Britbike forum Forums British Motorcycle specific Brands Bsa bulletin board The A70 Lightning story (rebuild)

Lloyd - I have a badly mangled sludge trap plug. It's not use that I think you want to use, but it will be a good pattern for making a correct one.
Also I have a sludge trap tube, I think. It's mangled too: someone didn't know that the bolt needed to be removed.But it's repairable.

I'll look for a spare thrust washer too. Not sure about that.

I'm out of town through Monday. I'll try to drop these off with Sam mid next week.

Okay moving on, the winner is 5/8”- 18 tpi UNF.

Next question, I am having the con rods checked for ovality. What is the correct size for the big end eye diameter complete with tolerance.

Answer: 1.770" , no tolerance yet.

Ray thanks for checking for these parts. I found the thrust washer and sludge trap tube NOS. All I need is the plug to use as a pattern. Lloyd L

I am attaching the BSA correction bulletin for those who want to know the correct colors for the 1972 models

Here is a link to the video Allan mentioned, as we've been discussing it in the Office today. If you watch from about 10m 40 you'll see a few bikes that were filmed around October 1971. The first A65 (we think) had the brick red 4 gallon tank with white stripe and a little later there's what looks to be an A65 (maybe?) with Dove Grey frame and the Red and Chromed tank at about 10m 46. Our speculation is that a BSA worker bought this tank and fitted it to his earlier bike perhaps?

I am not sure of the colors available in the UK. This bulletin is put out by BSA in Maryland specific for the US market.

I have managed to find some unobtainium items. NOS sludge trap and thrust washer. I will be making some drawings and posting for others that might be in the same situation. Lloyd L

Don't bother buying any lottery tickets. You've already used your quota of good luck.

Detailed drawings will be greatly appreciated. 😊

I have attached the drawing for the A70 thrust washer and sludge trap. Sorry for the quality of the drawing. I had to take a photo of it to be able to attach it to the post. Lloyd L

The sludge trap was made from 1/2" steel tubing with 0.035" wall with end large end swedged to size.

Thank you for posting the sludge trap dimension. I have an a70 engine that was run in a trackmaster frame (have the frame too). I will be rebuilding it once I finish my Gold Star project.

Happy to help another vintage bike person.

Now, for the next question. What is the balance factor that I should use for general riding. This bike will never see a race track. Also, what is the best way to reduce the compression without having new pistons. The reason for the compression reduction is that these engine seem to like very high octane fuel. I will be fueling up at the local gas stations in the area, ie 94 octane.

There is a really nice discussion on balance factors here, with some technical detail from some of the gurus.
The Triumph T140 uses essentially the same frame with a similar bore and stroke, and apparently has a balance factor of 74% A65s seem happy with around 70%, give or take a bit, but that might be for the dry frame.

The ideal way is probably a compression plate under the cylinder, but a thicker head gasket would also work. There's a bloke in the western US who does copper gaskets in various thicknesses. Copper Gaskets Unlimited in Phoenix?
Ed V does A70 work as well, so should be able to help.

If your only backing off the compression to reduce pinging, I would instead go round with a needle file in all the piston corners on the crown, then either polish off with some wet and dry, wire wool or a metal polishing wheel (on the finishing mop)

My A65 is pretty high compression and doesn’t pink even with ordinary fuel, yet the unfinished low compression pistons it had in before it would pink even under light throttle.

Or you can turn the crown off a little in a lathe if you need it lower. A70 domes are quite shallow though compared to the A65, so they get most their compression from the long stroke.

Shane, thanks for the lead on the gaskets. They are closed. I will contact them next week. LloydL

Balance factor I am going for is 66%. Solid mount Nortons do well with this balance factor. I personally would not lower the compression on the A70, you have access to 94 octane which is ample. I am limited to 91 Octane and get away with 10:1 on air cooled bikes. If you are concerned about octane take a degree or two out of timing. If you absolutely need to get lower compression get a 0.060" head gasket.

MR, Thanks for the comments. I will consider all ideas given. Lloyd L

I should update my statement on balance factor as I only mentioned wet balance. I am chasing 66% wet factor which is closer to 72% dry. Jim schmidt (JS Motorsport) has done a ton of research and spintron testing on solid mount Nortons to come to these numbers. If you are looking for smooth the A70 in theory is a lot smoother due to the raised height of the piston pin. Additionally you may want to look into lightening the piston as much as you feel comfortable doing as that will further help reducing vibration.

I am in the process of sending rod specs into Carillo to get rods made without a small end bushing. I will use a DLC coated pin instead. The weight saving is small, but I am rebuilding an A70 race engine and want to be able to run it up high in the rev range as reliably as I can.

Here are the latest photos of the bike. It looks a little different than when it arrived. These parts are off the the powder coater while I am getting the engine ready to assemble.

The early frame does not have the re-enforcement of the centre stand tabs so do not use the stand for starting.

No bsa frame did as standard. I have added supports to my frames which also braces against the centre stand and keeps it more upright than way over centre like most stands. You can then get away with the shorter triumph stand which will make going on and off stand even more a breeze.

The scribed line on the stand is 90° to the frame tube.

Lloyd,
It’ll all look good and like new before too long. Before you get those parts painted, just measure the length of those foot rests, they look like triumph triple ones which were longer, but I’ll double check your original images to confirm.

Again before you paint, a worth while upgrade is to buy the T140 footrests which are a bolt through design (1/2” UNF). Then weld a nut to the frame gusset plate. If ever you need to do anything on the bike that involves taking the primary or timing cover off (or like me you want to make some space to squeeze it and other bikes into a van) then you’ll find the standard way of getting these footrests off is a pain in the backside, even if you cut a slot into the end of the bolt to aid winding it out with a screw driver (it’s an improvement but still a pain).

Apart from the engine capacity (and the above listed mods plus some hidden ones) I made my thunderbolt as close to original as I could, Resto-mod I think is the correct term. Sensible mods without detracting from the original appearance of the bike.

It's worth re-enforcing the large tube welds as well, they always crack.
That is if the bike is ridden hard (which is what it should be, it's an a70 after all.)
Bollox to originality, make the thing a rider.

This document has suggested repairs. I found this on the Triumph forum some years ago but cannot find it now so this link below will download it. This is not my work & I'm grateful to whomever provided it.
Suggested OIF Reinforcements

Every OIF that I've stripped has had cracks.

Before painting the main frame do the pressure test shown in the manual.

Pressure test, hmm, I have not not seen that. Back to the powder coated to retrieve the frame.