Bantam piston.

[scousebantam]

rhyatt, please excuse if you think that this is a stupid question, just need to be clear in my head. I have no rings on the piston so am I correct in assuming that if the piston was touching one side of the bore then I should have a clearance of either 0.008" and 0.010" on the opposite side? 

[rhyatt]

Hi,
No.
If you have no rings on the piston, just drop the piston into the bore and enter a 4 or 5 thou feeler gauge between the 2 components, you will have to enter the feeler sideways around the piston to wrap the gauge around it, not straight down.

Then


If you have rings, place 1 into the bore and push it down square in the bore using the piston, remove the piston and then using feelers measure the end gaps of the rings.
Should be about .009" maybe a shade more max .013"

[scousebantam]

rhyatt, got you, I understand. I don't have rings, I emailed Hepolite and they don't stock them anymore.
I will do as you say and check that I have the correct piston, as I said I bought both barrel and piston at a bike autojumble.
Need to check that it is the correct piston first, will do that today.
Thanks.

[rhyatt]

Cox and turner in Yeovil may be able to help you with ring.
Hepolite, once a solid British company , now exists in China or Japan in name only sadly

[scousebantam]

rhyatt, I done exactly as you said and the results are, the  0.004 went in easily, the  0.005 went in with a push and the  0.006 wouldn't go in at all.
So I guess I have the correct piston/barrel, I will now try your advice for the rings, many thanks.

[rhyatt]

Well done, getting there, they're very helpful, chap called Kit is the man

[scousebantam]

Cheers rhyatt, I will write that down.

[Allan G]

Appologies ScouseBantam, I didn't see your reply.

I'm hoping to get in the garage this weekend so will have a look, as long as i have 1x D5/D7 barrel for myself for a trials motor I plan to eventually build, I will be happy. Your new aquisition is a curious one, with it having the later domed piston (which needs the later head to go with it - or careful machining to cut a squish band into the head to prevent the piston and head connecting), I am wondering if the barrel is also a later one? If so the fransfer ports which meet the crank case are bigger, but the carb is bigger too, 26m as opposed to what ever the D5 uses? I seem to recall it works out at something like 22 or 24mm.

[scousebantam]

Thanks Allan G,  I have just purchased the rings for £36.00 for the piston, so do I have to look for a head to suit now. What's a squish band?
What advice would you give me now, the original piston is badly scored, I have tried to clean it with wet & dry but not sure if it would be
serviceable, completely lost now on how to go forward. Two questions;
1   Would it make a big difference using the later barrel that I have, ie the ports.
2   Should I look for a standard barrel. (and I will have to purchase new rings for my original barrel).
I read the tech details for all D5/D7's and the stroke and bore are the same throughout and it's a question of a couple of mil difference in the ports.   

[rhyatt]

I refer you to my original post re the piston and rings in the bore.
Do what I suggested with the rings to ascertain their end gap.
If its correct then you have no need for another barrel.
Again re piston.
You can clean them up surprisingly well and then check in the bore

[rhyatt]

Have you ascertained, by measurements , whether the bore or piston is standard or + X thou overboard.
If you have bought standard rings and their end gaps are true to spec , then try cox and turner for a new piston if yours is scored terminally.
Either was , New or cleaned up piston , you must deglaze the cylinder bore , to allow the new rings to bed in to their new home.

[scousebantam]

Hi rhyatt, yes I did what you said, the gap for the new (to me) barrel and the gap for the piston in the bore is spot on.
And when I get my new piston rings I will measure them in the bore as you suggest
Referring to what AllanG said I have measured all the gaps and for the record are the following;
Gap in the skirt of the barrel is  23.62mm
Port gap in the bottom of the barrel is  20.19mm
Port gap in the engine is  21.23mm
Inlet port gap is  22.20mm
Exhaust port gap is  30.24
My question is are the gaps above to differential for the motor to run efficiently? 

[scousebantam]

Hi rhyatt, received my piston rings, tried them in the bore 0.005" is a comfortable fit and 0.006" is a tight fit, I would guess that that
is not acceptable. 

[rhyatt]

My post just above indicated concern if your bore was standard or + a rebore.
Ring end gaps should be between .009 and .013 thou , irrespective of std or rebore.
If you only have .005 or so, I'm wondering if you have oversize rings in a std bore or
Plus something rings in a smaller bore.
So, you need as suggested earlier I think to ascertain your bore diameter , hopefully you have ordered the right rings for the right bore

[scousebantam]

I ordered the rings from as you instructed me and he certainly knew he was talking about.
I now think someone has just stuck any old piston in a used cylinder that they had and the piston does not belong to that cylinder.
That is the problem, you take your chance when you buy from an autojumble. Even if the person selling tells you they match.
Can I rub the end of the rings with fine emery to gain a few thou.