I have yet another question about my Bantam resto, this time regarding advice on pistons.
The engine was seized and I have managed to free it, when I got the barrel off everything looked good until I picked the barrel up and the
lower skirt was broken. I have taken it to a local engineer to see if he can weld it and grind flush. My question is, the number on the piston is;
90-1432 and below that is a number 1. In the spare parts book for D3 and D7 are;
D3 - 90-337, 338, 388, 789, 791, 868, 871 and 873 8 pistons in all.
D7 - 4 15 90 - 1311, 1312 and 1313 total of 3
Anyone with an idea on the number?
I can't do anything like buy new rings (because they may not match the D3 and D7 numbers above), and if the barrel is a write off I will have
to buy a new barrel, with maybe piston and rings to match.
Don't weld the barrel, it's cast iron so it's a good chance that if it doesn't distort it will separate again.
It's hard to find this part number in one of spares catalogues although it seems to be a standard 61.45 mm, 2-ring Bantam piston. The only reason I can think of for it differing from the STD D5 piston (90-1305), is if it is slightly higher compression ratio. However, it's a fair guess that both use the same rings.
In terms of the damaged skirt, if this is only a relatively chuck missing from the bottom, it may not make much difference to the way the engine runs and therefore does not need to be repaired (probably impossible) or replaced. Any chance of a pic?
Hi Guys,
Allan G, Yes I know it's cast iron that is why I didn't do it my self, I have taken it to a cast iron welding company (excuse the pun), they said that if they can do it they will get it ground so that it will work, if not it is a right off anyway.
Spaceman, it broke where the skirt and barrel meet so as I said above it was pretty much a right off. In reply to your other remarks, it's a D7 motor with two piston rings, no oil scraper ring.
The barrel is still in the welders and I never thought to take a picture, I initially thought that I had broken it as it was a seized motor but when I looked at the broken piece I noticed that either side of the broken piece it was black with about only 3/4 of an inch nice shiny cast, so it may have gone while I was riding it causing even more damage. Lucky in that respect I guess.
I agreed with the welding company in that it was a right off any way so nothing lost in trying to weld it.
I thought that it may be an original piston with having the number 1 stamped on it, but I am not an expert.
The same comments apply to the D7 pistons. I believe the D5 and D7 essentially used the same pistons except the later D7 pistons changed to 3 rings. I am sure the piston rings were the same on all the 61.45 mm 2-ring pistons and may well have been the same as the compression rings in the 3-ring sets.
Spaceman that's useful information anyway, because if the barrel is repaired I will only have to get a couple of rings, thanks.
Quote from: scousebantam on 03 Jul, 2023, 11:22
Spaceman that's useful information anyway, because if the barrel is repaired I will only have to get a couple of rings, thanks.
Check it for clearance and have it honed or bored.
D5/D7 barrels are (or at least used to be) pence especially compared to D14 barrels (which had the larger transfer ports), I probably have a spare one blocking up my garage.. (amongst the other collection of Bantam and A65 bits I have collected over the years).
Allan G, that would be great, if you do have one let me know and I will PM you, cheers.
Allan G, don't let me stop you looking in your garage but, at a bike Auto Jumble at the week end I picked up a barrel for my Bantam.
There was a piston in the barrel, the piston had the No 17952 embossed on the inside and .020 R8 stamped on the crown, I am guessing
that it has been re-bored.
The piston in the barrel that I bought has no rings on it, I tried to see if the rings off my original would fit and it doesn't, the thickness of
my original piston is 0.092" (2.33mm) and will not fit in the grooves of the piston that I picked up at the week end.
Can anyone advise please?
17952 is Hepolite number for D10/D14 piston.
These take rings 1/16 1nch wide.
The D7 piston takes rings 3/32 inch wide.
Thanks JulianS, that's good info.
Hi all, just had an after thought, I have been on to Heplolite about rings but, how can I be sure that the piston in the barrel is the right one, after all is was an Auto Bike Jumble.
Anyone know what the gap should be between the piston and the inside of the bore should be?
I forgot to say in the last 2 posts, got my cylinder barrel back from the welders, going to make a plant pot out of it for the wife.
From memory, the piston skirt to bore clearance is .004 to .005"
Many thanks rhyatt.
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?
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"
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.
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
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.
Well done, getting there, they're very helpful, chap called Kit is the man
Cheers rhyatt, I will write that down.
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.
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.
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
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.
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?
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.
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
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.
Do you have an engine reconditioner near you that you can take the barrel, rings and piston to.
They'll be able to measure what you have and what you need.
Yes , rings can be filed to achieve the correct end gap , but 2 stroke rings are pegged making a bit more difficult
Measured the bore yesterday with a vernier gauge and the bore is 2.432" piston is 2.427". Which measures up to the 0.005" gap that you advised me to take earlier of piston to bore.
I looked at the instruction manual I have and it gives measurements in mm, so I will do it again today to see if it measures up to standard bore of
61.5mm.
Just measured the bore and it reads, 61.76mm the standard bore is 61.5mm. The difference .26mm equates to 0.010".
The piston I have that fits the bore reads, 0.020 with the letter R stamped on the crown, the piston has a domed crown,
unlike the original piston which was flat across the crown.
