in my 58 VB; it vibrated quite a lot, especially on tickover; I sleeved the barrel back to standard and fitted a 5631 piston I had, result - much less vibration.
Roger

[nevhunter]

The later Pistons are far better as concerns weight and a more contemporary design. The Balance factor a would start at is 65%. Whenever a piston is heavier it will vibrate more even though you have balanced it correctly as an up and down shake cannot be canceled by a weight going round and round. . Both flywheels should be counter weighted equally even IF the total weight of each varies. Don't remove weight in a way which will weaken the flywheels. Nev

[david.anderson]

When a .060” oversize piston is fitted the piston will be heavier than std size and the engine will vibrate more.
JP pistons are generally a bit heavier than originals. With the JP 9 to 1 piston for a VH/HS, I reduced the weight by scalloping the piston skirt at the sides a little more, as well as reducing the sides of the gudgeon boss. It was an easy job with a rotary burr and a semi round file. I also used a tapered gudgeon. The final result was the same weight as an original HS 9 to 1 piston. I am not sure how much can be removed from the JP VB piston but certainly some scalloping of the skirt at the sides can be done.
However at any time that a different weight piston is fitted, the engine should be balanced to suit. 65% is the correct balance factor.
Running in the VB may not be easy. I obtained a new, old stock +.030” split skirt piston, for my VB, which was bored to specification split skirt clearance. The engine was very tight and I did not exceed 40mph, 10 mile runs for 150 miles. I then lifted the cylinder and removed the high spots from the piston and hoped it would free up a bit. It remained very tight so I kept the speed down until 700 miles from rebore at which stage I increased speed to 50mph. In no time the engine started to tighten so I stopped to let it cool before continuing home at 40mph. I lifted the cylinder again and while all was ok I honed an extra thou of relief and after that I had no problems. I would not attempt to run a solid skirt VB piston at less than .007” clearance at the bottom of the skirt.
David

[nevhunter]

Yes generally any oversize piston will retain the original core dimensions. For large oversize you might try another piston that has an original larger diameter. . JP are now under a new owner and a lot of redesign has made them more like any other. Some of the earlier ones had extra large pin bosses and thick skirts I prefer a solid skirt without much cam as it has more area in contact with the cylinder wall to cool better. Don't try to run very close fits or you'll be running it in forever even IF it doesn't lock up tight and scratch the bore first. . Nev