Totally baffled.
Ok- I think I see the issue
you have a +10 cylinder barrel and a + 20 piston(is this the piston that you mention was scored etc??)
if you have ordered +20 piston rings and then are now trying to squeeze them in to a +10 bore, then this is the reason that you have .005 end gaps on the rings.
I think I see 2 options - maybe 3
1) exchange the rings for some +10 and try to find a +10 piston to fit them onto( but you don't actually know the "truth" or roundness of your cylinder barrel - so this route could be wasted time and money.
2) either speak to Cox and Turner to see if you can post your +10 cylinder barrel to them for them to measure and if necessary rebore and at the same time supply a correct brand new piston to fit the newly rebored barrel - this basically starts everything anew.
3) if you have an engine reconditioner near to you and they are conpetent on 2 strokes , approach them for them to measure and if necessary rebore close to you to deliver/collect
My own D7 has a hotchpotch of barrels and pistons and I eventually went down the route of (2) - but I live reasonably close to them
https://www.yell.com/ucs/UcsSearchAction.do?scrambleSeed=211022062&keywords=engine+reconditioners&location=yeovil
you'll have to put your own town or post code into the search box(cox and turner are down the list)
my rebore/piston was about £150
Brilliant, I didn't see that, I was told that they were matching Barrel/Piston so didn't think outside the box, well done for that rhyatt.
I do have a very good automotive engineers less than a mile away, or did have years ago (they done all my re-bores many years ago),
don't know if they are still there, I will know Monday.
As I said before it depends on the reliability of the source you buy from.
ryhatt, that's what I wanted to ask you, in one of your previous posts you said the gap between piston and bore should be 0.004" or 0.005",
which mine is already, if I get the barrel re-bored to +20 (from +10) that 4 or 5 thou gap will be a lot more?
The piston to bore clearance is a constant gap to allow for the aluminium piston to expand in a cast iron cylinder.
Irrespective of standard or plus 10 or plus 20 plus 40.
Your local chap may appreciate looking at the bsa specs to enable him to bore correctly.⁹
Go and see him, take your barrel and piston and rings, see what he can suggest as a way forwards.
I think my best option at this time is to get another barrel if I can, if anyone has a standard barrel that they want to get rid of I can assure them it will be going to a good home.
By the time I get my cylinder re-bored and the head cut to take the high compression piston it will cost me a fortune, I need to cut my costs, at least I have a plus 0.020" high compression piston with new rings (which cost me £37.00) if anyone is interested?
So, I am in the market for a standard D7 barrel, if there is one spare out there, I know I will have to buy more piston rings, but that is my cheapest option.
Thanks all for all your help, it has been appreciated. AND I have learnt a lot.
rhyatt, we were talking about automotive engineers and I said that there used to be, well I went to see them today they are still there and they now take up both sides of the road and they said they couldn't do any automotive stuff because they don't have the tooling to do such small work, they now do heavy industrial work. How times change.
Anyway, I went back to basics, I put a pair of inside callipers in the bore and fixed them, when I went across the tips of the callipers with the verniers they measured 61.76mm.
I am therefore fairly confident to say that they are +10 over standard, and I compared the head with FAQ's on the other site and I am happy that I have D7 barrel and head.
Thanks for your advice.
Ok
So if you're confident that a plus 10 piston will get it running, then go for finding 1 somewhere, you have Kidderminster 01562 66679
Or chasbike but I don't have his number sorry.
If you find that a plus 10 piston is completely unavailable , then you'll be looking at a plus 20 rebore and a new piston .
Of course, if that route happens , you already have plus 20 rings, so just need a plus 20 naked piston
Conclusion;
ryhatt I took your advice, took my barrel to a racing motorbike engineers, less than 3 miles from home, he measured the top, middle and bottom of the cylinder and informed me that there was different readings at all 3 points and advised me to bore out to +40.
I have ordered a piston and rings from Cox & Turner, when it arrives I have to take it to the engineers so that he can hone the cylinder to match the pistons.
Think I am now on the right road, thanks to everyone who helped along the way, much appreciated.
Glad progressing
Just one thing before I pack this thread in please, my engineer has got my barrel and piston and I am picking them up tomorrow but one final question as a matter of interest.
Is it normal that when a cylinder is re-bored the new piston is slightly domed, I ask because Cox & Turner from whom I purchased the new piston and rings said it was a standard flat top piston at 0.040", but it is not, it has a very slight domed crown, or at least not as flat as my original piston. If you know what I mean.
Difficult to suggest, but I think Rex Gaunt has alloy 0.5 thick and a 1.5 thick head gaskets if clearance is a problem
That is handy to know, spoke to my new friend (I think that is one sided though) the motor engineer, and he said try the old solder trick
to see how close the piston is to the head.
Just picked up my cylinder, honed for the piston and no sharp edges, he also loosened my flywheel nut, he has the right tools, all for
50 quid. I thought it was a good price but, no real experience to judge.
He can put a squish band in the head if I need it.
Onwards and upwards.
Yes solder into the plughole after nipping head down and and turn it over by hand ,and see if it gets squashed.
At least you know where you are then.
Use an ally head gasket though